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bug#6131: [PATCH]: fiemap support for efficient sparse file copy

From: Jim Meyering
Subject: bug#6131: [PATCH]: fiemap support for efficient sparse file copy
Date: Fri, 28 May 2010 18:44:33 +0200

Tao Ma wrote:

> Hi Jim
> On 05/27/2010 06:30 PM, Jim Meyering wrote:
>> jeff.liu wrote:
>>> This is the revised version, it fixed the fiemap-start offset calculation
>>> approach to remove it out
>>> of the 'for (i = 0; i<  fiemap->fm_mapped_extents; i++)' loop.
>> Hi Jeff,
>> I've included below the state of my local changes.
>> Unfortunately, with that 5-patch series, there is always a test failure
>> on F13/ext4.  Maybe someone who knows more about extents can provide an
>> explanation?
> Just want to clarify why ocfs2 didn't work here. I guess the reason
> also works for ext4 since both ext4 and ocfs2 use block group to
> organize their blocks in the volume.

Hi Tao,

Thank you for the explanation.
I'm glad to hear that there is no underlying problem.

> I checked the perl test script to create sparse src file, it will
> create contiguous bytes(around 20-24k) at an interval of around 40k.So
> in general, these 20-24k should be contiguous. But that does exist
> some scenario that they could be separately into 2 extents. Consider
> one block group is used to allocate blocks to this file, when the
> block group only has 10K left while you are requiring 20K, it will use
> the left 10K in this group and allocate 10K from another block
> group. That would become 2 extents since they can't be contiguous.

> So I guess the right step is to check the holes by using filefrag if
> you prefer this tool and want to make sure cp doesn't copy holes(I get

Do you know of a tool other than filefrag that I could use?

It looks like a small script could filter filefrag -v output, detect
split extents and rewrite to make the output match what's expected.
Probably not worth it, though, since this is already a very fragile test.

It would be nice to be able to perform the test in non-root
mode on any of ext4, ocfs2, btrfs, xfs file systems, but for
now, due to this difference, I can use only the latter two.

> this point from another e-mail written by you). How to find holes with
> filefrag? I guess it is quite simple since it also use fiemap and we
> can calculating holes easily by comparing the 2 consecutive records. I
> guess we can get what you want with ext4 after this update.

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