Re: [dm-devel] [Lsf-pc] [LSF/MM TOPIC] a few storage topics

[Andreas Dilger <adilger dilger ca>]

On 2012-01-24, at 5:23 PM, NeilBrown wrote:
> On Thu, 19 Jan 2012 01:42:12 +0200 Boaz Harrosh <bharrosh panasas com> wrote:
> 
>>>> Or possibly modify md-raid5 not to snapshot dirty pages prior to xor/write?
>>>> (I never really bothered to find out if it really does this.)
>> 
>> md-raid5/1 currently copies all pages if that what you meant.
>> 
> 
> Small correction:  RAID5 and RAID6 copy all pages.
>                   RAID1 and RAID10 do not.
> 
> If the incoming bios had nicely aligned pages which were somehow flagged to
> say that they would not change until the request completed, then it should be
> trivial to avoid that copy.

Lustre has a patch to that effect that we've been carrying for several years.
It avoids copying of the pages submitted to the RAID5/6 layer, and provides
a significant improvement in performance and efficiency.

A version of the patches for RHEL6 is available at:

http://review.whamcloud.com/1142

though I don't know how close it is to working with the latest kernel.

Cheers, Andreas