From: Dave Chinner <david@fromorbit.com>
To: Qu Wenruo <quwenruo@cn.fujitsu.com>
Cc: linux-btrfs@vger.kernel.org, fstests@vger.kernel.org
Subject: Re: [PATCH v2] fstests: generic: Check if a bull fallocate will change extent number
Date: Wed, 30 Sep 2015 07:51:35 +1000 [thread overview]
Message-ID: <20150929215135.GZ3902@dastard> (raw)
In-Reply-To: <1443519264-19184-1-git-send-email-quwenruo@cn.fujitsu.com>
On Tue, Sep 29, 2015 at 05:34:24PM +0800, Qu Wenruo wrote:
> Normally, a bull fallocate call on a fully written and synced file
> should not add an extent.
Why not? Filesystems can do whatever they want with extents during
a fallocate call. e.g. if the blocks are shared, then fallocate
might break the block sharing so future overwrites don't get
ENOSPC. This is a requirement set down by posix_fallocate(3)
"After a successful call to posix_fallocate(), subsequent writes to
bytes in the specified range are guaranteed not to fail because of
lack of disk space."
Hence if you've got a file with shared blocks, a "full fallocate"
must change the extent layout to break the sharing. As such, the
premise of this test is wrong.
That's not to say that btrfs has a bug:
> Btrfs has a bug to always truncate the last page if the fallocate start
> offset is smaller than inode size.
But it' not clear that this behaviour is actually a bug if it's not
changing the file data.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
next prev parent reply other threads:[~2015-09-29 21:51 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-29 9:34 [PATCH v2] fstests: generic: Check if a bull fallocate will change extent number Qu Wenruo
2015-09-29 9:55 ` Eryu Guan
2015-09-29 10:16 ` Qu Wenruo
2015-09-29 10:33 ` Eryu Guan
2015-09-29 10:46 ` Qu Wenruo
2015-09-29 10:00 ` Hugo Mills
2015-09-29 10:13 ` Qu Wenruo
2015-09-29 10:24 ` Filipe Manana
2015-09-29 10:48 ` Qu Wenruo
2015-09-30 6:42 ` Duncan
2015-09-29 10:24 ` Hugo Mills
2015-09-29 21:51 ` Dave Chinner [this message]
2015-09-30 1:05 ` Qu Wenruo
2015-09-30 1:45 ` Tsutomu Itoh
2015-09-30 1:49 ` Qu Wenruo
2015-09-30 4:20 ` Dave Chinner
2015-09-30 5:48 ` Qu Wenruo
2015-10-02 8:35 ` Qu Wenruo
2015-10-06 1:31 ` Dave Chinner
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