Re: md question re: max_hw_sectors_kb

["fibreraid@gmail.com" <fibreraid@gmail.com>]

Hi Mike,

Thank you for the patch. I've tested it though on 2.6.38 and do not
see any significant change in md RAID 5 write performance compared to
the unpatched kernel. Can you clarify how you may have achieved a
demonstrable improvement?

-Tommy


On Wed, May 11, 2011 at 8:51 PM, Martin K. Petersen
<martin.petersen@oracle.com> wrote:
>>>>>> "Neil" == NeilBrown  <neilb@suse.de> writes:
>
> Neil,
>
>>> Your fix is functionally correct. However, another case just popped
>>> up this week where we need to distinguish between stacking driver and
>>> LLD defaults.
>
> Neil> What case is this?
>
> This particular case involved the need to set different defaults for
> discard depending on whether it was a stacking or a low level driver.
>
>
> Neil> If you have FS -> DM -> MD, then any change that MD makes to
> Neil> max_hw_sectors_kb will not be visible to the FS.  So adding and
> Neil> activating a hot spare with smaller max_hw_sectors_kb cause cause
> Neil> it to receive requests that are too big.
>
> Yeah, this issue pops up occasionally. Alasdair and I were discussing it
> just a couple of weeks ago.
>
>
> Neil> So we really need a propery resolution to this problem first.
> Neil> i.e. A way for 'dm' to notice when 'md' changes its parameters -
> Neil> or in general any stacking deivce to find out when an underlying
> Neil> device changes in any way.
>
> Neil> I would implement this by having blkdev_get{,_by_path,_by_dev}
> Neil> take an extra arg which is a pointer to a struct of functions.  In
> Neil> the first instance there would be just one which tells the claimer
> Neil> that something in queue.limits has changed.  Later we could add
> Neil> other calls to help with size changes.
>
> I agree we need a way to propagate queue limit and capacity changes up
> the stack. I'll put in on my todo list.
>
> --
> Martin K. Petersen      Oracle Linux Engineering
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