| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com> |
| Cc: | robert(at)interactive(dot)co(dot)uk, pgsql-docs(at)postgresql(dot)org |
| Subject: | Re: Needs discussion of pg_xlog |
| Date: | 2016-12-01 18:50:13 |
| Message-ID: | 29385.1480618213@sss.pgh.pa.us |
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| Lists: | pgsql-docs |
"Joshua D. Drake" <jd(at)commandprompt(dot)com> writes:
> On 12/01/2016 07:00 AM, robert(at)interactive(dot)co(dot)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
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