Re: Raid 50

On Tuesday 06 July 2004 10:39, David wrote:
> I am well aware of the pros and cons of RAID 0, 1, 0+1, 1+0(raid 10), 5,
> etc. But, here is a new one on me...RAID 50.
>
> Thoughts, experience...please elaborate asI just found out it's our RAC IO
> backend.

Raid50 is a compromise between raid10 and raid5. Basically you create
small raid5 sets (usually 4 or 8 disks) and then stripe across the small raid5 
sets rather than across mirror pairs.

10,000 foot comparison with raid10:

Pro: you get more storage, and somewhat better streaming read performance

con: somewhat worse small/random write performance for non full stripe or non
stripe aligned operations (you go from a crc+write to a read, modify, crc, 
write)

con: overall write performance can be constrained by the speed of your 
checksummer

con: you lose some level of redundancy. Instead of mirror pairs of two disks,
you have raid5 sets of N (usually 4 or 8) disks. A double fault is more         
    
likely with a raid5 set. It is still a possibility with raid10, too.

con: degraded mode will cause a stripe will require additional compute 
(usually via the checksummer) to reconstruct the missing data which can have 
an unpredictable impact upon io performance as well as potentially impairing 
io to the rest of the array. a raid1 mirror fault will cause degraded 
performance only to the impacted set, and that usually is represented by a  
50% loss of effective read iops if the array load balances (it should) but 
full write io capacity is retained. (i'm excluding the io cost of folding in 
a new disk/hot spare for both cases) 
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