Re: [Bug 1875762] [NEW] Poor disk performance on sparse VMDKs

[Stefan Hajnoczi]

On Tue, Apr 28, 2020 at 10:45:07PM -0000, Alan Murtagh wrote:
> QEMU appears to suffer from remarkably poor disk performance when
> writing to sparse-extent VMDKs. Of course it's to be expected that
> allocation takes time and sparse VMDKs peform worse than allocated
> VMDKs, but surely not on the orders of magnitude I'm observing.

Hi Alan,
This is expected behavior. The VMDK block driver is not intended for
running VMs. It is primarily there for qemu-img convert support.

You can get good performance by converting the image file to qcow2 or
raw instead.

The effort required to develop a high-performance image format driver
for non-trivial file formats like VMDK is quite high. Therefore only
qcow2 goes through the lengths required to deliver good performance
(request parallelism, metadata caching, optimizing metadata update
dependencies, etc).

The non-native image format drivers are simple and basically only work
well for sequential I/O with no parallel requests. That's all qemu-img
convert needs!

If someone volunteers to optimize VMDK then I'm sure the patches could
be merged. In the meantime I suggest using QEMU's native image formats:
qcow2 or raw.

Stefan

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