Re: limits on raid

[David Chinner <dgc@sgi.com>]

On Fri, Jun 15, 2007 at 01:58:20PM +1000, Neil Brown wrote:
> Certainly.  But the raid doesn't need to be tightly integrated
> into the filesystem to achieve this.  The filesystem need only know
> the geometry of the RAID and when it comes to write, it tries to write
> full stripes at a time.

XFS already knows this (stripe unit, stripe width) and already
does stripe unit sized and aligned allocation where it can.

> If that means writing some extra blocks full
> of zeros, it can try to do that.  This would require a little bit
> better communication between filesystem and raid, but not much.  If
> anyone has a filesystem that they want to be able to talk to raid
> better, they need only ask...

I'm interested in what you think is necessary here, Neil.

But I think there's more to it than this - the filesystem only
writes back what the generic writeback code passes it (i.e. a page
at a time). XFs writes back extra adjacent pages in each I/O
if they are in the same state, but it might take multiple
I/Os to write out a full stripe units if we are doing things
like writing across a hole.

Also, there is no guarantee that the first page the filesystem
receives lies at the start of a stripe boundary, so that
sort of information really needs to be propagated into
the generic writeback code above the filesystem as well....

IOWs, the files can easily end I/Os on stripe boundaries
but it is much harder to start them there because that is
not in the control of the filesystem.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group