Daniel Carosone writes:
> On Sat, Dec 20, 2003 at 12:17:56PM -0600, Greg Oster wrote:
> >
> > The "row" code in RAIDframe would behave, in theory, much like doing 
> > a concatenation ccd of RAID sets.
> 
> This can be very very useful from a performance and reliability point
> of view.  If I have, say, 28 disks, then making a single raid5 of all
> of them (if I even am allowed to by any implementation) will truly
> suck for any filesystem usage because the read-modify-write cycle is
> horrendous. (Using them as a pure sequential device might be better,
> just).  And, of course, I'll need several spares and I'll be highly
> exposed to a second failure while rebuilding to a spare.

Right.

> > I forgot to mention, but had the row functionality actually worked in 
> > RAIDframe, we could *still* replace it with a concatenation ccd and 
> > regular "1D" (one-row) RAID sets.
> 
> There's even an example of this in the manpage (RAID on RAID) using RF
> RAID0 rather than ccd(4).

Not quite... the 2D stuff is a "concatenation" of the RAID sets, not 
a stripe across the RAID sets.

> Would there be any other (hypothetical) functionality lost doing this
> vs multi-row?  One example that comes to mind would be in dealing with
> spares. 

Spares could be one issue, yes.

> RF doesn't (seem to) nicely support the hot-spare-pool
> concept, where a spare device can be used by whichever of several sets
> fails first, at least without manual intervention (assiging the disk
> as spare to that set after failure).

"yet" :)

> If removing rows makes adding shared spares easier, then all the
> better :)

It will.  The "extra dimension" of components made handling spares 
even more of a hassle.. (never mind that spares were only ever 
assigned to the first row of the array!).  I'm hoping these changes 
will make a lot of things easier to fix. (In fact, I know they will, 
cause I'm already using a tree with the changes to start work on 
other stuff, and it's been quite nice having the row stuff out of the 
way :) )

Later...

Greg Oster