[Sussex] Recovering from RAID 5

[Steve Dobson]

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Hi Dave

On 09/11/10 11:02, D.Morris at brighton.ac.uk wrote:
> Steve Dobson wrote:
>> 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.

Nice to know that that works.  But I feel that when one disk of a RAID
set goes then the other are likely to be near the end of the reliable
life too, or close to it.  Best to replace the lot anyway.


> 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.

That doesn't appear to be a useful use of RAID 6 to me.  You loose two
disks to parity with RAID 6 (as I understand it) so with only four disks
you would be better off striping and mirroring.  Same capacity, 3TB and
not performance lose in calculating two parity blocks.

Steve
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