linux-kernel - Re: btrfs: compression breaks cp and cross-FS mv, FS_IOC

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Date:	Sun, 13 Feb 2011 10:57:30 -0500
From:	Josef Bacik <josef@...hat.com>
To:	Marti Raudsepp <marti@...fo.org>
Cc:	btrfs hackers <linux-btrfs@...r.kernel.org>,
	Kernel hackers <linux-kernel@...r.kernel.org>
Subject: Re: btrfs: compression breaks cp and cross-FS mv, FS_IOC_FIEMAP
	bug?

On Sun, Feb 13, 2011 at 05:49:42PM +0200, Marti Raudsepp wrote:
> Hi list!
> 
> It seems I have found a serious regression in compressed btrfs in
> kernel 2.6.37. When creating a small file (less than the block size)
> and then cp/mv it to *another* file system, an appropriate number of
> zeroes gets written to the destination file. Case in point:
> 
> % echo foobar > foobar
> % hexdump -C foobar
> 00000000  66 6f 6f 62 61 72 0a                              |foobar.|
> 00000007
> % mv foobar /tmp
> % hexdump -C /tmp/foobar
> 00000000  00 00 00 00 00 00 00                              |.......|
> 00000007
> % cp foobar foobar2
> % hexdump -C foobar2
> 00000000  00 00 00 00 00 00 00                              |.......|
> 00000007
> 
> Via strace I found that mv doesn't even attempt to read anything:
> 
> open("foobar", O_RDONLY|O_NOFOLLOW)     = 3
> fstat(3, {st_mode=S_IFREG|0664, st_size=7, ...}) = 0
> open("/tmp/foobar", O_WRONLY|O_CREAT|O_EXCL, 0600) = 4
> fstat(4, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
> ioctl(3, FS_IOC_FIEMAP, 0x7fff62f6bfa0) = 0
> write(4, "\0\0\0\0\0\0\0", 7)           = 7
> 
> What's that, is FS_IOC_FIEMAP telling it that it's a sparse file?
> Compare with ext4:
> 
> ioctl(3, FS_IOC_FIEMAP, 0x7fff2c576a90) = 0
> lseek(3, 0, SEEK_SET)                   = 0
> read(3, "foobar\n", 4096)               = 7
> write(4, "foobar\n", 7)                 = 7
> 
> I'm currently running on 2.6.37, x86_64 using Arch Linux -testing with
> coreutils 8.10. Filesystem is mounted from LVM2 to /usr/src with -o
> noatime,compress
> 
> This only seems to occur with compressed file systems (either zlib or
> LZO). A person on IRC also reproduced the same problem in 2.6.28-rc.
> I'm pretty sure this used to work correctly around 2.6.35 or 2.6.36.
> 
> This is 100% reproducible here. If anyone has trouble reproducing
> this, I can dig further and provide information as needed.
>

Does the same problem happen when you use cp --sparse=never?  Thanks,

Josef 
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