"Joshua D. Drake" <j...@commandprompt.com> writes:
> On 12/01/2016 07:00 AM, rob...@interactive.co.uk wrote:
>> The only mention of this that I&#39;ve seen is in Section 29.5 (WAL 
>> Internals),
>> and that just says &quot;it is advantageous...&quot;, 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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