From: Jeff Liu <jeff.liu@oracle.com>
To: dave@jikos.cz
Cc: "Liu Bo" <liub.liubo@gmail.com>, "Gábor Nyers" <gnyers@suse.com>,
linux-btrfs@vger.kernel.org
Subject: Re: How to find (out if) files sharing content?
Date: Wed, 31 Oct 2012 21:02:15 +0800 [thread overview]
Message-ID: <50912157.7090708@oracle.com> (raw)
In-Reply-To: <20121031113154.GG3102@twin.jikos.cz>
On 10/31/2012 07:31 PM, David Sterba wrote:
> On Wed, Oct 31, 2012 at 10:30:22AM +0800, Jeff Liu wrote:
>> One idea is to mark those cloned extents as FIEMAP_EXTENT_SHARED so that
>> we can go through a file to figure out how many extents are shared
>> through fiemap(2), and calculate the real storage(fs/subvolume) footprint
>> in the end.
>
> This will cost at least one more seek per extent to find out that the
> extent is shared, could be quite expensive.
I propose this because OCFS2 report shared space in this way combine with du(1).
An old patch set to teach du(1) aware of reflinked file:
https://oss.oracle.com/pipermail/ocfs2-devel/2010-September/007293.html
Do you means that the costs is very expensive for userland extent status checkup per file?
If yes, I have once tested an 50Gb OCFS2 partition filled with reflinked files on an old laptop,
it spent around 4 minutes to show the totally results if I recalled correct, but this definitely
depending on the real world scenarios.
> And without any possibility to turn this off,I'm afraid this will render FIEMAP unusable in practice.
For OCFS2, the FIEMAP_EXTENT_SHARED flag will be set upon fiemap ioctl(2) if an extent
is OCFS2_EXT_REFCOUNTED(i.e. reflinked or cloned), which means that FIEMAP_EXTENT_SHARED
is not a persistent flag, but I have no idea how Btrfs would be in this point. :(
Thanks,
-Jeff
>
> david
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next prev parent reply other threads:[~2012-10-31 13:04 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-30 15:20 How to find (out if) files sharing content? Gábor Nyers
2012-10-30 15:39 ` Hugo Mills
2012-10-30 15:58 ` Jan Schmidt
2012-10-31 0:40 ` Liu Bo
2012-10-31 2:30 ` Jeff Liu
2012-10-31 11:31 ` David Sterba
2012-10-31 13:02 ` Jeff Liu [this message]
2012-11-05 22:45 ` David Sterba
2012-11-06 3:53 ` Jeff Liu
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