Message-ID: <535E98DA.7080702@denninger.net>
Date: Mon, 28 Apr 2014 13:07:22 -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>
In-Reply-To: <535E9812.1090605@vmware.com>
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On 4/28/2014 1:04 PM, Heikki Linnakangas wrote:
> On 04/28/2014 06:47 PM, Karl Denninger wrote:
>> What I am curious about, however, is the xlog -- that appears to suffe=
r
>> 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.
>
> The WAL is fsync'd frequently. My guess is that that causes a lot of=20
> extra work to repeatedly recompress the same data, or something like=20
> that.
>
> - Heikki
>
It shouldn't as ZFS re-writes on change, and what's showing up is not=20
high I/O *count* but rather percentage-busy, which implies lots of head=20
movement (that is, lots of sub-allocation unit writes.)

Isn't WAL essentially sequential writes during normal operation?

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


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