Re: incremental backups

And here is the real million dollar question.  Let's say for some
reason I don't have the last WAL file I need for my backup to be
valid.  Will it die and tell me it's bad or will it just start up
with a screwed up data directory?


On Jan 30, 2006, at 4:29 PM, Rick Gigger wrote:

> Yes!  Thanks you!  That is exactly what I was looking for.
>
> So I take it that this means that it is save to copy the current in
> use WAL file even as it is being written to?
> And it also means that if I copy it with my physical file system
> backup then I should have the last file that I need to restore from
> that physical backup?
>
> So if I write my own backup_latest_WAL_file.sh script (I think I
> found one on the list from Simon Riggs) then I can do what I need
> to do before those todo items get done?  Or will I need to wait
> till postgres gives me the ability to safely copy the file?
>
>
>
> On Jan 30, 2006, at 11:13 AM, Bruce Momjian wrote:
>
>>
>> Unfortunately, I think I understand your question.  :-)
>>
>> These TODO items are what you need:
>>
>>     * Point-In-Time Recovery (PITR)
>>
>>           o Allow point-in-time recovery to archive partially filled
>>             write-ahead logs [pitr]
>>
>>             Currently only full WAL files are archived. This means
>> that the
>>             most recent transactions aren't available for recovery
>> in case
>>             of a disk failure. This could be triggered by a user
>> command or
>>             a timer.
>>
>>           o Automatically force archiving of partially-filled WAL
>> files when
>>             pg_stop_backup() is called or the server is stopped
>>
>>             Doing this will allow administrators to know more
>> easily when
>>             the archive contains all the files needed for point-in-
>> time
>>             recovery.
>>
>> I will try to push to have them done for 8.2.
>>
>> ---------------------------------------------------------------------
>> ------
>>
>> Rick Gigger wrote:
>>> I guess my email wasn't all that clear.  I will try to rephrase.  I
>>> am moving from using the old style pg_dump for backups to using
>>> incrementals and want to make sure I understand the process before I
>>> go about writing a bunch of scritps.
>>>
>>> To me setting up incremental backup consists of the following
>>> components:
>>>
>>> 1) Setting up the WAL archiving.  This one is trivial.
>>> 2) Doing physical dumps of the $PGDATA directory.  This one is once
>>> again trivial.
>>> 3) Knowing which physical dumps are Good and Not Good.  For a given
>>> physical dump D there is are WAL archive files Dstart and Dend for
>>> which you much have Dstart and Dend and all files in between.  If
>>> you
>>> have all those files then the physical dump is Good.  If you don't
>>> have them then the dump is worthless to you.
>>> 4) Knowing which dumps and which archive files can be deleted.  This
>>> depends on a number of factors.
>>>     a) How far back do you want to be able to do PITR
>>>     b) How much space do you have / want to use for PITR
>>>     c) Which physical dumps are Good and which are Not Good. (see #3)
>>>
>>> Now I think I have a pretty good plan here except for #3 (and so #4
>>> then also suffers).
>>>
>>> Just as an example lets say I'm not concerned so much with PITR as I
>>> am recovering from a db crash. I've got all the backups files saved
>>> to my backup db server so I can failover to it if my primary db
>>> server dies.  I just want to make sure I've got one physical dump
>>> that is good.  (This is not my actual situation but it
>>> illustrated my
>>> point better.)
>>>
>>> Now when I do a physical dump it is not a Good dump.  That is I
>>> don't
>>> have the end archive file necessary to recover from that physical
>>> dump.  That is to say that  when I call pg_backup_start() then copy
>>> $PGDATA then call pg_backup_stop() postgres might be on say WAL
>>> archive file #5.  Once the physical dump is completed WAL archive
>>> file #5 hasn't been archived yet.  I only have up to #4.  So if I
>>> delete my old physical dumps and all I've got is this most recent
>>> one
>>> and my database crashes before #5 gets archived then I am hosed.  I
>>> have no good physical backups to start from.
>>>
>>> My main question is about the best way to figure out when a physical
>>> dump is Good.
>>>
>>> One strategy is to always keep around lots of physical dumps.  If
>>> you
>>> keep around 100 dumps you can be pretty sure that in the space of
>>> time that those physical dumps take place that at least one WAL file
>>> was archived.  In fact if you keep 2 physical dumps you can be
>>> fairly
>>> certain of this.  If not then you really need to space our your
>>> dumps
>>> more.
>>>
>>> Is this making sense at this point?
>>>
>>> The problem is that the WAL archiving is triggered by postgres and
>>> the rate at which the db is updated.  The physical dumps are
>>> triggered by cron and on a purely time based schedule.  So in theory
>>> if you had the physical dumps happening once a day but for some odd
>>> reason no one updated the database for 4 days then all of a sudden
>>> you'd have 2 physical backups and neither of them are good.  If
>>> you're db crashes during that time you are hosed.
>>>
>>> Maybe I am arguing a point that is just stupid because this will
>>> never happen in real life.  But in that it is my backups system that
>>> I will be using to recover from complete and total disaster I just
>>> want to have all my bases covered.
>>>
>>> So my ideas on how to determine if a physical dump is Good are as
>>> follows.
>>>
>>> 1) When you do the physical backup (after dumping the $PGDATA dir
>>> but
>>> before calling pg_stop_backup() ) determine the current WAL archive
>>> file.  Mark somewhere in the backed up physical dump the last file
>>> needed for the dump to be considered good.  Then your deletion
>>> scripts can look at the WAL archive files you have and the last one
>>> required for the dump to be Good and determine if the dump is
>>> Good or
>>> not.
>>>
>>> 2) After doing the physical dump but before calling pg_stop_backup()
>>> just copy the current WAL file to the physical dump.  If that file
>>> later gets archived then the restore commands overwrites your
>>> partially completed one so it doesn't hurt but you know that when
>>> you
>>> call pg_stop_backup() that that physical dump is good.  (Is it ok to
>>> copy the current WAL file while it is still in use?)
>>>
>>> Is anyone taking one of these or any other precautions to make sure
>>> they've got a good physical dump or does everyone just keep a whole
>>> bunch of dumps around, and then actually restore the dump to see if
>>> it is good and if not go back to a previous dump?
>>>
>>> I hope that makes more sense.
>>>
>>> Thanks,
>>>
>>> Rick
>>>
>>> On Jan 27, 2006, at 3:33 AM, Richard Huxton wrote:
>>>
>>>> Rick Gigger wrote:
>>>>> Um, no you didn't read my email at all.  I am aware of all of that
>>>>> and it is clearly outlined in the docs.  My email was about a
>>>>> specific detail in the process.  Please read it if you want to
>>>>> know what my actual question was.
>>>>
>>>> I'm not sure your email is quite right as regards the process. You
>>>> need:
>>>>   1. the filesystem backup
>>>>   2. the WAL file indicated in the history-file
>>>>   3. all the WAL files later than that
>>>> to get up to "now".
>>>>
>>>> If you don't want to replay up to "now" then you will not need some
>>>> of the more recent WAL files. You can't afford to throw them away
>>>> though since you've got a rolling backup system running and the
>>>> whole point is so you can recover to any point you like.
>>>>
>>>> You can however throw away any WAL files older than that indicated
>>>> in the history file for your current filesystem-backup. You can
>>>> then only restore from that point in time forward.
>>>>
>>>> There is no "last one" in the WAL set unless you know the time you
>>>> want to restore to. Indeed, the "last one" might not be "full" yet
>>>> and therefore archived if you want to restore to 10 seconds ago.
>>>>
>>>> Or am I mis-understanding your email too?
>>>>
>>>> --
>>>>   Richard Huxton
>>>>   Archonet Ltd
>>>>
>>>
>>>
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>>
>> --
>>   Bruce Momjian                        |  http://candle.pha.pa.us
>>   pgman@candle.pha.pa.us               |  (610) 359-1001
>>   +  If your life is a hard drive,     |  13 Roberts Road
>>   +  Christ can be your backup.        |  Newtown Square,
>> Pennsylvania 19073
>>
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