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
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