[mythtv-users] Why RAID 5 stops working in 2009

[Yeechang Lee]

Mache Creeger <mache at creeger.com> says:
> Lally Singh recommends a ZDNet piece predicting 
> the imminent demise of RAID 5

Depends on the circumstances, of course. I've had reason to be glad my
16-disk MythTV-recording array is RAID 6, not 5. My three-, four-, and
currently out-of-commission eight-disk arrays are RAID 5, and in my
view the risks are acceptable.

If and when I ever spring for a new recording array based on whatever
is the highest-capacity drive at the time, the number of drives will
determine how it's configured. If 16, I'd likely again go RAID 6. If
20 (Or even more?), I'd likely multiple RAID 5 or 6 arrays.

> This reader adds, "Apparently, RAID 6 isn't far behind. I'll keep
> the ZFS plug short. Go ZFS. There, that was it."

I'd love to use ZFS, but there's no sign it's going to be incorporated
into the Linux kernel anytime soon. I'd have to build and maintain an
OpenSolaris box just for ZFS, and although I'm open to it given ZFS's
benefits it'd mean I wouldn't be able to run a slave backend on it.

> As the RAID controller is busily reading through those 6 disks to
> reconstruct the data from the failed drive, it is almost certain it
> will see an [unrecoverable read error]. So the read fails ...

The new Seagate 1.5TB drive's unexpectedly-impressive performance for
a first-generation model priced at $190 is evidence that drives are
getting faster as well as larger, although the improvement isn't as
fast as on the CPU side. (The rebuilds and preens of my 16-disk array
are always CPU, not disk, bound, as Linux software RAID is as yet
unable to take advantage of multiple cores.)

> Oh, you didn't back it up to tape? Bummer!"

As has been noted several times here, tape backup of terabyte arrays
of MythTV recordings is simply uneconomical even were it desired. The
cheapest way, by far, is another identical array.

-- 
Frontend:		P4 3.0GHz, 1.5TB software RAID 5 array
Backend:		Quad-core Xeon 1.6GHz, 6.6TB sw RAID 6
Video inputs:		Four high-definition over FireWire/OTA
Accessories:		47" 1080p LCD, 5.1 digital, and MX-600