Hi,

the (B) part is new to me - I just had my rhce course and they didn´t mention it in any way... Ok, I always do raid in hardware when using servers, for desktop class hardware I prefer doing mirroring.

Regarding (A): the 1 GB rebuild rate is a (hp) worst-case szenario when sizing solutions - in fact, it may well be a dozen times faster.
Nevertheless, I prefer to think of the worst possible case - in your case, it would be at least 8 hours before the rebuild was done. That would mean 8 hours without protection when using raid5; enough time for the next drive to fail... Furthermore the impact on your CPU will be rather big...

Thanks for the info anyway
rainer

Gilboa Davara wrote:

On Thu, 2007-11-29 at 21:39 +0100, moi wrote:

RAID5 (if it really is one) ALWAYS has one drive´s capacity as spare... 
the spare blocks are just distributed on the disks, thus avoiding the 
bottleneck of a single spare drive (these would be raid levels 3 and 4).

what you meant was RAID6/ADG, a semi-proprietary stuff rather found on 
hardware controllers, e.g. hp smartarrays. these do calculate a parity 
for each n blocks, and for "n blocks+parity" generate a second parity 
block. All these blocks are distributed evenly on all drives in the array.

The thing with ADG is the rebuild time - for example the RAIDs at work 
have about 20 drives each (300 gig); the rebuild time on those is about 
1 gb per hour minimum (when there is heavy activity on the raid set). 
that would mean 300 hours without any protection (when using raid5) ! 
instead, with raid6/adg there still is one parity left.
bad thing, though, is the raid controller has to calculate a lot of 
parities. furthermore, the cost is rather high with 2 disks´  worth  of 
parity. Most of the time, such setups use RAID10 (mirror and stripe), 
which uses much cheaper controllers and offers more performance.

sorry for off-topic :)


Two remarks:
A. Modern RAID5 (be that software and/or hardware controller) build far
faster then 1GB/h (291KB/s!??!?!).
I timed my own sever (6 250GB drives in software RAID5) at ~12MBps
(42GB/h) load and ~90MB/s (324GB/h) idle.
B. The Linux kernel has built in software RAID6 support; while slower
then the RAID5 implementation, the performance hit is noticeable but not
devastating and given the added price (1 250/320/etc GB SATA drive)
RAID6 is indeed a fair option if you require two-failed-disk support.

- Gilboa