Re: linear writes to raid5

[Michael Tokarev <mjt@tls.msk.ru>]

Neil Brown wrote:
[]
> raid5 shouldn't need to merge small requests into large requests.
> That is what the 'elevator' or io_scheduler algorithms are for.  There
> already merge multiple bio's into larger 'requests'.  If they aren't
> doing that, then something needs to be fixed.
> 
> It is certainly possible that raid5 is doing something wrong that
> makes merging harder - maybe sending bios in the wrong order, or
> sending them with unfortunate timing.  And if that is the case it
> certainly makes sense to fix it.  
> But I really don't see that raid5 should be merging requests together
> - that is for a lower-level to do.

Hmm.  So where's the elevator level - before raid level (between e.g.
a filesystem and md), or after it (between md and physical devices) ?

I mean, mergeing bios into larger requests makes alot of sense between
a filesystem and md levels, but it makes alot less sense to do that
between md and physical (fsvo "physical" anyway) disks.

Thanks.

/mjt