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

From: Jim Meyering
Subject: Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP related?)
Date: Fri, 15 Apr 2011 10:53:48 +0200

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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