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

[Max Reitz]

For whatever reason, using an empty image now works for me, too:

$ ./qemu-img create -f vdi test.vdi 64M; ./qemu-nbd -c /dev/nbd0 test.vdi; dd 
if=/dev/urandom of=/dev/nbd0 bs=1K count=16384; md5sum /dev/nbd0; sync; echo 1 
> /proc/sys/vm/drop_caches; md5sum /dev/nbd0; ./qemu-nbd -d /dev/nbd0
Formatting 'test.vdi', fmt=vdi size=67108864 static=off
16384+0 records in
16384+0 records out
16777216 bytes (17 MB) copied, 0.982225 s, 17.1 MB/s
216f7abbf90bf2539163396bdb7fd7b9  /dev/nbd0
a42faf71124c1f6102fa39cea82a1c86  /dev/nbd0
/dev/nbd0 disconnected

Writing less than 16384 kB, the issue is not always reproducible; for
me, it disappears around 16160 kB (it's fuzzy, sometimes it appears,
sometimes it doesn't).

So far I was only able to reproduce the issue by connecting qemu-nbd to
the the Linux NBD interface; connecting to qemu-nbd via TCP worked fine.

So, a couple of test cases:

VDI and NBD over /dev/nbd0:
# for i in $(seq 0 9); do ./qemu-img create -f vdi test.vdi 64M > /dev/null; 
./qemu-nbd -c /dev/nbd0 test.vdi; sleep 1; ./qemu-img convert -n blob.raw 
/dev/nbd0; ./qemu-img convert /dev/nbd0 test1.raw; sync; echo 1 > 
/proc/sys/vm/drop_caches; ./qemu-img convert /dev/nbd0 test2.raw; ./qemu-nbd -d 
/dev/nbd0 > /dev/null; if ! ./qemu-img compare -q test1.raw test2.raw; then 
md5sum test1.raw test2.raw; echo "$i failed"; break; fi; done; echo 'done'
e5185b807948d65bb4e837d992cea429  test1.raw
9907ca700f6ee4d4cdb136bb90fd8df1  test2.raw
6 failed
done

VDI and NBD over TCP:
# for i in $(seq 0 9); do ./qemu-img create -f vdi test.vdi 64M > /dev/null; 
(./qemu-nbd -t test.vdi &); sleep 1; ./qemu-img convert -n blob.raw 
nbd://localhost; ./qemu-img convert nbd://localhost test1.raw; sync; echo 1 > 
/proc/sys/vm/drop_caches; ./qemu-img convert nbd://localhost test2.raw; killall 
qemu-nbd; if ! ./qemu-img compare -q test1.raw test2.raw; then md5sum test1.raw 
test2.raw; echo "$i failed"; break; fi; done; echo 'done'      
done

VDI and NBD over a Unix socket:
# for i in $(seq 0 9); do ./qemu-img create -f vdi test.vdi 64M > /dev/null; 
(./qemu-nbd -k /tmp/nbd -t test.vdi &); sleep 1; ./qemu-img convert -n blob.raw 
nbd+unix:///\?socket=/tmp/nbd; ./qemu-img convert nbd+unix:///\?socket=/tmp/nbd 
test1.raw; sync; echo 1 > /proc/sys/vm/drop_caches; ./qemu-img convert 
nbd+unix:///\?socket=/tmp/nbd test2.raw; killall qemu-nbd; if ! ./qemu-img 
compare -q test1.raw test2.raw; then md5sum test1.raw test2.raw; echo "$i 
failed"; break; fi; done; echo 'done'                                     
done

VDI without NBD:
# for i in $(seq 0 9); do ./qemu-img create -f vdi test.vdi 64M > /dev/null; 
./qemu-img convert -n -O vdi blob.raw test.vdi; ./qemu-img convert test.vdi 
test1.raw; sync; echo 1 > /proc/sys/vm/drop_caches; ./qemu-img convert test.vdi 
test2.raw; if ! ./qemu-img compare -q test1.raw test2.raw; then md5sum 
test1.raw test2.raw; echo "$i failed"; break; fi; done; echo 'done'
done

qcow2 and NBD over /dev/nbd0:
# for i in $(seq 0 9); do ./qemu-img create -f qcow2 test.qcow2 64M > 
/dev/null; ./qemu-nbd -c /dev/nbd0 test.qcow2; sleep 1; ./qemu-img convert -n 
blob.raw /dev/nbd0; ./qemu-img convert /dev/nbd0 test1.raw; sync; echo 1 > 
/proc/sys/vm/drop_caches; ./qemu-img convert /dev/nbd0 test2.raw; ./qemu-nbd -d 
/dev/nbd0 > /dev/null; if ! ./qemu-img compare -q test1.raw test2.raw; then 
md5sum test1.raw test2.raw; echo "$i failed"; break; fi; done; echo 'done'
done

raw and NBD over /dev/nbd0:
# for i in $(seq 0 9); do ./qemu-img create -f raw test.raw 64M > /dev/null; 
./qemu-nbd -f raw -c /dev/nbd0 test.raw; sleep 1; ./qemu-img convert -n 
blob.raw /dev/nbd0; ./qemu-img convert /dev/nbd0 test1.raw; sync; echo 1 > 
/proc/sys/vm/drop_caches; ./qemu-img convert /dev/nbd0 test2.raw; ./qemu-nbd -d 
/dev/nbd0 > /dev/null; if ! ./qemu-img compare -q test1.raw test2.raw; then 
md5sum test1.raw test2.raw; echo "$i failed"; break; fi; done; echo 'done'
done

In conclusion, the only combination I can reproduce the issue with is
VDI with NBD over the Linux NBD interface. It doesn't seem to be the
kernel's fault because other file formats work fine; it doesn't seem to
be qemu-nbd's fault because not using the kernel interface works fine;
and it doesn't seem to be VDI's fault because not using NBD or at least
using NBD over TCP or Unix sockets works fine, too.

I'll keep looking into it.

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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