From: | Francisco Olarte <folarte(at)peoplecall(dot)com> |
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To: | Weiping Qu <qu(at)informatik(dot)uni-kl(dot)de> |
Cc: | "pgsql-general(at)postgresql(dot)org" <pgsql-general(at)postgresql(dot)org> |
Subject: | Re: Question regarding logical replication |
Date: | 2017-10-27 12:03:24 |
Message-ID: | CA+bJJbyNw6BNAeVpXwYGy+D-Q21tALHCcYfD=B7qr5hSDuxECw@mail.gmail.com |
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Lists: | pgsql-general |
On Fri, Oct 27, 2017 at 12:04 PM, Weiping Qu <qu(at)informatik(dot)uni-kl(dot)de> wrote:
> That's a good point and we haven't accounted for disk caching.
> Is there any way to confirm this fact in PostgreSQL?
I doubt, as it names indicates cache should be hidden from the db server.
You could monitor the machine with varying lags and see the disk-cache
hit ratio , or monitor the throughput loss, a disk-cache effect should
exhibit a constant part for little lags, where you mostly do cache
reads, then a rising part as you begin reading from disks stabilizing
asyntotically ( as most of the fraction of reads comes from disk, but
it could also exhibit a jump if you are unlucky and you evict pages
you'll need soon ), but it is not a simple thing to measure, specially
with a job mix and long delays.
The xlog can do strange things. IIRC it is normally write-only ( only
used on crash recovery, to archive (ship) it and for log based
replication slots ), but postgres recycles segments ( which can have
an impact on big memory machines ). I do not know to what extent a
modern OS can detect the access pattern and do things like evict the
log pages early after sync.
Francisco Olarte.
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