Re: [GENERAL] Shared WAL archive between master and standby: WALs notalways identical - Mailing list pgsql-general

On 02/27/2017 05:52 PM, Sasa Vilic wrote:
> Because standby is running in syncronous replication, whereby wal
> archiver is asynchronous. Therefore there is a small window where slave
> has received the data but master has not pushed it yet to wal archive.

Exactly. The standby already has the latest information, it would gain
nothing from fetching it from the archive and anything it wrote to the
archive would only have the information it got from the master at the
point of failure. That is before you promoted it, after it would be on
its own path independent of the master.

See here for more info:

https://www.postgresql.org/docs/9.6/static/warm-standby-failover.html

>
> Regards,
> Sasa
>
> Am 28.02.2017 02:48 schrieb "Adrian Klaver" <adrian.klaver@aklaver.com
> <mailto:adrian.klaver@aklaver.com>>:
>
>     On 02/27/2017 05:29 PM, Sasa Vilic wrote:
>
>         Master is streaming directly to standby. Both master and standby are
>         pushing WALs to archive.
>
>         My point is that in case that master crashed completely (and we
>         failover
>         to standby) and wal archiver on master didn't push everything to wal
>         archive, we would still have a wal pushed from slave. Therefore
>         there is
>         no interruption in WAL stream.
>
>
>     Still failing to see how the standby can have more information then
>     what the master had sent to it at the time of the crash.
>
>
>         Regards,
>         Sasa
>
>         On 28 February 2017 at 01:57, Adrian Klaver
>         <adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>
>         <mailto:adrian.klaver@aklaver.com
>         <mailto:adrian.klaver@aklaver.com>>> wrote:
>
>             On 02/27/2017 04:40 PM, Sasa Vilic wrote:
>
>                 Hallo,
>
>                 I am trying to setup shared WAL archive between master
>         and standby.
>                 Standby is synchronously streaming from master and both
>         servers
>                 run with
>                 archive_mode = always. The ideas is that when promoting
>         standby to
>                 master we would not missed WALs.
>
>
>             I seem to be missing the point of duplicating your effort.
>
>             You are doing this, correct?:
>
>             Master WAL --> WAL archive <--
>                                           |
>             Master stream --> Standby --> |
>
>             I can't see how the Standby contributes anything to the
>         archive that
>             it does not already have from the Master?
>
>
>
>                 My problem is that sometimes WAL uploaded from master
>         and from
>                 slave are
>                 not 100% identical. In most cases they are but
>         occasionally they are
>                 not. I have written small script that ensures that
>         upload is free of
>                 race condition and I log md5 sum of each WAL. Aren't
>         WALs from
>                 master
>                 and standby supposed to be identical? After all, standby
>         is just
>                 consuming WAL that it is receiving from master ...
>
>                 Or do you have any better suggestion on how to achieve
>         continuous
>                 incremental backup?
>
>                 Thanks in advance
>
>
>
>             --
>             Adrian Klaver
>             adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>
>         <mailto:adrian.klaver@aklaver.com
>         <mailto:adrian.klaver@aklaver.com>>
>
>
>
>
>     --
>     Adrian Klaver
>     adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>
>


--
Adrian Klaver
adrian.klaver@aklaver.com