From: Jeff Liu <jeff.liu@oracle.com>
To: dave@jikos.cz, "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: Tue, 06 Nov 2012 11:53:51 +0800 [thread overview]
Message-ID: <509889CF.30302@oracle.com> (raw)
In-Reply-To: <20121105224504.GN3102@twin.jikos.cz>
On 11/06/2012 06:45 AM, David Sterba wrote:
> On Wed, Oct 31, 2012 at 09:02:15PM +0800, Jeff Liu wrote:
>> 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
>
> Patch looks ok, the shared size is requested by an option.
>
>> Do you means that the costs is very expensive for userland extent status checkup per file?
>
> The most expensive part is IMO not in userspace, it does in-memory lookups.
>
>>> 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. :(
>
> After some research, I think this could work for btrfs without
> unwanted performance penalties.
>
> There's the fiemap::fm_flags field that can be extended to request the
> shared extent info from fiemap, so the information is not computed
> unconditionally (that was my concern before). The rest is only
> implementation details how to speed up the file extent -> refcount info
> lookups.
Thanks for your confirmation.
-Jeff
>
> david
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prev parent reply other threads:[~2012-11-06 3:54 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
2012-11-05 22:45 ` David Sterba
2012-11-06 3:53 ` Jeff Liu [this message]
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