Re: Shared WAL archive between master and standby: WALs not always identical

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.

Regards,
Sasa

Am 28.02.2017 02:48 schrieb "Adrian Klaver" <adrian(dot)klaver(at)aklaver(dot)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(dot)klaver(at)aklaver(dot)com
>> <mailto:adrian(dot)klaver(at)aklaver(dot)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(dot)klaver(at)aklaver(dot)com <mailto:adrian(dot)klaver(at)aklaver(dot)com>
>>
>>
>>
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com
>