Well, I've received a lot of responses to my original question. All of them
say the same solution.
Original post:
>I remember to have read in some book the relation between number of
>disks
>used to make a RAID 5, their capacity and the final capacity >obtained.
>This was exposed in terms of a given formula.
>The problem is that I can't find that expression in my books.
And here is the solution:
total capacity is = (( Number of disk ) - 1 ) * single disk capacity
Thanks to: Chuck Taylor, Paccapeli Marco, Alan, Mike D Cross, Thomas M.
Payerle, Joseph Thvedt, Daniel Monjar, Ron Bramblett, Joe Ryals, Gert
Winterhalter, Lars Bro and Rajesh Kapoor.
And special thanks to Tom Brand, who answered:
Raid 0 = striping
2 or more disks and data is split over all disks.
With two 9Gb drives, you get 18Gb of data space.
Main benefit: speed, I/Os are split into 2 chunks and written
concurrently
Con: the mean time between failures (MBTF) decreases. If the disks
are rated 100,000 hours MBTF, the 2 disk stripe set has a MBTF
of 50,000 hours and a 4 disk stripe set has a MBTF of 25,000
hours. If any disk crashes, the whole stripe set is broken.
------------------
Raid 1 = mirroring =
2 or more disks, each with the same copy of the data.
With 9Gb drives, you get 9Gb of data space.
Main benefit: data security. You can lose one drive and still
be fully ok.
Cons: price; you need twice as many disks
------------------
Raid 0+1 (or 1+0) = mirror + striping.
Needs 4 or more disks. Data is mirrored on 2 disks, and striped over
the mirrors. You mirror disk1+disk2=M1 and disk3+disk4=M2, and then
stripe the two mirrors M1+M2.
Benefits: you get the speed of Raid 0, the data security of Raid 1.
If you are doing Raid 0, and you want to secure your data, you better
be doing raid 0+1
Cons: as with raid 1, price.
------------------
Raid 3 and Raid 5 is striping + parity.
Needs (2 or more data) + 1 parity disks.
This means that you need a minimum of 3 disks (2 for striping
the data and 1 used as a parity disk. Not as fast as Raid 0, but
you can lose one drive in the stripe set without losing the whole
stripe set. If you lose a drive, the data is "rebuilt" on the fly
from the parity information. While in this "degraded" mode, your I/O
speed is very much slowed down as each bit of data has be be recreated
from a calculated parity.
Benefits: improves speed, requires only 1 additional disks for
data security.
Cons: parity overhead - decreases speed (compared to Raid 0).
See also:
http://www.uni-mainz.de/~neuffer/scsi/what_is_raid.html
http://www.raidweb.com/whatis.html
http://www.stealthraid.com/raidex.html
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Received on Wed Mar 08 2000 - 09:17:04 NZDT