'Normal' Raid 5 V's Online Spare

[David Farmer]

Hi All,

A quick question for you to ease my mind!

An ML570 (with Integrated Dual Wide Ultra 3 Controller) may be our next server, and to start with, we will purchase 4 x 36.4GB drives.

Do I use RAID 5 or RAID 5 with the online spare? With the spare I imagine I get a total capacity of 109GB (3 x 36) and a little bit extra re-assurance that the spare will kick in on disk failure.
Without the spare, do I get 145GB, and on disk failure, suffer a performance hit, while the data is rebuilt using the parity info?

Or have I not got it all!

[Leif Halvarsson_2]

Hi,
With a raid 5 set of 3 disks you get about 72GB capacity not 109. To get 109GB you need a Raid set of 4 drives, and if you want 1 online spare disk you need a total of 5 disks.

[Vincent Fleming]

Figure it this way:

Usable space = (Total # of drives - spares -1) * 36

So, you have either:

usable space with spare = (4 - 1 -1) * 36 = 2*36 = 72

usable space no spare = (4 -0 -1 ) * 36 = 3 * 36 = 108

For me, I would use RAID 1/0 with 4 drives and no spare. You would get 72GB, it's faster, and you don't need a spare anyway. Keep a spare on the shelf if you are nervous about losing a drive.

Good luck,

Vince

[David Farmer]

Leif,

I will have 4 36GB drives, and according to the documentation I've read that's the minimun number of disks I need to use the online spare?

If I get you right, you are saying that using RAID 5, I'll get 109GB from 4 x 36's.

Dumbing it down a bit, what do I get from 4 x 36's using RAID 5 with the online spare?

[Vincent Farrugia]

Hello,

Two of us answered already: that would be 72Gb.

HTH,
Vince

[Vincent Farrugia]

Hello,

You'd get 3 disks used in RAID 5... so 1 for redundancy and the other 2 for data = 72Gb.

The other one will only be used when a disk fails, which would start the rebuild process automatically to it.

HTH,
Vince

[David Farmer]

Duh!

Thanks Guys.

Could not see the sense in that at first, but I think I was looking for a more complicated answer than that what I got.

I get it now.

Thanks for your help.

[Michel M.]

David,
It look like you didn't get the exact principle of RAID5 and use of a spare (you should use "hot spare").
1) You will always suffer a performance hit when a disk fail.
2) You need at least 1 disk to be use for the parity of the RAID5. So, the highest capacity you can get is 109GB (3*36). Otherwise, on a disk fail, you loose all your datas.
3) "Hot spare" disk is a optional feature that will give you more security. With that disk online, the rebuild process start at once (even when your are sleeping at night...), so you don't spend time before changing the faulty drive(if a second disk fail before the end of the rebuilding of the first one, you loose all you datas). But, in this case you will have 2 disks used for redondancy and only 2 for storing datas (72GB). This should be used only for critical servers located on a distant location.
Michel