linux-kernel - Re: [patch v2 0/6] Add TRIM support for raid linear/0/1/10

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Message-ID: <CADNH=7EfmNRXq1bSyuUcRGy-8ksfNhJuGnopubTRomq8-oSmJw@mail.gmail.com>
Date:	Wed, 21 Mar 2012 02:29:21 +0000
From:	Mathias Burén <mathias.buren@...il.com>
To:	Shaohua Li <shli@...nel.org>
Cc:	Holger Kiehl <Holger.Kiehl@....de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-raid <linux-raid@...r.kernel.org>, neilb@...e.de,
	axboe@...nel.dk, vgoyal@...hat.com, martin.petersen@...cle.com
Subject: Re: [patch v2 0/6] Add TRIM support for raid linear/0/1/10

On 21 March 2012 02:08, Shaohua Li <shli@...nel.org> wrote:
> 2012/3/20 Shaohua Li <shli@...nel.org>:
>> 2012/3/20 Holger Kiehl <Holger.Kiehl@....de>:
>>> Hello,
>>>
>>>
>>> On Tue, 20 Mar 2012, Shaohua Li wrote:
>>>
<snip>
> Thanks for the testing. The trace data is very helpful. In the intel
> SSD, trace data
> shows a discard request uses about 1 ~ 3 ms. The filesystem suffers from
> fragmentation too, so lots of small discard requests. When ext4 starts doing
> discard, it usually uses more than 1 minutes. That's too bad.
> If just looking one disk's trace data, there are some extra latencies between
> two discard requests. The combined trace data of two disks show the latency
> comes from waiting for another disk, so nothing abnormal. I thought we could
> do an optimization for this case in the future.
> So in summary, discard from the SSDs is slow. When your filesystem is
> fragmented, the performance will be terrible.
>
> Thanks,
> Shaohua
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Many people prefer to have the filesystems mounted without discard,
then they run a TRIM command/script at night (or whenever the disk
isn't used very much). This way the TRIM commands don't slow down the
'overall' performance.

Mathias
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