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

From: Stefan Hajnoczi
Subject: Re: [ovirt-users] very very bad iscsi performance
Date: Fri, 24 Jul 2020 11:26:18 +0100

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

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.


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