Re: [BULK] Re: [ovirt-users] very very bad iscsi performance

[Stefan Hajnoczi]

On Tue, Jul 21, 2020 at 07:14:53AM -0700, Philip Brown wrote:
> Thank you for the analysis. I have some further comments:
> 
> First off, filebench pre-writes the files before doing oltp benchmarks, so I 
> dont think the thin provisioning is at play here.
> I will double check this, but if you dont hear otherwise, please presume that 
> is the case :)
> 
> Secondly, I am surprised at  your recommendation to use virtio instead of 
> virtio-scsi. since the writeup for virtio-scsi claims it has equivalent 
> performance in general, and adds better scaling
> https://www.ovirt.org/develop/release-management/features/storage/virtio-scsi.html
> 
> As far as your suggestion for using multiple disks for scaling higher:
> We are using an SSD. Isnt the whole advantage of using SSD drives, that you 
> can get the IOP/s performance of 10 drives, out of a single drive?
> We certainly get that using it natively, outside of a VM.
> SO it would be nice to see performance approaching that within an ovirt VM.

Hi,
At first glance it appears that the filebench OLTP workload does not use
O_DIRECT, so this isn't a measurement of pure disk I/O performance:
https://github.com/filebench/filebench/blob/master/workloads/oltp.f

If you suspect that disk performance is the issue please run a benchmark
that bypasses the page cache using O_DIRECT.

The fio setting is direct=1.

Here is an example fio job for 70% read/30% write 4KB random I/O:

  [global]
  filename=/path/to/device
  runtime=120
  ioengine=libaio
  direct=1
  ramp_time=10            # start measuring after warm-up time

  [read]
  readwrite=randrw
  rwmixread=70
  rwmixwrite=30
  iodepth=64
  blocksize=4k

(Based on 
https://blog.vmsplice.net/2017/11/common-disk-benchmarking-mistakes.html)

Stefan

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