Re: Question about LVM and RAID

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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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