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Re: Things to do this week instead of arguing about mixers



Daniel P. Berrange wrote:
> On Mon, Apr 27, 2009 at 02:10:10PM -0500, Eric Sandeen wrote:
>> Now that we have ext4 as the new default filesystem, it'd be nice if we
>> can get more applications to take advantage of some of the features.
>>
>> One big feature that has already been brought up on the list[1] is file
>> preallocation, which allows an application to pre-allocate blocks it
>> knows that it will eventually write into, thereby making sure it won't
>> run out of space, and also generally getting a more efficient/contiguous
>> file layout.
>>
>> * Come up with a list of apps which could benefit:
> [snip]
>>   - virt image tools?
> 
> We're one step ahead of you !  libvirt supports it as of 0.6.3 for its
> raw file storage management APIs, and this is in Fedora 11 trees

Cool!

> http://git.et.redhat.com/?p=libvirt.git;a=commit;h=70e7672184ff639856d5f2f3bf7849464031dff9
> 
> Thanks to Amit Shah for doing the patch for us, and providing us some
> compelling performance figures at the same time:
> 
> http://www.amitshah.net/2009/03/comparison-of-file-systems-and-speeding.html

Ah, that explains the call to posix_fallocate() I saw in the library :)

Just FWIW, this will still be slow on ext3, but fast on
ext4/xfs/btrfs/ocfs2 - as Amit's blog entry shows, I guess.

If you didn't want to fall back to the slow behavior on ext3, you might
consider only using fallocate() if it's there, rather than
posix_fallocate().  But the behavior is up to you of course.  :)

-Eric


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