From: Heikki Linnakangas <hlinnakangas@vmware.com>
To: Karl Denninger <karl@denninger.net>
To: pgsql-performance@postgresql.org
Subject: Re: Revisiting disk layout on ZFS systems
Date: Mon, 28 Apr 2014 21:22:36 +0300
Message-ID: <535E9C6C.7090407@vmware.com> (raw)
In-Reply-To: <535E98DA.7080702@denninger.net>
References: <535E781A.5030708@denninger.net>
<535E9812.1090605@vmware.com>
<535E98DA.7080702@denninger.net>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-performance>
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
--
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: hlinnakangas@vmware.com, karl@denninger.net
Subject: Re: Revisiting disk layout on ZFS systems
In-Reply-To: <535E9C6C.7090407@vmware.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox