[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Bug 1875762] Re: Poor disk performance on sparse VMDKs
From: |
Thomas Huth |
Subject: |
[Bug 1875762] Re: Poor disk performance on sparse VMDKs |
Date: |
Thu, 06 May 2021 14:30:08 -0000 |
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
--
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
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`
To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1875762/+subscriptions
[Prev in Thread] |
Current Thread |
[Next in Thread] |
- [Bug 1875762] Re: Poor disk performance on sparse VMDKs,
Thomas Huth <=