Re: Very long raid5 init/rebuild times

[Stan Hoeppner <stan@hardwarefreak.com>]

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.

-- 
Stan