From: Tom Lane <tgl@sss.pgh.pa.us>
To: Joshua D. Drake <jd@commandprompt.com>
Cc: robert@interactive.co.uk
Cc: pgsql-docs@postgresql.org
Subject: Re: Needs discussion of pg_xlog
Date: Thu, 01 Dec 2016 13:50:13 -0500
Message-ID: <29385.1480618213@sss.pgh.pa.us> (raw)
In-Reply-To: <5414c450-db82-8169-d5cf-f33339d0944c@commandprompt.com>
References: <20161201150006.4377.8523@wrigleys.postgresql.org>
<5414c450-db82-8169-d5cf-f33339d0944c@commandprompt.com>
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"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
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To: pgsql-docs@postgresql.org
Cc: tgl@sss.pgh.pa.us, jd@commandprompt.com, robert@interactive.co.uk
Subject: Re: Needs discussion of pg_xlog
In-Reply-To: <29385.1480618213@sss.pgh.pa.us>
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