Обсуждение: WAL directory size calculation
Hi folks! :-)
I'm about to bring up my brand new production server and I was wondering
if it's possible to calculate (approx.) the WAL directory size.
I have to choose what's better in terms of cost vs. performance (we are
on Google Cloud Platform) between a ramdisk or a separate persistent
disk. Obviously ramdisk will be times faster disk, but having a, say,
512 GB ramdisk will be a little too expensive :-)
I've read somewhere that the formula should be 16 MB * 3 *
checkpoint_segment in size. But won't it be different depending on the
type of /wal_level/ we set? And won't it also be based on the volume of
transactions in the cluster?
And, in place of not-anymore-used-in-9.5 /checkpoint_segment/ what
should I use? /max_wal_size/?
Aside of this, I'm having 350 DBs that sum up a bit more than 1 TB, and
plan to use wal_level=archive because I plan to have a backup server
with barman.
Using the above formula I have:
16 MB * 3 * 1 GB
that leads to to ... uh .. 48000 TB?
Any ideas?
Thanks
Moreno.-
I've read somewhere that the formula should be 16 MB * 3 * checkpoint_segment in size.
Using the above formula I have:
16 MB * 3 * 1 GB
that leads to to ... uh .. 48000 TB?
Am 28.07.2016 um 15:25 schrieb Moreno Andreo: > Hi folks! :-) > I'm about to bring up my brand new production server and I was > wondering if it's possible to calculate (approx.) the WAL directory size. > I have to choose what's better in terms of cost vs. performance (we > are on Google Cloud Platform) between a ramdisk or a separate > persistent disk. Obviously ramdisk will be times faster disk, but > having a, say, 512 GB ramdisk will be a little too expensive :-) > Don't use a RAM-Disk for WAL! DON'T! NEVER! You really need only some GB, if you are using Replication. Without Replication 1 GB would be fine, even with replication. But it must be realible! Andreas
Il 28/07/2016 15:54, Andreas Kretschmer ha scritto: > Am 28.07.2016 um 15:25 schrieb Moreno Andreo: >> Hi folks! :-) >> I'm about to bring up my brand new production server and I was >> wondering if it's possible to calculate (approx.) the WAL directory >> size. >> I have to choose what's better in terms of cost vs. performance (we >> are on Google Cloud Platform) between a ramdisk or a separate >> persistent disk. Obviously ramdisk will be times faster disk, but >> having a, say, 512 GB ramdisk will be a little too expensive :-) >> > > Don't use a RAM-Disk for WAL! DON'T! NEVER! OK OK OK, got the point...... :-) > > You really need only some GB, if you are using Replication. Without > Replication 1 GB would be fine, even with replication. But it must be > realible! I'll get what seems to be the minumum persistent disk in size among GCP, 10 GB. Thanks! > > Andreas > >
Il 28/07/2016 15:33, David G. Johnston
ha scritto:
<blockquote
cite="mid:CAKFQuwaMGb-qMnf0T0F35NQg_FidzVugnBYhSz3AybMwkwppOg@mail.gmail.com"
type="cite">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif"><span
style="font-family:arial,sans-serif">On Thu, Jul 28, 2016 at
9:25 AM, Moreno Andreo <span dir="ltr"
style="font-family:arial,sans-serif"><<a
moz-do-not-send="true"
href="mailto:moreno.andreo@evolu-s.it" target="_blank">moreno.andreo@evolu-s.it><span
style="font-family:arial,sans-serif"> wrote:
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">I've
read somewhere that the formula should be 16 MB * 3 *
checkpoint_segment in size.
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif;display:inline">â[...]â
Â
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">Using
the above formula I have:
  16 MB * 3 * 1 GB
that leads to to ... uh .. 48000 TB?
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif">âYou seem
to be mis-remembering the formula.â
The result is too big to be sane... so there *must* be something
wrong
<blockquote
cite="mid:CAKFQuwaMGb-qMnf0T0F35NQg_FidzVugnBYhSz3AybMwkwppOg@mail.gmail.com"
type="cite">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif;display:inline">âcheckpoiint_segments
is a quantity (i.e., unit-less), not a size. Saying its
"1GB" makes no sense.
Yes, my "1 GB" is related to max_wal_file_size, that from 9.5 came
over checkpoint_segment.
<blockquote
cite="mid:CAKFQuwaMGb-qMnf0T0F35NQg_FidzVugnBYhSz3AybMwkwppOg@mail.gmail.com"
type="cite">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif;display:inline">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif;display:inline">âI'm
also doubting you multiply it by 3 - add three to it
maybe...
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif;display:inline">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif">âEach
segment takes 16MB. The total space required is that
times whatever maximum count of segments you expect to
have.â
That's the variable. How many segments do I expect to have? (jumping
from 9.1 (current) to 9.5 (new) documentation and having a bit of
confusion in my head...)
<blockquote
cite="mid:CAKFQuwaMGb-qMnf0T0F35NQg_FidzVugnBYhSz3AybMwkwppOg@mail.gmail.com"
type="cite">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif">I'd
suggest waiting for better responses on these lists
...[snip]...
Got one, in fact. The statement to *absolutely* not use ramdisk with
wal files resolved almost all of my doubts.
<blockquote
cite="mid:CAKFQuwaMGb-qMnf0T0F35NQg_FidzVugnBYhSz3AybMwkwppOg@mail.gmail.com"
type="cite">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif">David J.
Thanks
Moreno
On Thu, Jul 28, 2016 at 3:25 PM, Moreno Andreo <moreno.andreo@evolu-s.it> wrote: > Obviously ramdisk will be times faster disk, but having a, say, 512 GB > ramdisk will be a little too expensive :-) Besides defeating the purpose of WAL, if you are going to use non persistent storage for WAL you could as well use minimal level, fsync=off and friends. > Aside of this, I'm having 350 DBs that sum up a bit more than 1 TB, and plan > to use wal_level=archive because I plan to have a backup server with barman. Is this why you plan using RAM for WAL ( assuming fast copies to the archive and relying on it for recovery ) ? Francisco Olarte.
Without Replication 1 GB would be fine, even with replication. But it must be realible!
Andreas
--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general
Senior Postgres Architect
Il 28/07/2016 20:45, Francisco Olarte ha scritto: > On Thu, Jul 28, 2016 at 3:25 PM, Moreno Andreo <moreno.andreo@evolu-s.it> wrote: >> Obviously ramdisk will be times faster disk, but having a, say, 512 GB >> ramdisk will be a little too expensive :-) > Besides defeating the purpose of WAL, if you are going to use non > persistent storage for WAL you could as well use minimal level, > fsync=off and friends. After Andreas post and thinking about it a while, I went to the decision that it's better not to use RAM but another persistent disk, because there can be an instant between when a WAL is written and it's fsync'ed, and if a failure happens in this instant the amount of data not fsync'ed is lost. Am I right? > >> Aside of this, I'm having 350 DBs that sum up a bit more than 1 TB, and plan >> to use wal_level=archive because I plan to have a backup server with barman. > Is this why you plan using RAM for WAL ( assuming fast copies to the > archive and relying on it for recovery ) ? Yes, but having to deal with the risk of having loss of data, I think I'll go on a bigger persistent disk, have bigger checkpoint intervals and end up having a longer rescue time, but the main thing is *no data loss* > > Francisco Olarte. > > Thanks Moreno.
Aside of this, I'm having 350 DBs that sum up a bit more than 1 TB, and plan
to use wal_level=archive because I plan to have a backup server with barman.
With that many databases with that so many objectsand undoubtable client connections, I'd want to spread that across a cluster of smaller servers.
just sayin...
-- john r pierce, recycling bits in santa cruz
Il 29/07/2016 10:43, John R Pierce ha
scritto:
<blockquote
cite="mid:5f27cf5e-666d-501e-f250-b4bae0a3f3a3@hogranch.com"
type="cite">
<blockquote
cite="mid:5f43e1cd-9348-64bc-ff0c-1906db671277@evolu-s.it"
type="cite">
Aside of this,
I'm having 350 DBs that sum up a bit more than 1 TB, and
plan
to use wal_level=archive because I plan to have a backup
server with barman.
With that many databases with that so many objects
350 DBs with about 130 tables and a bunch of sequences each, for the
sake of precision.
With extensive use of BLOBs.
<blockquote
cite="mid:5f27cf5e-666d-501e-f250-b4bae0a3f3a3@hogranch.com"
type="cite">
and undoubtable client connections,
Yes, that's another big problem... we run normally between 500 and
700 concurrent connections... I had to set max_connections=1000, the
whole thing grew up faster than we were prepared for...
<blockquote
cite="mid:5f27cf5e-666d-501e-f250-b4bae0a3f3a3@hogranch.com"
type="cite">
I'd want to spread that across a cluster of smaller servers.
That will be step 2... while migration is running (and will run for
some months, we have to plan migration with users) I'll test putting
another one or two machines in cluster, make some test cases, and
when ready, databases will be migrated on other machines, too.
I posted a question about this some months ago, and I was told that
one solution would be to set the servers to be master on some
databases and slave on others, so we can have a better load
balancing (instead of having all writes on the sole master, we split
among all masters depending on which database is getting the write
command, especially when having to write BLOBs that can be some
megabytes in size).
I don't know to achieve this, but I will find a way somewhere.
<blockquote
cite="mid:5f27cf5e-666d-501e-f250-b4bae0a3f3a3@hogranch.com"
type="cite">
just sayin...
ideas are always precious and welcome.
<blockquote
cite="mid:5f27cf5e-666d-501e-f250-b4bae0a3f3a3@hogranch.com"
type="cite">
--
john r pierce, recycling bits in santa cruz
The question to ask is what benefit would you gain by saving BLOB object on a database than on say a flat file server or url on an ftp server? Specially larger ones.
BLOB’s cause a lot problem for all DBs. Not unless the DB engine can understand their structure and process them. It is not worth the effort.
It can hit the DB performance in Indexing, backups, migrations and load balancing.
Hope this helps.
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Moreno Andreo
Sent: 29 July 2016 10:19
To: pgsql-general@postgresql.org
Subject: Re: [SPAM] Re: [GENERAL] WAL directory size calculation
Il 29/07/2016 10:43, John R Pierce ha scritto:
Aside of this, I'm having 350 DBs that sum up a bit more than 1 TB, and plan
to use wal_level=archive because I plan to have a backup server with barman.
With that many databases with that so many objects
350 DBs with about 130 tables and a bunch of sequences each, for the sake of precision.
With extensive use of BLOBs.
and undoubtable client connections,
Yes, that's another big problem... we run normally between 500 and 700 concurrent connections... I had to set max_connections=1000, the whole thing grew up faster than we were prepared for...
I'd want to spread that across a cluster of smaller servers.
That will be step 2... while migration is running (and will run for some months, we have to plan migration with users) I'll test putting another one or two machines in cluster, make some test cases, and when ready, databases will be migrated on other machines, too.
I posted a question about this some months ago, and I was told that one solution would be to set the servers to be master on some databases and slave on others, so we can have a better load balancing (instead of having all writes on the sole master, we split among all masters depending on which database is getting the write command, especially when having to write BLOBs that can be some megabytes in size).
I don't know to achieve this, but I will find a way somewhere.
just sayin...
ideas are always precious and welcome.
--john r pierce, recycling bits in santa cruz
Another option which is growing in popularity is distributed in memory cache. There are quite a few companies providing such technology.
Pricing can range from free to quite expensive.
One recommendation with these technologies is to test them under heavy load conditions.
Good luck.
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Moreno Andreo
Sent: 29 July 2016 10:19
To: pgsql-general@postgresql.org
Subject: Re: [SPAM] Re: [GENERAL] WAL directory size calculation
Il 29/07/2016 10:43, John R Pierce ha scritto:
Aside of this, I'm having 350 DBs that sum up a bit more than 1 TB, and plan
to use wal_level=archive because I plan to have a backup server with barman.
With that many databases with that so many objects
350 DBs with about 130 tables and a bunch of sequences each, for the sake of precision.
With extensive use of BLOBs.
and undoubtable client connections,
Yes, that's another big problem... we run normally between 500 and 700 concurrent connections... I had to set max_connections=1000, the whole thing grew up faster than we were prepared for...
I'd want to spread that across a cluster of smaller servers.
That will be step 2... while migration is running (and will run for some months, we have to plan migration with users) I'll test putting another one or two machines in cluster, make some test cases, and when ready, databases will be migrated on other machines, too.
I posted a question about this some months ago, and I was told that one solution would be to set the servers to be master on some databases and slave on others, so we can have a better load balancing (instead of having all writes on the sole master, we split among all masters depending on which database is getting the write command, especially when having to write BLOBs that can be some megabytes in size).
I don't know to achieve this, but I will find a way somewhere.
just sayin...
ideas are always precious and welcome.
--john r pierce, recycling bits in santa cruz
Il 29/07/2016 11:44,
FarjadFarid(ChkNet) ha scritto:
<blockquote
cite="mid:002101d1e97d$c19a6540$44cf2fc0$@checknetworks.com"
type="cite">
<meta name="Generator" content="Microsoft Word 15 (filtered
medium)">
<!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:Consolas;
panose-1:2 11 6 9 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",serif;
color:black;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
p
{mso-style-priority:99;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:12.0pt;
font-family:"Times New Roman",serif;
color:black;}
pre
{mso-style-priority:99;
mso-style-link:"HTML Preformatted Char";
margin:0cm;
margin-bottom:.0001pt;
font-size:10.0pt;
font-family:"Courier New";
color:black;}
p.msonormal0, li.msonormal0, div.msonormal0
{mso-style-name:msonormal;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:12.0pt;
font-family:"Times New Roman",serif;
color:black;}
span.moz-txt-tag
{mso-style-name:moz-txt-tag;}
span.HTMLPreformattedChar
{mso-style-name:"HTML Preformatted Char";
mso-style-priority:99;
mso-style-link:"HTML Preformatted";
font-family:"Consolas",serif;
color:black;}
span.EmailStyle22
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:72.0pt 72.0pt 72.0pt 72.0pt;}
div.WordSection1
{page:WordSection1;}
-->
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">The
question to ask is what benefit would you gain by saving
BLOB object on a database than on say a flat file server or
url on an ftp server? Specially larger ones.
Privacy. Blobs are stored encrypted, since they are health-related
images or documents.
You should be right if all of this data would be resident only on
our server (that can only be accessed by application), but every
user has a small PG cluster in his PC with his patients data and
images that replicates continuously with our server.
Our application runs on Windows. To get into patient data from
another user (say, someone that stole the computer) is a bit tricky,
because you have to know how to exclude authentication in postgres
and even after this, you have to know where to search and what to
search and sometines what is the meaning on the encodings.
Imagine if we have a folder containing all images.... double click
and open...
Another point is a bit of self-defense. Our users are far to be
smart computer users, and in the past we had some cases in which
someone, trying to clean up a filled-up disk, deleted a directory
under his Paradox database (!!!) and then asked us why the app was
not loading anymore....
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<blockquote
cite="mid:002101d1e97d$c19a6540$44cf2fc0$@checknetworks.com"
type="cite">
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">BLOBâs
cause a lot problem for all DBs. Not unless the DB engine
can understand their structure and process them. It is not
worth the effort.
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">It
can hit the DB performance in Indexing, backups, migrations
and load balancing.
Regarding backups I disagree. Files related to database must be
consistent to the database itself, so backup must be done saving
both database and images. AFAIK there's not a big difference in
backing up image files versus BLOBS in a database.
I agree about load balancing, but only in case of a bulk load of
several megabytes. (our actual server got an overload 2 months ago
when a client we were activating sent a transaction with the
insertion of 50 blobs sizing about 300 megabytes)
<blockquote
cite="mid:002101d1e97d$c19a6540$44cf2fc0$@checknetworks.com"
type="cite">
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Hope
this helps.
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"
lang="EN-US">From:<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"
lang="EN-US"> pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On
Behalf Of Moreno Andreo
Sent: 29 July 2016 10:19
To: pgsql-general@postgresql.org
Subject: Re: [SPAM] Re: [GENERAL] WAL directory
size calculation
Â
Il 29/07/2016 10:43, John R Pierce ha
scritto:
Â
Aside of this, I'm having 350 DBs
that sum up a bit more than 1 TB, and plan
to use wal_level=archive because I plan to have a
backup server with barman.
Â
With that many databases with that so many objects
350 DBs with about 130 tables and a bunch
of sequences each, for the sake of precision.
With extensive use of BLOBs.
and undoubtable client connections,
Yes, that's another big problem... we run
normally between 500 and 700 concurrent connections... I had
to set max_connections=1000, the whole thing grew up faster
than we were prepared for...
I'd want to spread that across a cluster of smaller
servers.
That will be step 2... while migration is
running (and will run for some months, we have to plan
migration with users) I'll test putting another one or two
machines in cluster, make some test cases, and when ready,
databases will be migrated on other machines, too.
I posted a question about this some months ago, and I was told
that one solution would be to set the servers to be master on
some databases and slave on others, so we can have a better
load balancing (instead of having all writes on the sole
master, we split among all masters depending on which database
is getting the write command, especially when having to write
BLOBs that can be some megabytes in size).
I don't know to achieve this, but I will find a way somewhere.
just sayin...
ideas are always precious and welcome.
Â
--
john r pierce, recycling bits in santa cruz
Â
Actually you can increase the over-all performance of your system several fold by distributing the source of data with encryption. CDN services use this old technique consistently all the time.
If you add a URL to an ftp with SSL certificate. Your backup will be much quicker and if someone stole the computer the images are still encrypted as before. It is just the source where data comes from that changes.
Of course for small amount of data, say encrypted user name, password or id credential, db engine is still the best. But for larger files you could benefit substantially by looking at hybrid solutions.
Check out companies like www.maytech.com, not related to me at all. But they have secure network used for NHS (UK).
Their ftp service does have user name password protection which could be customised for different customer. They also distributed servers around the world.
Hope this helps.
From: Moreno Andreo [mailto:moreno.andreo@evolu-s.it]
Sent: 29 July 2016 12:08
To: FarjadFarid(ChkNet) <farjad.farid@checknetworks.com>; pgsql-general@postgresql.org
Subject: Re: [SPAM] Re: [GENERAL] WAL directory size calculation
Il 29/07/2016 11:44, FarjadFarid(ChkNet) ha scritto:
The question to ask is what benefit would you gain by saving BLOB object on a database than on say a flat file server or url on an ftp server? Specially larger ones.
Privacy. Blobs are stored encrypted, since they are health-related images or documents.
You should be right if all of this data would be resident only on our server (that can only be accessed by application), but every user has a small PG cluster in his PC with his patients data and images that replicates continuously with our server.
Our application runs on Windows. To get into patient data from another user (say, someone that stole the computer) is a bit tricky, because you have to know how to exclude authentication in postgres and even after this, you have to know where to search and what to search and sometines what is the meaning on the encodings.
Imagine if we have a folder containing all images.... double click and open...
Another point is a bit of self-defense. Our users are far to be smart computer users, and in the past we had some cases in which someone, trying to clean up a filled-up disk, deleted a directory under his Paradox database (!!!) and then asked us why the app was not loading anymore....
BLOB’s cause a lot problem for all DBs. Not unless the DB engine can understand their structure and process them. It is not worth the effort.
It can hit the DB performance in Indexing, backups, migrations and load balancing.
Regarding backups I disagree. Files related to database must be consistent to the database itself, so backup must be done saving both database and images. AFAIK there's not a big difference in backing up image files versus BLOBS in a database.
I agree about load balancing, but only in case of a bulk load of several megabytes. (our actual server got an overload 2 months ago when a client we were activating sent a transaction with the insertion of 50 blobs sizing about 300 megabytes)
Hope this helps.
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Moreno Andreo
Sent: 29 July 2016 10:19
To: pgsql-general@postgresql.org
Subject: Re: [SPAM] Re: [GENERAL] WAL directory size calculation
Il 29/07/2016 10:43, John R Pierce ha scritto:
Aside of this, I'm having 350 DBs that sum up a bit more than 1 TB, and plan
to use wal_level=archive because I plan to have a backup server with barman.
With that many databases with that so many objects
350 DBs with about 130 tables and a bunch of sequences each, for the sake of precision.
With extensive use of BLOBs.and undoubtable client connections,
Yes, that's another big problem... we run normally between 500 and 700 concurrent connections... I had to set max_connections=1000, the whole thing grew up faster than we were prepared for...
I'd want to spread that across a cluster of smaller servers.
That will be step 2... while migration is running (and will run for some months, we have to plan migration with users) I'll test putting another one or two machines in cluster, make some test cases, and when ready, databases will be migrated on other machines, too.
I posted a question about this some months ago, and I was told that one solution would be to set the servers to be master on some databases and slave on others, so we can have a better load balancing (instead of having all writes on the sole master, we split among all masters depending on which database is getting the write command, especially when having to write BLOBs that can be some megabytes in size).
I don't know to achieve this, but I will find a way somewhere.just sayin...
ideas are always precious and welcome.
--john r pierce, recycling bits in santa cruz
Sorry the URL should have been https://www.maytech.net/
Of course there are other companies in this space.
From: Moreno Andreo [mailto:moreno.andreo@evolu-s.it]
Sent: 29 July 2016 12:08
To: FarjadFarid(ChkNet) <farjad.farid@checknetworks.com>; pgsql-general@postgresql.org
Subject: Re: [SPAM] Re: [GENERAL] WAL directory size calculation
Il 29/07/2016 11:44, FarjadFarid(ChkNet) ha scritto:
The question to ask is what benefit would you gain by saving BLOB object on a database than on say a flat file server or url on an ftp server? Specially larger ones.
Privacy. Blobs are stored encrypted, since they are health-related images or documents.
You should be right if all of this data would be resident only on our server (that can only be accessed by application), but every user has a small PG cluster in his PC with his patients data and images that replicates continuously with our server.
Our application runs on Windows. To get into patient data from another user (say, someone that stole the computer) is a bit tricky, because you have to know how to exclude authentication in postgres and even after this, you have to know where to search and what to search and sometines what is the meaning on the encodings.
Imagine if we have a folder containing all images.... double click and open...
Another point is a bit of self-defense. Our users are far to be smart computer users, and in the past we had some cases in which someone, trying to clean up a filled-up disk, deleted a directory under his Paradox database (!!!) and then asked us why the app was not loading anymore....
BLOB’s cause a lot problem for all DBs. Not unless the DB engine can understand their structure and process them. It is not worth the effort.
It can hit the DB performance in Indexing, backups, migrations and load balancing.
Regarding backups I disagree. Files related to database must be consistent to the database itself, so backup must be done saving both database and images. AFAIK there's not a big difference in backing up image files versus BLOBS in a database.
I agree about load balancing, but only in case of a bulk load of several megabytes. (our actual server got an overload 2 months ago when a client we were activating sent a transaction with the insertion of 50 blobs sizing about 300 megabytes)
Hope this helps.
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Moreno Andreo
Sent: 29 July 2016 10:19
To: pgsql-general@postgresql.org
Subject: Re: [SPAM] Re: [GENERAL] WAL directory size calculation
Il 29/07/2016 10:43, John R Pierce ha scritto:
Aside of this, I'm having 350 DBs that sum up a bit more than 1 TB, and plan
to use wal_level=archive because I plan to have a backup server with barman.
With that many databases with that so many objects
350 DBs with about 130 tables and a bunch of sequences each, for the sake of precision.
With extensive use of BLOBs.and undoubtable client connections,
Yes, that's another big problem... we run normally between 500 and 700 concurrent connections... I had to set max_connections=1000, the whole thing grew up faster than we were prepared for...
I'd want to spread that across a cluster of smaller servers.
That will be step 2... while migration is running (and will run for some months, we have to plan migration with users) I'll test putting another one or two machines in cluster, make some test cases, and when ready, databases will be migrated on other machines, too.
I posted a question about this some months ago, and I was told that one solution would be to set the servers to be master on some databases and slave on others, so we can have a better load balancing (instead of having all writes on the sole master, we split among all masters depending on which database is getting the write command, especially when having to write BLOBs that can be some megabytes in size).
I don't know to achieve this, but I will find a way somewhere.just sayin...
ideas are always precious and welcome.
--john r pierce, recycling bits in santa cruz
Il 29/07/2016 15:13,
FarjadFarid(ChkNet) ha scritto:
<blockquote
cite="mid:000c01d1e99a$f2825c40$d78714c0$@checknetworks.com"
type="cite">
<meta name="Generator" content="Microsoft Word 15 (filtered
medium)">
<!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:Consolas;
panose-1:2 11 6 9 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",serif;
color:black;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
p
{mso-style-priority:99;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:12.0pt;
font-family:"Times New Roman",serif;
color:black;}
pre
{mso-style-priority:99;
mso-style-link:"HTML Preformatted Char";
margin:0cm;
margin-bottom:.0001pt;
font-size:10.0pt;
font-family:"Courier New";
color:black;}
p.msonormal0, li.msonormal0, div.msonormal0
{mso-style-name:msonormal;
mso-style-priority:99;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:12.0pt;
font-family:"Times New Roman",serif;
color:black;}
span.HTMLPreformattedChar
{mso-style-name:"HTML Preformatted Char";
mso-style-priority:99;
mso-style-link:"HTML Preformatted";
font-family:Consolas;
color:black;}
span.moz-txt-tag
{mso-style-name:moz-txt-tag;}
span.EmailStyle22
{mso-style-type:personal;
font-family:"Calibri",sans-serif;
color:windowtext;}
span.EmailStyle23
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:72.0pt 72.0pt 72.0pt 72.0pt;}
div.WordSection1
{page:WordSection1;}
-->
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">If
you add a URL to an ftp with SSL certificate. Your backup
will be much quicker and if someone stole the computer the
images are still encrypted as before. It is just the source
where data comes from that changes.Â
... and if while working the Internet connection drops....? or my
office is not covered by broadband at all (and, still in 2016, in
Italy there are so many places not covered by broadband... no adsl,
no wi-max, low-performing mobile)?
Local copies of databases that we synchronize are made to permit
users to work even if no internet connection is available (since
they're doctors, they have to have their data available almost all
the time)
This architecture is made by design. Some years ago, when we started
designing our software, we went into this situation, and the
question was "Why don't we have just a remote server and users
connect remotely, instead of having replicas in their places?"
This can ease updates, troubleshooting, almost everything.
After a while, the answer we went into is exactly as above. Until we
have slow and unreliable Internet connections (fiber coverage is
growing, but it's still very sparse) so we can't count on them, we
can't rely only to a remote server.
Thanks
Moreno
<blockquote
cite="mid:000c01d1e99a$f2825c40$d78714c0$@checknetworks.com"
type="cite">
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"
lang="EN-US">From:<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"
lang="EN-US"> Moreno Andreo
[mailto:moreno.andreo@evolu-s.it]
Sent: 29 July 2016 12:08
To: FarjadFarid(ChkNet)
<farjad.farid@checknetworks.com>;
pgsql-general@postgresql.org
Subject: Re: [SPAM] Re: [GENERAL] WAL directory
size calculation
Â
Il 29/07/2016 11:44, FarjadFarid(ChkNet)
ha scritto:
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">The
question to ask is what benefit would you gain by saving
BLOB object on a database than on say a flat file server
or url on an ftp server? Specially larger ones.
Privacy. Blobs are stored encrypted, since
they are health-related images or documents.
You should be right if all of this data would be resident only
on our server (that can only be accessed by application), but
every user has a small PG cluster in his PC with his patients
data and images that replicates continuously with our server.
Our application runs on Windows. To get into patient data from
another user (say, someone that stole the computer) is a bit
tricky, because you have to know how to exclude authentication
in postgres and even after this, you have to know where to
search and what to search and sometines what is the meaning on
the encodings.
Imagine if we have a folder containing all images.... double
click and open...
Another point is a bit of self-defense. Our users are far to
be smart computer users, and in the past we had some cases in
which someone, trying to clean up a filled-up disk, deleted a
directory under his Paradox database (!!!) and then asked us
why the app was not loading anymore....
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">BLOBâs
cause a lot problem for all DBs. Not unless the DB engine
can understand their structure and process them. It is not
worth the effort.
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">It
can hit the DB performance in Indexing, backups,
migrations and load balancing.
Regarding backups I disagree. Files related
to database must be consistent to the database itself, so
backup must be done saving both database and images. AFAIK
there's not a big difference in backing up image files versus
BLOBS in a database.
I agree about load balancing, but only in case of a bulk load
of several megabytes. (our actual server got an overload 2
months ago when a client we were activating sent a transaction
with the insertion of 50 blobs sizing about 300 megabytes)
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Hope
this helps.
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext;mso-fareast-language:EN-US">Â
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"
lang="EN-US">From:<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"
lang="EN-US"> <a moz-do-not-send="true"
href="mailto:pgsql-general-owner@postgresql.org">pgsql-general-owner@postgresql.org
[<a moz-do-not-send="true"
href="mailto:pgsql-general-owner@postgresql.org">mailto:pgsql-general-owner@postgresql.org]
On Behalf Of Moreno Andreo
Sent: 29 July 2016 10:19
To: <a moz-do-not-send="true"
href="mailto:pgsql-general@postgresql.org">pgsql-general@postgresql.org
Subject: Re: [SPAM] Re: [GENERAL] WAL directory
size calculation
Â
Il 29/07/2016 10:43, John R Pierce ha
scritto:
Â
Aside of this, I'm having 350 DBs
that sum up a bit more than 1 TB, and plan
to use wal_level=archive because I plan to have a
backup server with barman.
Â
With that many databases with that so many objects
350 DBs with about 130 tables and a bunch
of sequences each, for the sake of precision.
With extensive use of BLOBs.
and undoubtable client connections,
Yes, that's another big problem... we run
normally between 500 and 700 concurrent connections... I had
to set max_connections=1000, the whole thing grew up faster
than we were prepared for...
I'd want to spread that across a cluster of smaller
servers.
That will be step 2... while migration is
running (and will run for some months, we have to plan
migration with users) I'll test putting another one or two
machines in cluster, make some test cases, and when ready,
databases will be migrated on other machines, too.
I posted a question about this some months ago, and I was
told that one solution would be to set the servers to be
master on some databases and slave on others, so we can have
a better load balancing (instead of having all writes on the
sole master, we split among all masters depending on which
database is getting the write command, especially when
having to write BLOBs that can be some megabytes in size).
I don't know to achieve this, but I will find a way
somewhere.
just sayin...
ideas are always precious and welcome.
Â
--
john r pierce, recycling bits in santa cruz
Â
Â
Regarding backups I disagree. Files related to database must be consistent to the database itself, so backup must be done saving both database and images.
Hi: On Fri, Jul 29, 2016 at 10:35 AM, Moreno Andreo <moreno.andreo@evolu-s.it> wrote: > After Andreas post and thinking about it a while, I went to the decision > that it's better not to use RAM but another persistent disk, because there > can be an instant between when a WAL is written and it's fsync'ed, and if a > failure happens in this instant the amount of data not fsync'ed is lost. Am > I right? With the usual configuration, fsync on, etc.. what postgres does is to write and sync THE WAL before commit, but it does not sync the table pages. Should anything bad (tm) happen it can replay the synced wal to recover. If you use a ram disk for WAL and have a large enough ram cache you can lose a lot of data, not just from the last sync. At the worst point you could start a transaction, create a database, fill it and commit and have everything in the ram-wal and the hd cache, then crash and have nothing on reboot. Francisco Olarte.
Il 29/07/2016 15:30, David G. Johnston
ha scritto:
<blockquote
cite="mid:CAKFQuwaD7hyCg-zdMhMF6djfz+XzuM0hOSexXW50k7N4orFU0Q@mail.gmail.com"
type="cite">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif"><span
style="font-family:arial,sans-serif">On Fri, Jul 29, 2016 at
7:08 AM, Moreno Andreo <span dir="ltr"
style="font-family:arial,sans-serif"><<a
moz-do-not-send="true"
href="mailto:moreno.andreo@evolu-s.it" target="_blank">moreno.andreo@evolu-s.it><span
style="font-family:arial,sans-serif"> wrote:
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif;display:inline">âRâ
egarding backups I disagree. Files related to
database must be consistent to the database itself,
so backup must be done saving both database and
images.Â
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif;display:inline">âI'd
suggest you consider that such binary data be defined as
immutable. Then the only problem you have to worry
about is existence - versioning consistency goes away.Â
You only need focus on the versioning of associations -
which remains in the database and is very lightweight.Â
It is then a separate matter to ensure that all
documents you require are accessible given the
identifying information stored in the database and
linked to the primary records via those versioned
associations.
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif;display:inline">
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif;display:inline">David
J.
<div class="gmail_default"
style="font-family:arial,helvetica,sans-serif;display:inline">â
Â
I think you are right on this point, there are only some kind of
bytea that are not immutable, and that's where we store bytea
instead of images (many of these fields have been already
converted to text type, though)
Moreno
Il 29/07/2016 17:26, Francisco Olarte ha scritto: > Hi: > > On Fri, Jul 29, 2016 at 10:35 AM, Moreno Andreo > <moreno.andreo@evolu-s.it> wrote: >> After Andreas post and thinking about it a while, I went to the decision >> that it's better not to use RAM but another persistent disk, because there >> can be an instant between when a WAL is written and it's fsync'ed, and if a >> failure happens in this instant the amount of data not fsync'ed is lost. Am >> I right? > With the usual configuration, fsync on, etc.. what postgres does is to > write and sync THE WAL before commit, but it does not sync the table > pages. Should anything bad (tm) happen it can replay the synced wal to > recover. If you use a ram disk for WAL and have a large enough ram > cache you can lose a lot of data, not just from the last sync. At the > worst point you could start a transaction, create a database, fill it > and commit and have everything in the ram-wal and the hd cache, then > crash and have nothing on reboot. > > Francisco Olarte. > > This is another good point. I'm ending up with a 10 GB SSD dedicated to WAL files. I'm starting with a small slice of my clients for now, to test production environment, and as traffic will grow, I'll see if my choice was good or has to be improved. Should I keep fsync off? I'd think it would be better leaving it on, right?
On Wed, Aug 3, 2016 at 8:07 PM, Moreno Andreo <moreno.andreo@evolu-s.it> wrote: > Should I keep fsync off? I'd think it would be better leaving it on, right? From the docs: https://www.postgresql.org/docs/9.6/static/runtime-config-wal.html#RUNTIME-CONFIG-WAL-SETTINGS While turning off fsync is often a performance benefit, this can result in unrecoverable data corruption in the event of a power failure or system crash. Thus it is only advisable to turn off fsync if you can easily recreate your entire database from external data. So if you care about your data, that's to set in any case to on, as is full_page_writes. -- Michael
Il 03/08/2016 14:12, Michael Paquier ha scritto: > On Wed, Aug 3, 2016 at 8:07 PM, Moreno Andreo <moreno.andreo@evolu-s.it> wrote: >> Should I keep fsync off? I'd think it would be better leaving it on, right? > >From the docs: https://www.postgresql.org/docs/9.6/static/runtime-config-wal.html#RUNTIME-CONFIG-WAL-SETTINGS > While turning off fsync is often a performance benefit, this can > result in unrecoverable data corruption in the event of a power > failure or system crash. Thus it is only advisable to turn off fsync > if you can easily recreate your entire database from external data. > > So if you care about your data, that's to set in any case to on, as is > full_page_writes. Got it. Thanks! Moreno
On Thu, Jul 28, 2016 at 6:33 AM, David G. Johnston <david.g.johnston@gmail.com> wrote: > On Thu, Jul 28, 2016 at 9:25 AM, Moreno Andreo <moreno.andreo@evolu-s.it> > wrote: >> >> I've read somewhere that the formula should be 16 MB * 3 * >> checkpoint_segment in size. > > [...] > >> >> Using the above formula I have: >> 16 MB * 3 * 1 GB >> that leads to to ... uh .. 48000 TB? > > > You seem to be mis-remembering the formula. > > checkpoiint_segments is a quantity (i.e., unit-less), not a size. Saying > its "1GB" makes no sense. Right, that doesn't make any sense. > I'm also doubting you multiply it by 3 - add three to it maybe... No, multiplying by 3 is roughly correct. A log file can't be recycled/removed until there have been 2 checkpoints started and completed. And if you have checkpoint completion target set to high value, the 2nd checkpoint will complete just before the 3rd one will be triggered to start. So that means you have 3 * checkpoint_segments worth of segment. Cheers, Jeff
On Thu, Jul 28, 2016 at 6:25 AM, Moreno Andreo <moreno.andreo@evolu-s.it> wrote: > Hi folks! :-) > I'm about to bring up my brand new production server and I was wondering if > it's possible to calculate (approx.) the WAL directory size. > I have to choose what's better in terms of cost vs. performance (we are on > Google Cloud Platform) between a ramdisk or a separate persistent disk. As others have said, there is almost no point in putting WAL on a ramdisk. It will not be there exactly at the time you need it. > Obviously ramdisk will be times faster disk, but having a, say, 512 GB > ramdisk will be a little too expensive :-) > I've read somewhere that the formula should be 16 MB * 3 * > checkpoint_segment in size. But won't it be different depending on the type > of /wal_level/ we set? And won't it also be based on the volume of > transactions in the cluster? Not in usual cases. If you have more volume, then checkpoint_segment will get exceeded more frequently and you will have more frequent checkpoints. As long as your system can actually keep up with the checkpoints, then the more frequent checkpoints will cancel the higher volume, leaving you with the same steady-state number of segments. > And, in place of not-anymore-used-in-9.5 /checkpoint_segment/ what should I > use? /max_wal_size/? max_wal_size doesn't just replace "checkpoint_segment" in the formula. It replaces the entire formula itself. That was the reason for introducing it. > Aside of this, I'm having 350 DBs that sum up a bit more than 1 TB, and plan > to use wal_level=archive because I plan to have a backup server with barman. > > Using the above formula I have: > 16 MB * 3 * 1 GB If you are getting the "1 GB" from max_wal_size, then see above. Note that max_wal_size is not a hard limit. It will be exceeded if your system can't keep up with the checkpoint schedule. Or if archive_command can't keep up. Cheers, Jeff
Il 03/08/2016 18:01, Jeff Janes ha scritto: > On Thu, Jul 28, 2016 at 6:25 AM, Moreno Andreo <moreno.andreo@evolu-s.it> wrote: >> Hi folks! :-) >> I'm about to bring up my brand new production server and I was wondering if >> it's possible to calculate (approx.) the WAL directory size. >> I have to choose what's better in terms of cost vs. performance (we are on >> Google Cloud Platform) between a ramdisk or a separate persistent disk. > As others have said, there is almost no point in putting WAL on a > ramdisk. It will not be there exactly at the time you need it. OK, got it, as I already stated. That was just a bad thought :-) > >> Obviously ramdisk will be times faster disk, but having a, say, 512 GB >> ramdisk will be a little too expensive :-) >> I've read somewhere that the formula should be 16 MB * 3 * >> checkpoint_segment in size. But won't it be different depending on the type >> of /wal_level/ we set? And won't it also be based on the volume of >> transactions in the cluster? > Not in usual cases. If you have more volume, then checkpoint_segment > will get exceeded more frequently and you will have more frequent > checkpoints. As long as your system can actually keep up with the > checkpoints, then the more frequent checkpoints will cancel the higher > volume, leaving you with the same steady-state number of segments. So if I want to keep checkpoint happening not frequently, the solution is to have a bigger checkpoint_segment (or max_wal_size), so value gets exceeded less frequently? >> And, in place of not-anymore-used-in-9.5 /checkpoint_segment/ what should I >> use? /max_wal_size/? > max_wal_size doesn't just replace "checkpoint_segment" in the formula. > It replaces the entire > formula itself. That was the reason for introducing it. Another point cleared. I did not get this in the docs. I'll go an read it again. > >> Aside of this, I'm having 350 DBs that sum up a bit more than 1 TB, and plan >> to use wal_level=archive because I plan to have a backup server with barman. >> >> Using the above formula I have: >> 16 MB * 3 * 1 GB > If you are getting the "1 GB" from max_wal_size, then see above. Exactly. I think it's its default value, since I didn't change it. > Note that max_wal_size is not a hard limit. It will be exceeded if > your system can't keep up with the checkpoint schedule. Or if > archive_command can't keep up. Got it. Thanks Moreno > > Cheers, > > Jeff > >
Hi Moreno: On Wed, Aug 3, 2016 at 1:07 PM, Moreno Andreo <moreno.andreo@evolu-s.it> wrote: It's already been answered, but as it seems to be answering a chunk of my mail... > Should I keep fsync off? I'd think it would be better leaving it on, right? Yes. If you have to ask wether fsync should be on, it should. I mean, you only take it off when you are absolutely sure of where you are doing, fsync off goes against the D in acid. You normally only turn it off in counted cases. As an example we have an special postgresql.conf for full cluster restores, with fsync=off. Wehen we need it we stop the cluster, boot it with that, restore, stop it again and reboot with the normal fsync=on config. In this case we do not mind losing data as we are doing a full restore anyway. But normally, its a bad idea. As a classic photo caption says, fsync=off => DBAs running with scissors. Francisco Olarte.