Re: raid 5 read performance

[Neil Brown <neilb@suse.de>]

On Sunday May 21, dan.j.williams@gmail.com wrote:
> > Please read
> >
> > http://www.spinics.net/lists/raid/msg11838.html
> >
> > and ask if you have further questions.
> >
> Does this implementation also need to do delayed updates to the stripe
> cache?  I.e. we bypass the cache and get the requester the data it
> needs but then schedule that data to also be copied into the cache by
> the work queue.  The thinking being to reduce the read penalties
> associated with subsequent writes to the stripe.
> 

I doubt that would be worth the effort.  I don't think data lives very
long in the stripe cache, and so a lot of the copying would be wasted.
Also, I gather that modern drives have a reasonable cache and if data
was read recently for a read, then reading it again for a write might
be very quick.

I think the gains you get on the read side will more than offset any
small loss on the write side.

NeilBrown