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

[Thomas Huth]

Ok, I'm closing this now, since this is the expected behavior according
to Stefan's description.

** Changed in: qemu
       Status: New => Won't Fix

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https://bugs.launchpad.net/bugs/1875762

Title:
  Poor disk performance on sparse VMDKs

Status in QEMU:
  Won't Fix

Bug description:
  Found in QEMU 4.1, and reproduced on master.

  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.

  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:

  `-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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