Re: [PATCH/RFC] md/raid10: optimize read_balance() for 'far copies' arrays

[Namhyung Kim <namhyung@gmail.com>]

Keld Jørn Simonsen <keld@keldix.com> writes:
> On Wed, Jun 08, 2011 at 04:42:27PM +0900, Namhyung Kim wrote:
>> Still can't understand why we choose the closest-to-the-start disk in
>> case we could have possible sequencial access on other disk. Probably
>> because of the lack of my understanding how md/disk works :(
>
> the nearest position was the case for the initial implementation of
> raid10-far.  But this had bad performance for an array with disks of
> varying specifications. And also it led to not using the faster
> outer sectors. Using the closest-to-beginning gave a spped-up of about
> 50 % in some cases.
>

Hi Keld,

Thanks for the explanation. That means lower sectors reside on the outer
tracks/cylinders in the disk, right? The 50% seems a huge improvement I
couldn't stand against. Although my patch tried to choose
closest-to-current-head disk if the disk head is in the lowest stripe -
in the (similar) hope that it'd be on the outer tracks - I don't have
the numbers, so I'll just give up on it.

Besides, I just noticed that the rationale behind read_balance()
pressumed that all components of the array are traditional disks. If we
could detect all/some of them are not (i.e. SSD, etc.), it would be
better off using some other criteria for the read balancing IMHO,
something like nr_pending?

-- 
Regards,
Namhyung Kim
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html