| From: | Kevin Grittner <kgrittn(at)ymail(dot)com> |
|---|---|
| To: | James Bottomley <James(dot)Bottomley(at)HansenPartnership(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Andres Freund <andres(at)2ndquadrant(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Hannu Krosing <hannu(at)2ndquadrant(dot)com>, Trond Myklebust <trondmy(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Dave Chinner <david(at)fromorbit(dot)com>, Joshua Drake <jd(at)commandprompt(dot)com>, Claudio Freire <klaussfreire(at)gmail(dot)com>, Mel Gorman <mgorman(at)suse(dot)de>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "lsf-pc(at)lists(dot)linux-foundation(dot)org" <lsf-pc(at)lists(dot)linux-foundation(dot)org>, Magnus Hagander <magnus(at)hagander(dot)net> |
| Subject: | Re: [Lsf-pc] Linux kernel impact on PostgreSQL performance |
| Date: | 2014-01-14 17:23:41 |
| Message-ID: | 1389720221.74969.YahooMailNeo@web122306.mail.ne1.yahoo.com |
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| Lists: | pgsql-hackers |
James Bottomley <James(dot)Bottomley(at)HansenPartnership(dot)com> wrote:
> you mean the order of write out, if we have to do it, is
> important. In the rest of the kernel, we do this with barriers
> which causes ordered grouping of I/O chunks. If we could force a
> similar ordering in the writeout code, is that enough?
Unless it can be between particular pairs of pages, I don't think
performance could be at all acceptable. Each data page has an
associated Log Sequence Number reflecting the last Write-Ahead Log
record which records a change to that page, and the referenced WAL
record must be safely persisted before the data page is allowed to
be written. Currently, when we need to write a dirty page to the
OS, we must ensure that the WAL record is written and fsync'd
first. We also write a WAL record for transaction command and
fsync it at each COMMIT, before telling the client that the COMMIT
request was successful. (Well, at least by default; they can
choose to set synchronous_commit to off for some or all
transactions.) If a write barrier to control this applied to
everything on the filesystem, performance would be horrible.
--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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