From: Karl Denninger <karl@denninger.net>
To: pgsql-performance@postgresql.org
Subject: Revisiting disk layout on ZFS systems
Date: Mon, 28 Apr 2014 10:47:38 -0500
Message-ID: <535E781A.5030708@denninger.net> (raw)
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I've been doing a bit of benchmarking and real-world performance
testing, and have found some curious results.
The load in question is a fairly-busy machine hosting a web service that
uses Postgresql as its back end.
"Conventional Wisdom" is that you want to run an 8k record size to match
Postgresql's inherent write size for the database.
However, operational experience says this may no longer be the case now
that modern ZFS systems support LZ4 compression, because modern CPUs can
compress fast enough that they overrun raw I/O capacity. This in turn
means that the recordsize is no longer the record size, basically, and
Postgresql's on-disk file format is rather compressible -- indeed, in
actual performance on my dataset it appears to be roughly 1.24x which is
nothing to sneeze at.
The odd thing is that I am getting better performance with a 128k record
size on this application than I get with an 8k one! Not only is the
system faster to respond subjectively and can it sustain a higher TPS
load objectively but the I/O busy percentage as measured during
operation is MARKEDLY lower (by nearly an order of magnitude!)
This is not expected behavior!
What I am curious about, however, is the xlog -- that appears to suffer
pretty badly from 128k record size, although it compresses even
more-materially; 1.94x (!)
The files in the xlog directory are large (16MB each) and thus "first
blush" would be that having a larger record size for that storage area
would help. It appears that instead it hurts.
Ideas?
--
-- Karl
karl@denninger.net
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To: pgsql-performance@postgresql.org
Cc: karl@denninger.net
Subject: Re: Revisiting disk layout on ZFS systems
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