Message-ID: <535E9C6C.7090407@vmware.com>
Date: Mon, 28 Apr 2014 21:22:36 +0300
From: Heikki Linnakangas <hlinnakangas@vmware.com>
Organization: VMware
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To: Karl Denninger <karl@denninger.net>,
 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>
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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 head
> 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 
means a lot of small writes.

- Heikki


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