Re: Very long raid5 init/rebuild times

[Larry Fenske <LFenske@SGI.com>]

On 02/03/2014 08:30 PM, Stan Hoeppner wrote:
> On 2/3/2014 8:42 AM, Phillip Susi wrote:
>
>> On 2/3/2014 1:34 AM, Stan Hoeppner wrote:
>>> Please reread my statement above.  Now let me restate that as:
>>>
>>> Measuring disk throughput when writing through the buffer cache
>>> isn't a measurement of disk throughput as much as it is a
>>> measurement of cache throughput.  Thus, such measurements do not
>>> demonstrate actual disk throughput.
>>>
>>> Do you disagree?
>> Yes, I do because cache throughput is >>>> disk throughput.
> It is because buffer cache throughput is greater that measurements of
> disk throughput are not accurate.  If one issues a sync after writing
> through buffer cache the measured throughput should be fairly close.
> But without issuing a sync you're measuring buffer cache throughput.
>
> Thus, as I said previously, it is better to do parallel O_DIRECT writes
> or use AIO with O_DIRECT for testing disk throughput as one doesn't have
> to worry about these buffer cache issues.
>
Perhaps Phillip is doing the obvious and only measuring throughput after
the cache is full.