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Re: several questions about pg_dumpall, pg_start_backup, pg_basebackup and WAL - Mailing list pgsql-admin

From Patrick Dung
Subject Re: several questions about pg_dumpall, pg_start_backup, pg_basebackup and WAL
Date
Msg-id 1378028013.14431.YahooMailNeo@web193502.mail.sg3.yahoo.com
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In response to Re: several questions about pg_dumpall, pg_start_backup, pg_basebackup and WAL  (Kevin Grittner <kgrittn@ymail.com>)
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'Crash consistent' is a term I heard in some oracle white papers..
If I am correct, it means the database would be consistent after the crash recovery is done.

>> Which means it is consistent at the time that the pg_start_backup
>> is run.

>No.  It will be consistent with the time that pg_stop_backup was
>run, or any later point in time that you choose, as long as you
>have WAL to that point in time.

In the manual, we where told to do three steps:
1) pg_start_backup
2) backup by tar
3) pg_stop_backup

If the backup is consistent when pg_stop_backup is called, I think it worth to mention it in the manual.

Thanks and regards,
Patrick Dung

From: Kevin Grittner <kgrittn@ymail.com>
To: Patrick Dung <patrick_dkt@yahoo.com.hk>; Albe Laurenz <laurenz.albe@wien.gv.at>; "pgsql-admin@postgresql.org" <pgsql-admin@postgresql.org>
Sent: Wednesday, August 28, 2013 3:31 AM
Subject: Re: [ADMIN] several questions about pg_dumpall, pg_start_backup, pg_basebackup and WAL

Patrick Dung <patrick_dkt@yahoo.com.hk> wrote:
> Albe Laurenz <laurenz.albe@wien.gv.at>
>> Patrick Dung wrote:

>>> It is possible that there is file changes (added or file size
>>> changed) between the pg_start_backup and pg_stop_backup.
>>
>> Yes.
>>
>>> Is the backup consistent?
>>
>> The tar or snapshot itself will not be consistent, it will have
>> to be recovered at least until the end on the online backup.
>
> I should ask: is the backup crash consistent?

PITR restore procedures will use the crash recovery mechanism to
make the database consistent, if that's what you mean.

> Which means it is consistent at the time that the pg_start_backup
> is run.

No.  It will be consistent with the time that pg_stop_backup was
run, or any later point in time that you choose, as long as you
have WAL to that point in time.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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