From: Eric Sandeen <sandeen@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Lukas Czerner <lczerner@redhat.com>,
linux-fsdevel@vger.kernel.org, tytso@mit.edu, adilger@dilger.ca
Subject: Re: [PATCH 0/1] Batched discard support
Date: Thu, 30 Sep 2010 11:11:04 -0500 [thread overview]
Message-ID: <4CA4B698.4@redhat.com> (raw)
In-Reply-To: <20100930005535.GV5665@dastard>
On 09/29/2010 07:55 PM, Dave Chinner wrote:
> 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)....
xfstests could use the scsi-debug module for testing too - at least
once it has http://www.spinics.net/lists/linux-scsi/msg46008.html
so that tests can be run even if you don't have the hardware.
-Eric
> Cheers,
>
> Dave.
prev parent reply other threads:[~2010-09-30 16:11 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 ` [PATCH 0/1] Batched discard support Dave Chinner
2010-09-30 12:17 ` Lukas Czerner
2010-09-30 16:11 ` Eric Sandeen [this message]
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