Re: [PATCH] copy: adjust fiemap handling of sparse files

[Jim Meyering]

Pádraig Brady wrote:
> Unfortunately, after checking BTRFS I see that fiemap
> behaves differently to EXT4. IMHO the EXT4 operation
> seems correct, and gives full info about the structure
> of a file, which cp for example can use to efficiently
> and accurately reproduce the structure at the destination.
>
> On EXT4 (on kernel-2.6.35.11-83.fc14.i686) there are no extents
> returned for holes in a file.
>
> However on btrfs it does return an extent for holes?
> So with btrfs there is no way to know an extent
> is allocated but unwritten (zero), so one must
> read and write all the data, rather than just
> fallocating the space in the destination.
>
> One can also see this with the following on btrfs:
>
> $ fallocate -l 100000000 unwritten
> $ truncate -s 100000000 sparse
> $ dd count=1000 bs=100000 if=/dev/zero of=zero
>
> $ filefrag -vs *
> Filesystem type is: 9123683e
> File size of sparse is 100000000 (24415 blocks, blocksize 4096)
>  ext logical physical expected length flags
>    0       0        0           24415 unwritten,eof
> sparse: 1 extent found
> File size of unwritten is 100000000 (24415 blocks, blocksize 4096)
>  ext logical physical expected length flags
>    0       0    68160           12207
>    1   12207    92560    80366  12208 eof
> unwritten: 2 extents found
> File size of zeros is 100000000 (24415 blocks, blocksize 4096)
>  ext logical physical expected length flags
>    0       0    19360           20678
>    1   20678    43760    40037   3737 eof
> zeros: 2 extents found
>
> So is this expected?

I heard it's a bug in F14's btrfs.
To test btrfs properly, I had to use rawhide/F15.

> Has this already been changed to match ext4?

Yes, with a new-enough kernel.

> For my own reference, I can probably skip performance
> tests on (older) btrfs by checking `filefrag` output.
> Also in `cp`, if we see an "unwritten extent" we should
> probably create a hole rather than an empty allocation
> by default.  It's better to decrease file allocation
> than increase it.

Makes sense.