Re: The case of PostgreSQL on NFS Server (II)

On Thu, Apr 2, 2015 at 7:09 PM, Octavi Fors <octavi@live.unc.edu> wrote:
> Thanks John for your extensive and helpful response.
>
<snip>
> You see that I used the ALTER from David in last message, instead your
> suggestion of creating the whole database again.

Looks good!

<snip>
>
> Two only questions remain for me:
>
> 1) could you confirm that I don't have to mount --bind
> /var/lib/postgresql/9.2/main ?

If it is working for you now, then you don't need to mess with the
fstab or the mount --bind at all.

>
> 2) on my my /etc/fstab here is the current line for my NAS:
> nas_ip:/volume1/data /home/ofors/Documents/nas nfs
>
> Which NFS client and server (on NAS side) options/arguments do you suggest
> for optimizing performance? Or in other words, for minimizing database
> corruption in case of NAS (note that NAS drives are in RAID6) or computer
> failure?

Hum, performance is a bit over my head on disk drives, especially NAS.
I have a small NAS, but my main 2Tb disk is eSATA. My fstab entry for
the NAS just looks like:

smartstor1:/VOLUME1/DV7T /SmartStor1/Volume1/DV7T nfs
user,rw,noauto,noexec,nosuid 0 0

which is basically what you have, performance wise, as best as I can tell.

Oh, are you going to try to share a single data base file with
different PostgreSQL servers on different machines __AT THE SAME
TIME__??? I think you're going to "Corruption City". There are locks
which work fairly well on local disks (to share something like a file
or directory) which don't work very well on some NAS boxes. At least,
that is what Dr. Richard Hipp, who wrote and maintains SQLite,
maintains in his support forum. And he is much brighter than I!

>
>
> Thanks so much in advance,
>
> --
> Octavi Fors
>
> On Thu, Apr 2, 2015 at 4:54 PM, John McKown <john.archie.mckown@gmail.com>
> wrote:
>>
>> On Thu, Apr 2, 2015 at 3:01 PM, Octavi Fors <octavi@live.unc.edu> wrote:
>> > Hi,
>> >
>> > this is somehow overlapping one thread which was already posted in this
>> > list
>> >here.
>> >
>> > However, I'm newbie in PostgreSQL and would need some help from experts
>> > on
>> > two aspects. I apologize if these were already implicitely mentioned in
>> > the
>> > thread above.
>> >
>> > My situation is the following:
>> >   1-running postgresql-9.2 server under my Ubuntu 12.04 LTS desktop,
>>
>> I'm running release 9.4 on Fedora 21, but hopefully what I can test
>> will be of some use to you.
>>
>> >   2-user postgres created on the system,
>>
>> Good.
>>
>> >   3-two databases with several tables created, populated and owned by
>> > postgres,
>> >   4-these databases can be queried (SELECT, INSERT, etc.) by any system
>> > user
>> > of my desktop.
>> >
>> > Now here are my two questions:
>> >
>> > a) how can I know where my two databases are stored (I presume in a
>> > directory somewhere in my desktop path)?
>>
>> You might want to read: https://help.ubuntu.com/community/PostgreSQL
>>
>> They will be stored in the default location. On Fedora, this is
>> /var/lib/pgsql/data. I don't know what it is on Ubuntu, but you can
>> find out by using psql. Perhaps something like:
>>
>> sudo su - postgres -c 'SHOW data_directory;'
>>
>> Just using my normal id on Fedora, which is a PostgreSQL (not UNIX)
>> super user, I can see:
>>
>> tsh009=# SHOW data_directory;
>>    data_directory
>> ---------------------
>>  /var/lib/pgsql/data
>> (1 row)
>>
>> tsh009=#
>>
>>
>> >
>> > b) this is the though one (at least for me ;)  For data size reasons, I
>> > want
>> > to store my two databases in a NAS which I have mounted via NFS, but
>> > still
>> > keeping the postgresql server on my desktop.
>>
>> Well, if you insist. But I would not move the default data there. I'd
>> keep it local. See below for an alternative.
>>
>> > I've seen in this message of the thread above, that NFS noac mounting
>> > option
>> > is highly recommended.
>> > Apart from this, I don't see how to migrate the databases from my
>> > desktop
>> > directory determined in a) to my NAS. Could someone please provide the
>> > steps
>> > to accomplish that?
>>
>> Well, there are a couple of approaches. Both will require UNIX root to
>> accomplish. If you are running SELinux enabled & enforcing, it is even
>> more complicated. But basically create an new, empty subdirectory on
>> your NAS box. Look at the owner & group on the directory shown above
>> (/var/lib/pgsql/data) and make it the new directory have the same
>> information. Something like:
>>
>> sudo mkdir -p /NAS-box/pgsql-data
>> sudo chown --reference /var/lib/pgsql/data /NAS-box/pgsql-data
>>
>> This sets up the new NAS-box/pgsql-data directory and assigns it the
>> proper owner:group for use by the server.
>>
>> === Moving the data - BAD IDEA, IMO ==
>>
>> #stop the postgresql server on your system, however you do that. I
>> don't know Ubuntu.
>>
>> sudo cp -a /var/lib/pgsql/data/* /NAS-box/pgsql-data
>> sudo cp -a /var/lib/pgsql/data/.* /NAS-box/pgsql-data
>> sudo mount --bind /var/lib/pgsql/data /NAS-box/pgsql-data
>> sudo restorecon -R /var/lib/pgsql/data # restore SELinux attributes
>>
>> Commands #1 and #2 copy all the data to the new directory.
>> Command #3 does a "bind" mount so that any reference to
>> /var/lib/pgsql/data is redirected to /NAS-box/pgsql
>> Command #4 is if you run with SELinux.
>>
>> Update your fstab to maintain the bind mount when you reboot. The
>> entry will look something like:
>>
>> /NAS-box/pgsql-data /var/lib/pgsql/data none bind
>>
>> === THE ABOVE IS A BAD IDEA ===
>>
>> === Decent idea: ===
>> Create the NAS directory as above. Do _NOT_ copy anything into it! In
>> psql, as a postgres super user, create a new TABLESPACE
>> ref:
>> http://www.postgresql.org/docs/9.2/interactive/sql-createtablespace.html
>>
>> CREATE TABLESPACE onNAS LOCATION '/NAS-box/pgsql-data';
>> CREATE DATABASE somedb TABLESPACE onNAS;
>> \c somedb --connect to new database stored on the NAS
>> CREATE TABLE sometable ... ;
>>
>>
>> Now all the stuff in the "somedb" data base will reside on your NAS
>> box. Now all you need worry about is that it is reliable and fast
>> enough. Hopefully on at least a 1Gb ethernet.
>> In this case, I don't know the SELinux commands to set the SELinux
>> attributes. It is complicated.
>>
>> === Opinion.
>>
>> I have a NAS box. But I would worry about responsiveness. What is
>> better, IMO, is an external SATA connected DAS box. DAS is "Direct
>> Attached Storage". Many PCs have a eSATA port on the back side. You
>> connect your DAS box to that will an eSATA cable. Now you have an
>> external disk expansion which runs at full SATA speed, just like an
>> internal disk drive. It's just an alternative. But you'd do the same
>> thing as with the NAS to get data onto it. Well, except that you could
>> dedicate an entire filesystem on the DAS to the /var/lib/pgsql/data
>> with something like:
>>
>> # /dev/sd?n is the value for the DAS box.
>> sudo mkfs.ext4 /dev/sd?n #create new filesystem on DAS
>> sudo mount /dev/sd?n /tmp #mount it
>> sudo cp -a /var/lib/pgsql/data/* /tmp
>> sudo cp -a /var/lib/pgsql/data/.* /tmp
>> sudo umount /tmp
>> sudo mount /dev/sd?n /var/lib/pgsql/data #mount it
>> sudo restorecon -R /var/lib/pgsql/data # update SELinux attributes
>>
>> The restorecon is only needed if you run SELinux
>>
>>
>> >
>> >
>> > Thanks so much in advance,
>> >
>> > --
>> > Octavi Fors
>> >
>>
>>
>>
>> --
>> If you sent twitter messages while exploring, are you on a textpedition?
>>
>> He's about as useful as a wax frying pan.
>>
>> 10 to the 12th power microphones = 1 Megaphone
>>
>> Maranatha! <><
>> John McKown
>
>
>
>
>



--
If you sent twitter messages while exploring, are you on a textpedition?

He's about as useful as a wax frying pan.

10 to the 12th power microphones = 1 Megaphone

Maranatha! <><
John McKown