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    Subjectbtrfs: compression breaks cp and cross-FS mv, FS_IOC_FIEMAP bug?
    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.

    Regards,
    Marti


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