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.

> 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).

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. 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).

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

--
Dan.