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qemu-img convert issue in a tmpfs partition
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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