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

[Alan Murtagh]

Public bug reported:

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`

** Affects: qemu
     Importance: Undecided
         Status: New

** Attachment added: "Two different empty VMDKs with vastly different 
performance."
   https://bugs.launchpad.net/bugs/1875762/+attachment/5363023/+files/vmdks.zip

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

Title:
  Poor disk performance on sparse VMDKs

Status in QEMU:
  New

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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