[ovirt-devel] [VDSM] All tests using directio fail on CI

[Yaniv Kaul]

On Thu, Sep 29, 2016 at 12:55 PM, Anton Marchukov <amarchuk at redhat.com>
wrote:

>
>> > The node will be offline for now until we agree on what to do.
>> > An option is to abandon RAM disks completely as we didn't find
>> > any performance benefits from using them so far.
>>
>> That's very surprising. On my case it doubles the performance,  at least.
>> But I assume my storage (single disk) is far slower than yours.
>>
>
> What amount of RAM you had available to Linux file system cache and were
> there any previous runs so Linux were able to put any mock caches into the
> RAM cache?
>

I don't do mock. And if I run everything in RAM (whether directly under
/dev/shm/<somewhere> or in a zram disk), I honestly don't need the Linux
system cache.


>
> Besides the possible difference in disk speeds I think the second factor
> is this Linux fs cache that basically create an analog of RAM disk on the
> fly.
>

Well, theoretically, if you have enough RAM and you keep re-running, many
of the data is indeed going to be cached. I'd argue that it's a better use
of RAM to just run it there.


>
> Those two things might explain why we do not see any performance
> improvement from RAM drives in our case.
>

Indeed.
Y.


>
> Anton.
>
>
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