GNU bug report logs - #8001
cp (8.10) sparse handling fails on compressed btrfs (cp/fiemap-2)

Previous Next

Full log

View this message in rfc822 format

From: Pádraig Brady <P <at> draigBrady.com>
To: Mike Frysinger <vapier <at> gentoo.org>
Cc: BTRFS MAILING LIST <linux-btrfs <at> vger.kernel.org>, 8001 <at> debbugs.gnu.org
Subject: bug#8001: cp (8.10) sparse handling fails on compressed btrfs (cp/fiemap-2)
Date: Wed, 09 Feb 2011 10:57:14 +0000

On 08/02/11 03:53, Mike Frysinger wrote:
> after upgrading from coreutils 8.9 to 8.10, the sparse handling in cp is 
> silently breaking on btrfs filesystems with the compressed option enabled.  
> using --sparse=never works fine, but "auto" or "always" tend to fail.  the 
> cp/fiemap-2 test catches the issue nicely.
> 
> to reproduce (i'm using linux-2.6.37):
> file=btrfs.img
> mntp=/mnt/tmp/
> dd if=/dev/zero of=$file bs=1M count=0 seek=1024
> mkfs.btrfs $file 
> mount -t btrfs -o compress $file $mntp
> cd $mntp
> tar xf ~/coreutils-8.10.tar.xz
> cd coreutils-8.10/tests
> ./cp/fiemap-2
> 
> and this last test shows:
> Filesystem    Type   1K-blocks      Used Available Use% Mounted on
> /dev/loop0   btrfs     1048576     20420    377028   6% /mnt/tmp
> 0+0 records in
> 0+0 records out
> 0 bytes (0 B) copied, 2.7598e-05 s, 0.0 kB/s
> k k2 differ: byte 1, line 1

Eek.

That doesn't trigger here (2.6.35.10-72.fc14.i686)
because I guess this kernel doesn't honor the compress attribute:

dd if=/dev/zero of=test.size count=1000
# du -B512 test.size
1000    test.size

But on a general note, we may read more (or possibly less)
than is stored in the extent. So how to detect that?
I suppose one could use lseek() to get the current position
and see if it's ext_start + ext_length, otherwise adjust accordingly.
That would add a little overhead though.
I also notice the FIEMAP_EXTENT_DATA_ENCRYPTED and FIEMAP_EXTENT_ENCODED
flags, which could mean we only need to handle these extents specially.
Does `filefrag -v` show those for you? I see nothing here.

I don't suppose there is any facility to read the raw data
to also avoid decompressing and compressing again (sendfile, mmap?).

cheers,
Pádraig.

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.