From: | Andres Freund <andres(at)anarazel(dot)de> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, 周正中(德歌) <dege(dot)zzz(at)alibaba-inc(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, 张广舟(明虚) <guangzhou(dot)zgz(at)alibaba-inc(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, 范孝剑(康贤) <funnyxj(dot)fxj(at)alibaba-inc(dot)com>, 曾文旌(义从) <wenjing(dot)zwj(at)alibaba-inc(dot)com>, 窦贤明(执白) <xianming(dot)dxm(at)alibaba-inc(dot)com>, 萧少聪(铁庵) <shaocong(dot)xsc(at)alibaba-inc(dot)com>, 陈新坚(惧留孙) <xinjian(dot)chen(at)alibaba-inc(dot)com> |
Subject: | Re: Re: [HACKERS] Re: [HACKERS] 答复:[HACKERS] 答复:[HACKERS] about fsync in CLOG buffer write |
Date: | 2015-09-09 15:14:01 |
Message-ID: | 20150909151401.GA25875@alap3.anarazel.de |
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Lists: | pgsql-hackers |
On 2015-09-09 10:46:47 -0400, Robert Haas wrote:
> Well, if you're filling ~1 clog page per second, you're doing ~1 fsync
> per second too. Or if you are not, then you are thrashing the
> progressively smaller and smaller number of clean slots ever-harder
> until no clean pages remain and you're forced to fsync something -
> probably, a bunch of things all at once.
And you're also triggering a lot of other fsyncs. At the very least
you're pretty constantly flushing the WAL, even with synchronous_commit
= off.
> > Like Andres, I'd want to see a more realistic problem case before
> > expending a lot of work here.
>
> I think the question here isn't whether the problem case is realistic
> - I am quite sure that a pgbench workload is - but rather how much of
> a problem it actually causes. I'm very sure that individual pgbench
> backends experience multi-second stallls as a result of this.
The fsync causes that? That seems odd. I mean we release the slru
control lock for writing out pages, don't we?
To me it seems the fsyncs are a red herring here, and the problems are,
uh, much bigger. ISTM that cache replacement locking granularity et al
have a much bigger effect than a fsync every now and then.
Greetings,
Andres Freund
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