Re: archive_command and streaming replication

Scott Briggs wrote:
> Hi, can someone please explain the purpose of archive_command on both
> the master and slave when it comes to streaming replication? From
> what I understand so far, what really matters is how many pg_xlog
> files are kept when it comes to reestablishing replication when it
> breaks for some reason.
>
> Let's say I shutdown one slave to create a second slave by copying all
> the files to that new slave. If I want to guarantee that the original
> slave is able to reestablish replication, I just need to make sure
> that the master keeps enough pg_xlog files (defined by the setting
> "checkpoint_segments") around in order for that to happen (especially
> in the event that the database is large).

First, there are other uses of archive_command than streaming
replication - you may want to recover the database in case of
problems.

Also, it is a good idea to have the setting the same (or
comparable) on master and slave so that promotion of the slave
to master is easier.

But in principle you are right; if you keep the WAL files
in pg_xlog long enough, you won't need the archives for streaming
replication. Note, however, that the parameter you must set
to retain them long enough is wal_keep_segments and not
checkpoint_segments.

I think that the main reason to have shared WAL archives in
streaming replication is that it still may happen that the
slave lags too long, like a server crash or a network problem.
That could then mean that you have to rebuild the slave if
you have no WAL archives that enable the slave to catch up.

Yours,
Laurenz Albe