[Sussex] Recovering from RAID 5

[D.Morris at brighton.ac.uk]

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Steve Dobson wrote:
> 
> 
> 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

I use software RAID 1, which means I just need to get a disk thats bigger, and a make the partition
the same size.  Actually I think it works if you add a partition which is bigger than the old one.
Obviously there a performance cost to this, not sure how much though but I don't notice it on my
desktop nor our lightly used development servers at work.

As for RAID 6 performance loss, it depends upon what the task is for.  I use software RAID 6 with 4
1.5TB drives for a NAS with my multimedia stored on it. The machine has a 1000Mbit card, however my
desktop which writes to it only has 100Mbit and the network IO of my desktop is the bottleneck.  My
mythtv frontends can all play back fine whilst I'm writing to it.


Dave

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