RAID Query...

Matt_Domsch@Dell.com Matt_Domsch at Dell.com
Thu Nov 15 21:51:00 CST 2001


> Correct me if I'm wrong, but this sounds like a big pile of 
> baloney.

No, you're exactly right. :-)

> And I'd also appreciate if 
> someone could summarise the pros and cons of RAID-1 vs RAID-5 
> (performance, space, reliability, etc...)

R0 - stripes, no redundancy, capacity of N disks.  Fast.
R1, exactly 2 disks, capacity of one.  If one disk fails, you have an exact
copy right there.  Non-degraded reads can go to either drive, thus
potentially 2x read throughput.  Non-degraded writes must go to both disks,
thus potentially somewhat slower (not necesssarily 2x slower).
R10 - stripes of R1 sets, even number of disks.  Up to N/2 disks can fail
before you lose data (if one disk from each mirror set fails).  Fastest
performance, expensive as you only get N/2 capacity.
R5, >=3 disks, capacity of N-1 disks.  If 1 disk fails, you don't lose data.
Stripe parity across all N disks for good non-degraded read performance just
a bit slower than R0.  Non-degraded write performance slowed by need to XOR
data, and if writing less than a whole stripe set, then it's
read-modify-xor-write so even slower.  Degraded R/W performance really bad,
as you have to read and xor whole stripes.  You don't run degraded longer
than it takes to replace the failed disk.

Hot spare disks are really nice, so if one disk fails, the data can be
reconstructed on a spare hopefully before the next disk fails.

That's the gist at least.

Thanks,
Matt

-- 
Matt Domsch
Sr. Software Engineer
Dell Linux Solutions
www.dell.com/linux
#1 US Linux Server provider with 24% (IDC Sept 2001)
#2 Worldwide Linux Server provider with 17% (IDC Sept 2001)
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