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Needs discussion of pg_xlog 3+ messages / 3 participants [nested] [flat]
* Needs discussion of pg_xlog @ 2016-12-01 15:00 robert@interactive.co.uk 0 siblings, 1 reply; 3+ messages in thread From: robert@interactive.co.uk @ 2016-12-01 15:00 UTC (permalink / raw) To: pgsql-docs The following documentation comment has been logged on the website: Page: https://www.postgresql.org/docs/9.4/static/runtime-config-file-locations.html Description: In various places (e.g. stackoverflow), I've seen people suggest having pg_xlog ion a different disk (i.e. not in /var/lib/pgsql/9.X/data). The only mention of this that I've seen is in Section 29.5 (WAL Internals), and that just says "it is advantageous...", with no explanation. If this IS a good idea, then "Server Configuration / File Locations" is where it should be discussed: in particular, why is it a good idea, and how does it affect backups (and in particular base backups to a standby server). -- Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-docs ^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Needs discussion of pg_xlog @ 2016-12-01 16:56 Joshua D. Drake <jd@commandprompt.com> parent: robert@interactive.co.uk 0 siblings, 1 reply; 3+ messages in thread From: Joshua D. Drake @ 2016-12-01 16:56 UTC (permalink / raw) To: robert@interactive.co.uk; pgsql-docs On 12/01/2016 07:00 AM, robert@interactive.co.uk wrote: > The following documentation comment has been logged on the website: > > Page: https://www.postgresql.org/docs/9.4/static/runtime-config-file-locations.html > Description: > > In various places (e.g. stackoverflow), I've seen people suggest having > pg_xlog ion a different disk (i.e. not in /var/lib/pgsql/9.X/data). > > The only mention of this that I've seen is in Section 29.5 (WAL Internals), > and that just says "it is advantageous...", with no explanation. > > If this IS a good idea, then "Server Configuration / File Locations" is > where it should be discussed: in particular, why is it a good idea, and how > does it affect backups (and in particular base backups to a standby server). The reason it can be advantageous is that pg_xlog has a different write profile that $PGDATA. The WAL is written sequentially versus randomly. JD > -- Command Prompt, Inc. http://the.postgres.company/ +1-503-667-4564 PostgreSQL Centered full stack support, consulting and development. Everyone appreciates your honesty, until you are honest with them. Unless otherwise stated, opinions are my own. -- Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-docs ^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Needs discussion of pg_xlog @ 2016-12-01 18:50 Tom Lane <tgl@sss.pgh.pa.us> parent: Joshua D. Drake <jd@commandprompt.com> 0 siblings, 0 replies; 3+ messages in thread From: Tom Lane @ 2016-12-01 18:50 UTC (permalink / raw) To: Joshua D. Drake <jd@commandprompt.com>; +Cc: robert@interactive.co.uk; pgsql-docs "Joshua D. Drake" <jd@commandprompt.com> writes: > On 12/01/2016 07:00 AM, robert@interactive.co.uk wrote: >> The only mention of this that I've seen is in Section 29.5 (WAL Internals), >> and that just says "it is advantageous...", with no explanation. > The reason it can be advantageous is that pg_xlog has a different write > profile that $PGDATA. The WAL is written sequentially versus randomly. Yeah. The traditional understanding of that was you wanted to keep a write head positioned over the current end-of-WAL, which of course only applies to spinning rust. It's still true that under heavy update loads, your I/O volume to WAL is probably comparable to your I/O volume to everything else, which might justify a separate SSD just on write bandwidth grounds. But seek delays aren't part of the calculation anymore. regards, tom lane -- Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-docs ^ permalink raw reply [nested|flat] 3+ messages in thread
end of thread, other threads:[~2016-12-01 18:50 UTC | newest] Thread overview: 3+ messages (download: mbox mbox.gz follow: Atom feed) -- links below jump to the message on this page -- 2016-12-01 15:00 Needs discussion of pg_xlog robert@interactive.co.uk 2016-12-01 16:56 ` Joshua D. Drake <jd@commandprompt.com> 2016-12-01 18:50 ` Tom Lane <tgl@sss.pgh.pa.us>
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