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+Poor disk performance on sparse VMDKs
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+Found in QEMU 4.1, and reproduced on master.
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+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. On my system, the fully allocated write speeds are approximately 1.5GB/s, while the fully sparse write speeds can be as low as 10MB/s. I've noticed that adding "cache unsafe" reduces the issue dramatically, bringing speeds up to around 750MB/s. I don't know if this is still slow or if this perhaps reveals a problem with the default caching method.
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+To reproduce the issue I've attached two 4GiB VMDKs. Both are completely empty and both are technically sparse-extent VMDKs, but one is 100% pre-allocated and the other is 100% unallocated. If you attach these VMDKs as second and third disks to an Ubuntu VM running on QEMU (with KVM) and measure their write performance (using dd to write to /dev/sdb and /dev/sdc for example) the difference in write speeds is clear.
+
+For what it's worth, the flags I'm using that relate to the VMDK are as follows:
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+`-drive if=none,file=sparse.vmdk,id=hd0,format=vmdk -device virtio-scsi-pci,id=scsi -device scsi-hd,drive=hd0`
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