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SubjectRe: limits on raid
On Fri, 22 Jun 2007, David Greaves wrote:

> That's not a bad thing - until you look at the complexity it brings - and
> then consider the impact and exceptions when you do, eg hardware
> acceleration? md information fed up to the fs layer for xfs? simple long term
> maintenance?
> Often these problems are well worth the benefits of the feature.
> I _wonder_ if this is one where the right thing is to "just say no" :)

In this case I think the advantages of a higher level system knowing what
efficiant blocks to do writes/reads in can potentially be a HUGE

if the uppper levels know that you ahve a 6 disk raid 6 array with a 64K
chunk size then reads and writes in 256k chunks (aligned) should be able
to be done at basicly the speed of a 4 disk raid 0 array.

what's even more impressive is that this could be done even if the array
is degraded (if you know the drives have failed you don't even try to read
from them and you only have to reconstruct the missing info once per

the current approach doesn't give the upper levels any chance to operate
in this mode, they just don't have enough information to do so.

the part about wanting to know raid 0 chunk size so that the upper layers
can be sure that data that's supposed to be redundant is on seperate
drives is also possible

storage technology is headed in the direction of having the system do more
and more of the layout decisions, and re-stripe the array as conditions
change (similar to what md can already do with enlarging raid5/6 arrays)
but unless you want to eventually put all that decision logic into the md
layer you should make it possible for other layers to make queries to find
out what's what and then they can give directions for what they want to
have happen.

so for several reasons I don't see this as something that's deserving of
an atomatic 'no'

David Lang
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