[vfio-users] vm lags/pauses when IO overloaded on host

[Samuel Holland]

On 07/20/2016 02:41 AM, Marcin Falkiewicz wrote:
> ZFS's zvol is really bad for virtual disk backend when you have only
> few HDDs (or even SSDs) - I had a lot problems with latency and
> throughput on my 2-disk mirrored testbench. Hard to tell if it's ZFS
> itself, ZoL or just not enough disks and slow controller.

I haven't had any problems with zvols on a mirror of 7200RPM drives.
It's not as snappy as my SSD, for sure, but it's reasonably fast, and I
haven't noticed it bogging down the host.

Some things to consider: ARC size and logbias. I have 16GiB of RAM, and
8GiB goes to the VM, so I limited my ARC to 4 GiB to avoid memory
pressure issues on the host. Normally it would use half of RAM, but that
would make host applications have to constantly reclaim memory from the
cache.

Second, I have logbias=throughput on the whole pool. I do not have a
ZIL. There's a few pathological cases (on the host side) where I can get
my desktop to lock up for a minute or two, such as rsyncing several
hundred GiB of files from the SSD, but generally the performance is
better than logbias=latency (which is the default). I find there's
actually less stuttering. Plus it decreases fragmentation and makes my
drives quieter (less seeking).

> On the other hand, LVM runs great on single SSD (directsync+native)
> or even RAID1 HDDs (mdadm; none+native), achieving close to bare
> metal performance with virtio-scsi.

Out of curiosity, are you using scsi-hd, scsi-block, or scsi-generic?

--
Regards,
Samuel