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

[Strahil Nikolov]

I think that the best test is to:
0. Set only 1  change in the infrastructure
1. Automatically create your VM
2. Install the necessary application on the VM from point 1
3. Restore from backup the state of the App
4. Run a typical workload on the app - for example a bunch of queries that are 
pushed against a typical DB
5. Measure performance during point 4 (for example time of execution)
6. Start over

Anything else  is a waste of time.

Best Regards,
Strahil Nikolov


На 24 юли 2020 г. 13:26:18 GMT+03:00, Stefan Hajnoczi <stefanha@redhat.com> 
написа:
>On Thu, Jul 23, 2020 at 07:25:14AM -0700, Philip Brown wrote:
>> Usually in that kind of situation, if you dont turn on sync-to-disk
>on every write, you get benchmarks that are artificially HIGH.
>> Forcing O_DIRECT slows throughput down.
>> Dont you think the results are bad enough already? :-}
>
>The results that were posted do not show iSCSI performance in isolation
>so it's hard to diagnose the problem.
>
>The page cache is used when the O_DIRECT flag is absent. I/O is not
>sent
>to the disk at all when it can be fulfilled from the page cache in
>memory. Therefore the benchmark is not an accurate indicator of disk
>I/O
>performance.
>
>In addition to this, page cache behavior depends on various factors
>such
>as available free memory, operating system implementation and version,
>etc. This makes it hard to compare results across VMs, different
>machines, etc.
>
>Stefan