[Qemu-devel] [Bug 1751264] [NEW] qemu-img convert issue in a tmpfs partition

[Teddy VALETTE]

Public bug reported:

qemu-img convert command is slow when the file to convert is located in
a tmpfs formatted partition.

v2.1.0 on debian/jessie x64, ext4: 10m14s
v2.1.0 on debian/jessie x64, tmpfs: 10m15s

v2.1.0 on debian/stretch x64, ext4: 11m9s
v2.1.0 on debian/stretch x64, tmpfs: 10m21.362s

v2.8.0 on debian/jessie x64, ext4: 10m21s
v2.8.0 on debian/jessie x64, tmpfs: Too long

v2.8.0 on debian/stretch x64, ext4: 10m42s
v2.8.0 on debian/stretch x64, tmpfs: Too long

It seems that the issue is caused by this commit :
https://github.com/qemu/qemu/commit/690c7301600162421b928c7f26fd488fd8fa464e

In order to reproduce this bug :

1/ mount a tmpfs partition : mount -t tmpfs tmpfs /tmp
2/ get a vmdk file (we used a 15GB image) and put it on /tmp
3/ run the 'qemu-img convert -O qcow2 /tmp/file.vmdk /path/to/destination' 
command

When we trace the process, we can see that there's a lseek loop which is
very slow (compare to outside a tmpfs partition).

** Affects: qemu
     Importance: Undecided
         Status: New


** Tags: qemu-img

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https://bugs.launchpad.net/bugs/1751264

Title:
  qemu-img convert issue in a tmpfs partition

Status in QEMU:
  New

Bug description:
  qemu-img convert command is slow when the file to convert is located
  in a tmpfs formatted partition.

  v2.1.0 on debian/jessie x64, ext4: 10m14s
  v2.1.0 on debian/jessie x64, tmpfs: 10m15s

  v2.1.0 on debian/stretch x64, ext4: 11m9s
  v2.1.0 on debian/stretch x64, tmpfs: 10m21.362s

  v2.8.0 on debian/jessie x64, ext4: 10m21s
  v2.8.0 on debian/jessie x64, tmpfs: Too long

  v2.8.0 on debian/stretch x64, ext4: 10m42s
  v2.8.0 on debian/stretch x64, tmpfs: Too long

  It seems that the issue is caused by this commit :
  https://github.com/qemu/qemu/commit/690c7301600162421b928c7f26fd488fd8fa464e

  In order to reproduce this bug :

  1/ mount a tmpfs partition : mount -t tmpfs tmpfs /tmp
  2/ get a vmdk file (we used a 15GB image) and put it on /tmp
  3/ run the 'qemu-img convert -O qcow2 /tmp/file.vmdk /path/to/destination' 
command

  When we trace the process, we can see that there's a lseek loop which
  is very slow (compare to outside a tmpfs partition).

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