| From: | Jim Nasby <jim(at)nasby(dot)net> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com>, Jeff Layton <jlayton(at)redhat(dot)com> |
| Cc: | Dave Chinner <david(at)fromorbit(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Kevin Grittner <kgrittn(at)ymail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Jeff Janes <jeff(dot)janes(at)gmail(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: | 2014-01-23 00:58:41 |
| Message-ID: | 52E06941.7000505@nasby.net |
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| Lists: | pgsql-hackers |
On 1/17/14, 7:57 AM, Robert Haas wrote:
> - WAL files are written (and sometimes read) sequentially and fsync'd
> very frequently and it's always good to write the data out to disk as
> soon as possible
> - Temp files are written and read sequentially and never fsync'd.
> They should only be written to disk when memory pressure demands it
> (but are a good candidate when that situation comes up)
> - Data files are read and written randomly. They are fsync'd at
> checkpoint time; between checkpoints, it's best not to write them
> sooner than necessary, but when the checkpoint arrives, they all need
> to get out to the disk without bringing the system to a standstill
For sake of completeness... there are also data files that are temporary and don't need to be written to disk unless the kernel thinks there's better things to use that memory for. AFAIK those files are never fsync'd.
In other words, these are the same as the temp files Robert describes except they also have random access. Dunno if that matters.
--
Jim C. Nasby, Data Architect jim(at)nasby(dot)net
512.569.9461 (cell) http://jim.nasby.net
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