Re: incremental backups - Mailing list pgsql-general

I've started writing some scripts to set up incremental backup to my
taste. I just discovered something and thought I would revisit this
thread briefly.

When you go to restore from a give base file system backup you need
to know  the start WAL file that you need and the end WAL file that
you need.  (You will most likely have many files beyond the "stop"
file but you must have at least up to the "stop" file for the restore
to work.

Now if you try to restore but you don't have the "stop" WAL file
postges will die on recovery and tell you that it can't recover
forward far enough to make the backup consistent.  But I wanted to
know the easiest way to verify if you indeed had the necessary files
without having to actually do a restore and have postgres tell you if
it succeeded or not.

Perhaps no one understood me because the answer I was looking for was
too obvious.   But what I really wanted to know was how do you know
what the "stop" file is.  It informs you of the start file all over
the place when doing the base backups but I thought I would have to
do something clever to figure out the stop file on my own.  But
luckily I don't.  The backup history file has too lines like this:

START WAL LOCATION: 0/88F21D0C (file 000000010000000000000088)
STOP WAL LOCATION: 0/88F21D50 (file 000000010000000000000088)

It was clear to me from the docs how to figure out what the start
file is but the end file was a mystery until I actually created a
backup history file and looked in it.  The only place I can find in
the Online Backup instructions where this is indicated is this
paragraph:

"To make use of this backup, you will need to keep around all the WAL
segment files generated during and after the file system backup. To
aid you in doing this, the pg_stop_backup function creates a backup
history file that is immediately stored into the WAL archive area.
This file is named after the first WAL segment file that you need to
have to make use of the backup. For example, if the starting WAL file
is 0000000100001234000055CD the backup history file will be named
something like 0000000100001234000055CD.007C9330.backup. (The second
number in the file name stands for an exact position within the WAL
file, and can ordinarily be ignored.) Once you have safely archived
the file system backup and the WAL segment files used during the
backup (as specified in the backup history file), all archived WAL
segments with names numerically less are no longer needed to recover
the file system backup and may be deleted. However, you should
consider keeping several backup sets to be absolutely certain that
you can recover your data. Keep in mind that only completed WAL
segment files are archived, so there will be delay between running
pg_stop_backup and the archiving of all WAL segment files needed to
make the file system backup consistent."

Reading it now it seems obvious that the file would contain not only
the start WAL file but also the Stop WAL file but when going over the
directions the first time it did not pick up on it.  And it left me
thinking I would have to use some hack to figure it out if I ever
wanted to test a base backup.  It would have been less confusing to
me if it just said right in the docs: "The backup history file
contains both the start WAL file name and the Stop WAL file name" or
something like that just to make it perfectly clear.

Now that I know this I can extract that filename from the backup
history file, check to see if it has been archived and copy it
somewhere if it hasn't been archived yet.  I'm pretty sure that I can
assume that all files before the stop file have already been
archived.  So once I backup the stop file I can be positive that the
base backup I just made will be valid when I try to restore from it.

This lessens my need for the "get current WAL file" functionality in
this context.  It will still be nice to have in the context of
backing it up every five minutes or so in case a WAL file takes a
long time to fill up.

Anyway I would have been less confused if the docs had made it more
clear that the name of the stop wal file was in the backup history file.

Rick


On Jan 30, 2006, at 10:20 PM, Bruce Momjian wrote:

>
> Yes, I think copying it while it is being written is safe.
>
> ----------------------------------------------------------------------
> -----
>
> 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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>
> --
>   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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>                http://archives.postgresql.org
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