On 02/23/2015 01:01 PM, Andres Freund wrote:
On 2015-02-22 21:24:56 -0500, Robert Haas wrote:
On Sat, Feb 21, 2015 at 11:29 PM, Petr Jelinek <p...@2ndquadrant.com> wrote:
I am wondering a bit about interaction with wal_keep_segments.
One thing is that wal_keep_segments is still specified in number of segments
and not size units, maybe it would be worth to change it also?
And the other thing is that, if set, the wal_keep_segments is the real
max_wal_size from the user perspective (not from perspective of the
algorithm in this patch, but user does not really care about that) which is
somewhat weird given the naming.
It seems like wal_keep_segments is more closely related to
wal_*min*_size. The idea of both settings is that each is a minimum
amount of WAL we want to keep around for some purpose. But they're
not quite the same, I guess, because wal_min_size just forces us to
keep that many files around - they can be overwritten whenever.
wal_keep_segments is an amount of actual WAL data we want to keep
around.
Would it make sense to require that wal_keep_segments <= wal_min_size?
I don't think so. Right now checkpoint_segments is a useful tool to
relatively effectively control the amount of WAL that needs to be
replayed in the event of a crash. wal_keep_segments in contrast doesn't
have much to do with the normal working of the system, except that it
delays recycling of WAL segments a bit.
With a condition like above, how would you set up things that you have
50k segments around for replication (say a good days worth), but that
your will never have to replay more than ~800 segments (i.e. something
like checkpoint_segments = 800)?
Right. While wal_keep_segments and wal_min_size both set a kind of a
minimum on the amount of WAL that's kept in pg_xlog, they are different
things, and a rule that one must be less than or greater than the other
doesn't make sense.
Everyone seems to be happy with the names and behaviour of the GUCs, so
committed.
- Heikki
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