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

From: Max Reitz
Subject: [Qemu-devel] [Bug 1422307] Re: qemu-nbd corrupts files
Date: Tue, 17 Feb 2015 14:50:08 -0000

Hi Pierre,

I can reproduce the bug with a 2 GB VDI image with a single
FAT32-formatted partition (on git master):

# cp src.vdi test.vdi
# ./qemu-nbd -c /dev/nbd0 test.vdi
# dd if=/dev/urandom of=/dev/nbd0 bs=1M count=64
64+0 records in
64+0 records out
67108864 bytes (67 MB) copied, 3.34091 s, 20.1 MB/s
# md5sum /dev/nbd0
bfa6726d0d8fe752c0c7dccbf770fae6  /dev/nbd0
# sync
# echo 1 > /proc/sys/vm/drop_caches
# md5sum /dev/nbd0
cb4762769e09ed6da5e327710bfb3996  /dev/nbd0
# ./qemu-nbd -d /dev/nbd0
/dev/nbd0 disconnected

Using qcow2 or not using NBD I cannot reproduce the issue. Using a qcow2
image and converting it to VDI, the issue appears again.

Using an empty VDI image, or one filled with random data, the issue does
not appear either.

I have attached a qcow2 image for others to test:

# ./qemu-img convert -O vdi src.qcow2 test.vdi; ./qemu-nbd -c /dev/nbd0 
test.vdi; dd if=/dev/urandom of=/dev/nbd0 bs=1M count=64; md5sum /dev/nbd0; 
sync; echo 1 > /proc/sys/vm/drop_caches; md5sum /dev/nbd0; ./qemu-nbd -d 
64+0 records in
64+0 records out
67108864 bytes (67 MB) copied, 3.33071 s, 20.1 MB/s
9f683b4a58cecdd8da04ec2f1b7abc4a  /dev/nbd0
efb1cdd5ebe1dd326056eb2f2e500944  /dev/nbd0
/dev/nbd0 disconnected

Unfortunately, I do not yet know the cause of this issue.


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  qemu-nbd corrupts files

Status in QEMU:

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.


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