Re: patch proposal

From: Venkata B Nagothi <nag1010(at)gmail(dot)com>
To: Stephen Frost <sfrost(at)snowman(dot)net>
Cc: Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, David Steele <david(at)pgmasters(dot)net>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: patch proposal
Date: 2016-11-08 22:47:36
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Attached is the 2nd version of the patch with some enhancements.

*Scenario 2 :*
> Generates Errors, Hints when the specified recovery target is prior to the
> backup's current position (xid, time and lsn). This behaviour is integrated
> with the parameters "recovery_target_time","recovery_target_lsn" and
> "recovery_target_xid".

In the second version of the patch, recovery_target_xid will be compared
with the "latestCompletedXid" instead of "oldestXid" of the backup and if
the specified recovery_target_xid is prior to the "latestCompletedXid" of
the backup, then errors/hints would be generated indicating the DBA to use
the appropriate backup. Now, with this patch, pg_controldata also displays
the "latestCompletedXid" of the cluster.

> Backup started at: 4/22D3B8E0, time: 2016-08-26 08:29:08
>>> Backup completed (consistency) at: 4/22D3B8EF, 2016-08-26 08:30:00
>>> recovery_target is not set
>>> recovery_target_time = 2016-08-26 08:29:30
>>> recovery_target_inclusive = false
>>> In such a case, we will necessairly commit transactions which happened
>>> between 8:29:30 and 8:30:00 and only stop (I believe..) once we've
>>> reached consistency. We could possibly detect that case and throw an
>>> error instead. The same could happen with xid.
>> Yes, currently, PG just recovers until the consistency is reached. It
>> makes sense to throw an error instead.
> This has not been addressed yet. Currently, the patch only considers
> generating an error if the recovery target position is prior the current
> backup's position and this is irrespective of weather backup_label is
> present or not.
> I will share my updates on how this can be addressed.

This does not seem to be a possibility as the information in the
backup_label is not enough to handle this scenario in specific cases like
xid or time and it does not seem possible
to add the backup stop info to the backup_label. I would prefer leaving
this to the original behaviour at the moment which is : PostgreSQL
errors and exits when *EndOfLog < minRecoveryPoint *during recovery

Documentation is still pending from my end and will be submitting the same
when the patch design/approach is agreed.


Venkata B N
Database Consultant

Fujitsu Australia

Attachment Content-Type Size
recoveryTargetIncomplete.patch-version2 application/octet-stream 17.7 KB

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