Thread: WAL archive on slave
Hello,
My understanding is that WAL archiving can not be enabled on the slave in a streaming replication pair.
If this is correct, is there a reason behind it? I can see logs showing up in pg_xlog, so could they not be archived?
The reason I ask is if this happened it would allow the following with a streaming replication pair (A,B):
- Start A as master
- Attach B as slave using basebackup
- work ....
- Promote B to master
- Restore A from a scheduled backup to a time before promotion
- Attach A as slave pointing at B's WAL archive
If we used A's WAL archive in this case and A had writes after the promotion then we would get timeline errors.
As far as I can tell, using the WAL archive from B would resolve this issue.
Or have I missed something here?
Cheers,
Cheers,
James Sewell,
PostgreSQL Team Lead / Solutions Architect
______________________________________
PostgreSQL Team Lead / Solutions Architect
______________________________________
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Thanks for the reply Albe.
I have WAL archiving enabled on both my servers, but it only happens on the master.
When I promote the slave to be the new master then it will start archiving automatically, which suggests that my configuration is correct.
Can you think of anything else which might cause this?
Cheers,
James Sewell,
PostgreSQL Team Lead / Solutions Architect
______________________________________
On Fri, Feb 7, 2014 at 7:30 PM, Albe Laurenz <laurenz.albe@wien.gv.at> wrote:
James Sewell wrote:It can be enabled. Did you try it?
> My understanding is that WAL archiving can not be enabled on the slave in a streaming replication
> pair.These are files containing the WAL data replicated from the master.
> If this is correct, is there a reason behind it? I can see logs showing up in pg_xlog, so could they
> not be archived?
They won't be archived.> 1. Start A as master
> The reason I ask is if this happened it would allow the following with a streaming replication pair
> (A,B):
>
>
> 2. Attach B as slave using basebackup
> 3. work ....
> 4. Promote B to master
>
> 5. Restore A from a scheduled backup to a time before promotion
> 6. Attach A as slave pointing at B's WAL archive>You shouldn't with 9.3, because in that case A would follow the
> If we used A's WAL archive in this case and A had writes after the promotion then we would get
> timeline errors.
timeline switch introduced by B's promotion rather than its old timelime.
http://www.postgresql.org/message-id/E1TjCRc-00084r-1H@gemulon.postgresql.org
I may be missing something there since I have never tried it.That should work in any event.
> As far as I can tell, using the WAL archive from B would resolve this issue.
Yours,
Laurenz Albe
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Hi again,
I just realised I asked this question around a year ago (for a different reason actually) and you answered me then! Amazing!
Your answer then was:
It works fine, only the server will not generate WAL while
it is in recovery. As soon as you promote the standby,
it will archive ist WALs.
Is this still correct? If it is the the only way that I could achieve what I wanted would be to set wal_keep_segments high enough then they will all be archived on promotion?
I'm still not sure why they wouldn't be archived on the slave, seen as they show up in the directory? Is there a limitation I'm not thinking about here?
Cheers,
James Sewell,
PostgreSQL Team Lead / Solutions Architect
______________________________________
On Mon, Feb 10, 2014 at 11:16 AM, James Sewell <james.sewell@lisasoft.com> wrote:
Thanks for the reply Albe.I have WAL archiving enabled on both my servers, but it only happens on the master.When I promote the slave to be the new master then it will start archiving automatically, which suggests that my configuration is correct.Can you think of anything else which might cause this?Cheers,
James Sewell,
PostgreSQL Team Lead / Solutions Architect
______________________________________
On Fri, Feb 7, 2014 at 7:30 PM, Albe Laurenz <laurenz.albe@wien.gv.at> wrote:James Sewell wrote:It can be enabled. Did you try it?
> My understanding is that WAL archiving can not be enabled on the slave in a streaming replication
> pair.These are files containing the WAL data replicated from the master.
> If this is correct, is there a reason behind it? I can see logs showing up in pg_xlog, so could they
> not be archived?
They won't be archived.> 1. Start A as master
> The reason I ask is if this happened it would allow the following with a streaming replication pair
> (A,B):
>
>
> 2. Attach B as slave using basebackup
> 3. work ....
> 4. Promote B to master
>
> 5. Restore A from a scheduled backup to a time before promotion
> 6. Attach A as slave pointing at B's WAL archive>You shouldn't with 9.3, because in that case A would follow the
> If we used A's WAL archive in this case and A had writes after the promotion then we would get
> timeline errors.
timeline switch introduced by B's promotion rather than its old timelime.
http://www.postgresql.org/message-id/E1TjCRc-00084r-1H@gemulon.postgresql.org
I may be missing something there since I have never tried it.That should work in any event.
> As far as I can tell, using the WAL archive from B would resolve this issue.
Yours,
Laurenz Albe
The contents of this email are confidential and may be subject to legal or professional privilege and copyright. No representation is made that this email is free of viruses or other defects. If you have received this communication in error, you may not copy or distribute any part of it or otherwise disclose its contents to anyone. Please advise the sender of your incorrect receipt of this correspondence.
I can understand it will create duplicates, but it would also allow for recovery from backups.
--
James Sewell,
PostgreSQL Team Lead / Solutions Architect
______________________________________
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If a backup was taken at midnight, and promotion happened at 6am then having archiving on the slave would allow log replay from the backup.
Log replay from the old master would potentially end up in the incorrect timeline.
Although potentially merging the archive logs favoring the new master's logs might fix this?
Either way, it seems the answer to my question is no.
Thanks for the help!
-James Sewell
On Monday, 10 February 2014, Albe Laurenz <laurenz.albe@wien.gv.at> wrote:
James Sewell wrote:
> If it is the the only way that I could achieve what I wanted would be to set
> wal_keep_segments high enough then they will all be archived on promotion?
Even if you set wal_keep_segments high I don't think that the replayed
WAL will be archived.
> I'm still not sure why they wouldn't be archived on the slave, seen as they show up in the directory?
> Is there a limitation I'm not thinking about here?
I guess that the idea is that a cluster will only archive the WAL it
generates. Typically, the primary would archive these (if you don't archive
WAL on the primary, you're probably doing something wrong).
So this would only lead to duplicates.
Yours,
Laurenz Albe
--
James Sewell,
PostgreSQL Team Lead / Solutions Architect
______________________________________
The contents of this email are confidential and may be subject to legal or professional privilege and copyright. No representation is made that this email is free of viruses or other defects. If you have received this communication in error, you may not copy or distribute any part of it or otherwise disclose its contents to anyone. Please advise the sender of your incorrect receipt of this correspondence.