[Qemu-devel] [Bug 1422307] Re: qemu-nbd corrupts files

[Max Reitz]

I succeeded to reproduce the bug without NBD:

$ ./qemu-img create -f vdi test.vdi 2G
Formatting 'test.vdi', fmt=vdi size=2147483648 static=off
$ ./qemu-img create -f raw test.raw 2G
Formatting 'test.raw', fmt=raw size=2147483648
$ x86_64-softmmu/qemu-system-x86_64 -enable-kvm -drive 
if=virtio,file=blkverify:test.raw:test.vdi,format=raw -drive 
if=virtio,file=data.img,format=raw,format=raw -cdrom ~/tmp/arch.iso -m 512 
-boot d
blkverify: read sector_num=810976 nb_sectors=256 contents mismatch in sector 
811008

Operations in the guest:
$ dd if=/dev/vdb of=/dev/vda
$ dd if=/dev/vda of=/dev/null

So it really looks like something's broken in qemu's VDI block driver.

Max

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https://bugs.launchpad.net/bugs/1422307

Title:
  qemu-nbd corrupts files

Status in QEMU:
  New

Bug description:
  Dear all,

  On Trusty, in certain situations, try to copy files over a qemu-nbd
  mounted file system leads to write errors (and thus, file corruption).

  Here is the last example I tried:
  -> virtual disk is a VDI disk
  -> It has only one partition, in FAT

  Here is my mount process:
  # modprobe nbd max_part=63
  # qemu-nbd -c /dev/nbd0 "virtual_disk.vdi"
  # partprobe /dev/nbd0
  # mount /dev/nbd0p1 /tmp/mnt/

  Partition is properly mounted at that point:
  /dev/nbd0p1 on /tmp/mnt type vfat (rw)

  Now, when I copy a file (rather big, ~28MB):
  # cp file_to_copy /tmp/mnt/ ; sync
  # md5sum /tmp/mnt/file_to_copy
  2efc9f32e4267782b11d63d2f128a363  /tmp/mnt/file_to_copy
  # umount /tmp/mnt 
  # mount /dev/nbd0p1 /tmp/mnt/
  # md5sum /tmp/mnt/file_to_copy
  42b0a3bf73f704d03ce301716d7654de  /tmp/mnt/file_to_copy

  The first hash was obviously the right one.

  On a previous attempt I did, I spotted thanks to vbindiff that parts of the 
file were just filed with 0s instead of actual data.
  It will randomly work after several attempts to write.

  Version information:
  # qemu-nbd --version
  qemu-nbd version 0.0.1
  Written by Anthony Liguori.

  Cheers,

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