From: Dave Chinner <david@fromorbit.com>
To: Lukas Czerner <lczerner@redhat.com>
Cc: linux-fsdevel@vger.kernel.org, tytso@mit.edu, sandeen@redhat.com,
adilger@dilger.ca
Subject: Re: [PATCH 0/1] Batched discard support
Date: Thu, 30 Sep 2010 10:55:35 +1000 [thread overview]
Message-ID: <20100930005535.GV5665@dastard> (raw)
In-Reply-To: <1285750466-3579-1-git-send-email-lczerner@redhat.com>
On Wed, Sep 29, 2010 at 10:54:25AM +0200, Lukas Czerner wrote:
> Hi,
>
> I am working on something I have called "batched discard support" for Ext3
> and Ext4 filesystems. Traditional discard support for filesystems like Ext4
> has been implemented the way that whenever the file is unlinked the
> disk-space that the file was using is trimmed (discarded) by
> sb_issue_discard() to let the device know that this portion of disk is no
> longer in use by the filesystem and can be safely used for wear-leveling.
>
> However, this approach comes with very noticeable performance loss on most
> of SSD devices and LUN's I have the opportunity to test it on. The fact is,
> that bigger discard ranges are more efficient than smaller ones, so it make
> sense try to batch the ranges together wherever it is possible.
>
> I have introduced new filesystem independent ioctl (FITRIM) which can be used
> to send the "trim this portion of filesystem" command down to the filesystem
> which (if implemented) discards all free extents in that range.
>
> The implementation for Ext3 and Ext4 is complete and you can see it here:
>
> http://www.spinics.net/lists/linux-ext4/msg21050.html
>
> Why I am sending it here to linux-fsdevel is because I am introducing new fs
> independent ioctl and new member of super_operations (trim_fs) and we would
> like let you know about this approach (which any filesystem can take
> advantage from) and we would like your comment on this patch before we
> send it upstream.
My first question is: how do you test a filesystem implements
->trim_fs correctly?
That is, if we are going to include a data-destroying ioctl, I
really want some filesystem independent tests written first so that
as filesystems implement ->trim_fs they can be tested for correct
implementation.
Perhaps adding FITRIM support to xfs_io, and a generic test to
xfstests would be the way to go. e.g. write a set of patterned files
to the filesystem, unlink a number of the files, then run some trim
commands on the filesystem exercising corner cases and check that
none of the data in still-active files is damaged (e.g. via md5sum
comparison)....
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
next prev parent reply other threads:[~2010-09-30 0:56 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-09-29 8:54 [PATCH 0/1] Batched discard support Lukas Czerner
2010-09-29 8:54 ` [PATCH] Add ioctl FITRIM Lukas Czerner
2010-09-30 0:55 ` Dave Chinner [this message]
2010-09-30 12:17 ` [PATCH 0/1] Batched discard support Lukas Czerner
2010-09-30 16:11 ` Eric Sandeen
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