[Sussex] Recovering from RAID 5

[Steve Dobson]

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On 08/11/10 20:57, David Morris wrote:
> Steve Dobson wrote:
>> I'm afraid I don't have a recovery solution for you.  I read this
>> article [1] a year or two ago and was convinced by it.  I will admit
>> that I haven't checked the maths behind it, but the principles do appear
>> sound to me.  As disk capacity increases for a given format then they
>> must be packing the tracks closer and closer together.  I can't see this
>> improving the read failure rate.  Couple this with the requirements that
>> RAID 5 requires for recovery and complete RAID 5 failure during rebuild
>> becomes a significant increasing probability as disk capacity increases.
>>
> 
> You could also look RAID 6, which will give you more tolerance against
> failure.  Also worth reminding people that RAID != Backup

But is the performance hit you take with calculating two parity blocks
worth it?

In the days of old when disks where small and RAID 5 was useful, because
you only paid 1/nth the cost for increased tolerance (where n is the
number of disks in the set).  Then disks where relatively expensive (my
first hard drive cost me around £500 (including the SCSI interface card
for my Amiga 2000) and it was only 46MB unformed.

Thursday I bought 2 500GB disks for less then £100 - because they were
the cheapest disks in the store.  I was planing to reinstall my
workstation and I wanted to mirror.  That gives me some real protection
for very little write cost.  No parity claculation at all, just the need
to write the same block to two different devices.

Mirroring (RAID 1) is a much better solution.  What are the changes the
the same block is bad on both disks at the same time?  I doubt that it
is that high.  RAID 1 works!  I had a disk failure in one of my previous
RAID 1 setups (the one with my home partition on has it happens) and I
lost nothing!  I just continued using the single disk (my risk) until I
when out and bought a replacement pair.

The problem I find with RAID 1 is that when one disk does fail you can't
get a replacement disk.  They just don't do them in that configuration
any more.

Steve
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