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Zim3r
Asked: 2012-09-17 04:04:19 +0800 CST

does KVM raw images support preallocation?

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I was trying to create a raw image with this command:

qemu-img create -f raw -o size=200G,preallocation=metadata file.img

but seem that it raw images doesn't support preallocation. if so why there is raw + preallocation in this chart?

# qemu-img create -f raw -o size=200G,preallocation=full file.img
Unknown option 'preallocation'
Invalid options for file format 'raw'.
linux

3 Answers

  1. raw images do not support preallocation, qcow2 and qed-images do:

    preallocation=full instead of preallocation=metadata does not only allocate metadata for the raw image, but also writes zeros to it, thus creating a non-sparse image file.

    If your filesystems supports sparse-files (e.g. ext, xfs, btrfs), blocks filled only with zeros are not stored physically, that saves space at the beginning of using this image.

    If you would like to allocate all space on the raw image, you can use the dd command: dd if=/dev/zero of=file.img bs=1M count=204800

    If you are working towards speed, also use cache=writethrough with e.g. libvirt, this enhanced speed without loss of data if your physical machine crashes.

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  2. Best Answer

    According to this manual, I don't see any preallocation options for raw images.

    And also it might be good to take a look at these benchmarks, It seems that any kind of raw images have better performance than qcow2.

    and also I've seen many suggestions to avoid write-through caching because of performance issues but I didn't test it myself.

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  3. qemu-img create -f qcow2 -o size=199G,preallocation=full file.img
mv file.img file.raw
qemi-img resize -f raw file.raw 200G

    ( The last command is needed to align the file to block size 512b )

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