TLDR: We want to be able to use streaming replication, WAL archiving, and
have the ability to restore from a backup made before a failover using the
WAL archive.
Setup:
We use postgresql 9.1 with streaming replication between a pair of nodes.
The nodes are identical, and can fail over and fail back between
master/slave pairs. We have separate reliable storage where we store
periodic base backups and WAL archives. On each node, archive_command and
restore_command write and read from this shared location.
Currently, WAL archives are written from the current master to a shared
location. On failover, the new master will write WAL files with the same
name as already exist in the archive. Currently we allow these to
overwrite the old files. (See more below)
When a failed node comes back, if it is able to resume streaming from the
new master, it does. Otherwise it does an rsync backup from the new master
and eventually gets back into sync by replaying archived WAL files.
WAL archiving by new master:
When the new master comes up, it wants to archive some of the WAL files it
has. These WAL files have the same name as files that have already been
archived by the old master, but different contents. Newer WAL files in the
new timeline cannot be archived until the old ones are archived. For this
reason we allow the old files to be overwritten. This works consistently
for *failover*, and is effectively the same thing as having each node
write archives to a separate location, and have each only restore from the
others' archives.
Note that some of the contents of the WAL files on the old slave were
never archived, and do need to be archived. The slave has transactions
that were part of the open WAL file on the master that the master had not
archived, yet, but that the slave received via streaming replication. So
the end of the old timeline can only come from the old slave/new master.
Restore:
We also want to be able to restore from the base backups. The problem we
are encountering is that a server restored from a base backup fails to
continue to restore once it hits a WAL file that was overwritten. I.E.
that was written by the new master after a failover.
Related discussions:
http://www.postgresql.org/message-id/CADKfymHjBa9=edv1z8qh8G9o44iA2WNcRMNuowqj90djL+Y9Pg@xxxxxxxxxxxxxx
http://www.postgresql.org/message-id/CAHGQGwHVYqbX=A+zo+AvFbVHLGoypO9G_QDKbabeXgXBVGd05g@xxxxxxxxxxxxxx
Questions:
- Is there a single series of WAL files that will be the full timeline for
the old timeline? I.E. if we had every WAL file from each node, could we
put them together in a way that would allow us to replay them from a
backup made before the failover?
- Is there a better way to handle any of this?
- Is there something we're missing?
Thanks,
-Alan Bryant
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