[Sussex] Recovering from RAID 5

[Steve Dobson]

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Hi Dave

On 09/11/10 11:46, D.Morris at brighton.ac.uk wrote:
> Hi Steve,
> 
> Steve Dobson wrote:
>> Hi Dave
>>
>> On 09/11/10 11:02, D.Morris at brighton.ac.uk wrote:
>>> 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.
> 
> I tend to use different manufactures to make sure they're not from the same batch.

Good planning, I'm not quite that perfect for my own RAIDs.  :-)

>>> 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.
> 
> I plan on growing the array as I start to fill it up.

Can you just add disks to a RAID 6?  Nice if you can but I'm not sure
that that is allowed.

> Also as I understand it.
> 
> RAID 6 with 2 disk parity, I can lose any 2 disks in the array and still recover everything.

True.

> RAID 10 (which I think your talking about) would give better performance etc, and still give some
> support for 2 disk failure. In the attached picture, if the 2 disks which failed where on the same
> side of the tree i.e. 1 & 3 or 2 & 4 then you've lost everything, therefore it doesn't provide quite
> as much resilience.

I don't like that diagram as it doesn't show the structures correctly
for me, and I like even less the term RAID 10 - you should be talking
RAID 0+1 [1] so you don't get configured with RAID 1+0 [2] which doesn't
give the same protection against disk failure.

In my mind there is a reason why there are a number of different RAID
levels.  Each offers something over the others.  The best RAID
configuration will depend upon a number of different factors, not all of
them technical.

You have your MythTV system dumping data to your RAID 6 NAS.  I would
never do that, video captured is not something I want (let along need)
to backup.  What's the point?  The repeat (re-transmission) schedule
these days for just about every program means that even if my MythTV box
failed I can just watch the repeat.

I also don't run a central file system.  That would be a single point of
failure so I would need to run a backup of it.  I prefer to have local
copies using rsync to keep them in line with each other.  I like off
system backup as once the cron jobs are configured to make the copies I
can just forget about them until a problem occurs.  Tape backup either
requires a very expensive robot to change tapes or for me do get of my
butt every day - and I am lazy.

Steve

[1] http://www.raids.co.uk/raid_0+1.htm
[2] http://www.raids.co.uk/raid_1+0.htm
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