Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP related?)

From: Jim Meyering <jim@meyering.net>
To: Dave Chinner <david@fromorbit.com>
Cc: "Eric Sandeen" <sandeen@sandeen.net>, xfs-oss <xfs@oss.sgi.com>,
	coreutils@gnu.org, linux-ext4@vger.kernel.org,
	"Pádraig Brady" <P@draigBrady.com>,
	"Markus Trippelsdorf" <markus@trippelsdorf.de>
Subject: Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP related?)
Date: Fri, 15 Apr 2011 10:53:48 +0200	[thread overview]
Message-ID: <87sjtjq4ir.fsf@rho.meyering.net> (raw)
In-Reply-To: <20110415000940.GL21395@dastard> (Dave Chinner's message of "Fri, 15 Apr 2011 10:09:40 +1000")

Dave Chinner wrote:
> On Fri, Apr 15, 2011 at 12:29:46AM +0100, Pádraig Brady wrote:
...
>> Well IMHO there should be a difference between
>> knowing where you are going to write, and actually writing to disk.
>> I.E. one shouldn't need to write the whole way to the device
>> before returning a valid fiemap.  If a particular file system
>> implementation needs to sync to return a valid fiemap,
>> then it should be implicit.
>
> No, this was explicitly laid out in the fiemap interface discussions
> - it's up to the applicaiton to decide if it needs to do a sync
> first. That's what the FIEMAP_FLAG_SYNC control flag is for.
> This forces the fiemap call to do a fsync _before_ getting the
> mapping. If you want to know the exact layout of the file is, then
> you must use this flag.
>
> Even so, it is recognised that this is racy - any use of the block
> map has a time-of-read-to-time-of-use race condition that means you
> have to _verify_ the copy after it completes. FYI, that's what
> xfs_fsr does when copying based on extent maps - if the inode has
> changed in _any way_ during the copy, it aborts the copy of that
> file.
>
> i.e. using fiemap for copying is at best a *hint* about the regions
> that need copying, and it is in no way a guarantee that you'll get
> all the information you need to make accurate copy even if you do
> use the synchronous variant.

Hi Dave,

Can you or anyone else point to authoritative documentation
(or even a summary of those "discussions") of FIEMAP semantics?
I'm hoping the semantics are the same for all file system types.

I had understood that cp's use of FIEMAP_FLAG_SYNC was not only
unnecessary, but even undesirable, given a new-enough kernel.
That's why coreutils-8.11 resorts to using the workaround of
FIEMAP_FLAG_SYNC only when uname says the kernel is 2.6.[0..38].

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