On 01/05/2015 12:06 PM, Andres Freund wrote:
On 2015-01-05 11:34:54 +0200, Heikki Linnakangas wrote:
On 01/04/2015 11:44 PM, Josh Berkus wrote:
On 01/03/2015 12:56 AM, Heikki Linnakangas wrote:
On 01/03/2015 12:28 AM, Josh Berkus wrote:
On 01/02/2015 01:57 AM, Heikki Linnakangas wrote:
wal_keep_segments does not affect the calculation of CheckPointSegments.
If you set wal_keep_segments high enough, checkpoint_wal_size will be
exceeded. The other alternative would be to force a checkpoint earlier,
i.e. lower CheckPointSegments, so that checkpoint_wal_size would be
honored. However, if you set wal_keep_segments high enough, higher than
checkpoint_wal_size, it's impossible to honor checkpoint_wal_size no
matter how frequently you checkpoint.
So you're saying that wal_keep_segments is part of the max_wal_size
total, NOT in addition to it?
Not sure what you mean. wal_keep_segments is an extra control that can
prevent WAL segments from being recycled. It has the same effect as
archive_command failing for N most recent segments, if that helps.
I mean, if I have these settings:
max_wal_size* = 256MB
wal_keep_segments = 8
... then my max wal size is *still* 256MB, NOT 384MB?
Right.
With that you mean that wal_keep_segments has *no* influence over
checkpoint pacing or the contrary? Because upthread you imply that it
doesn't, but later comments may mean the contrary.
wal_keep_segments does not influence checkpoint pacing.
If that's the case (and I think it's a good plan), then as a follow-on,
we should prevent users from setting wal_keep_segments to more than 50%
of max_wal_size, no?
Not sure if the 50% figure is correct, but I see what you mean: don't allow
setting wal_keep_segments so high that we would exceed max_wal_size because
of it.
I wasn't clear on my opinion here. I think I understood what Josh meant,
but I don't think we should do it. Seems like unnecessary nannying of
the DBA. Let's just mention in the manual that if you set
wal_keep_segments higher than [insert formula here], you will routinely
have more WAL in pg_xlog than what checkpoint_wal_size is set to.
That seems a unrealistic goal. I've seen setups that have set
checkpoint_segments intentionally, and with good reasoning, north of
50k.
So? I don't see how that's relevant.
Neither wal_keep_segments, nor failing archive_commands nor replication
slot should have an influence on checkpoint pacing.
Agreed.
- Heikki
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