Message-ID: <535E9D7B.6020706@denninger.net>
Date: Mon, 28 Apr 2014 13:27:07 -0500
From: Karl Denninger <karl@denninger.net>
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To: pgsql-performance@postgresql.org
Subject: Re: Revisiting disk layout on ZFS systems
References: <535E781A.5030708@denninger.net> <535E9812.1090605@vmware.com>
 <535E98DA.7080702@denninger.net> <535E9C6C.7090407@vmware.com>
In-Reply-To: <535E9C6C.7090407@vmware.com>
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On 4/28/2014 1:22 PM, Heikki Linnakangas wrote:
> On 04/28/2014 09:07 PM, Karl Denninger wrote:
>>> The WAL is fsync'd frequently. My guess is that that causes a lot of
>>> extra work to repeatedly recompress the same data, or something like
>>> that.
>>
>> It shouldn't as ZFS re-writes on change, and what's showing up is not
>> high I/O*count*  but rather percentage-busy, which implies lots of hea=
d
>> movement (that is, lots of sub-allocation unit writes.)
>
> That sounds consistent frequent fsyncs.
>
>> Isn't WAL essentially sequential writes during normal operation?
>
> Yes, it's totally sequential. But it's fsync'd at every commit, which=20
> means a lot of small writes.
>
> - Heikki

Makes sense; I'll muse on whether there's a way to optimize this=20
further... I'm not running into performance problems at present but I'd=20
rather be ahead of it....

--=20
-- Karl
karl@denninger.net


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