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RAID on a Dell server
http://www.x404.co.uk/forum/viewtopic.php?f=4&t=15723		 Page 1 of 1
Author:  DizietSma [ Fri Jan 27, 2012 12:41 pm ]
Post subject:  RAID on a Dell server
Hi all,

I have a 2nd hand Dell server I want to sell to a client.
It has 3 SAS drives, each a Maxtor 180 Gb.
What is the maximum size I could make a server drive while keeping it RAIDed between 3 drives? (and thus keeping some level of fault redundancy in play ie no server down time if 1 drive fails).
And which RAID should I use?

I know what RAID is and understand the concepts of the various RAIDs 0,1,2,3,4,5 & 6 but have had limited experience actually setting it up.

Many thanks,
Diz
Author:  JohnSheridan [ Fri Jan 27, 2012 1:00 pm ]
Post subject:  Re: RAID on a Dell server
Probably Raid5 as (I think) that should give you around 360Gb of storage with the 3rd drive as a hot-swap spare if one fails.
Author:  jonbwfc [ Fri Jan 27, 2012 2:49 pm ]
Post subject:  Re: RAID on a Dell server
Not exactly. RAID 5 (IIRC) is a parity-based system, and in that case all three drives would be part of the RAID set, and none of them would be a 'hot spare' as such. You could survive one drive failing but if you pulled one of the others to replace it all your data would go up in smoke. You have resilience, but no hot spare. If you do RAID-5 you tend to lose roughly one drive's worth of capacity, that's always been my rule of thumb. So you'll get decent capacity and some resilience, but if a drive pops you might find it hard to get a replacement depending on how old the server is.

If you want data resilience and a hot spare, your only real option is RAID 1 - i.e. use two of the drives as a mirrored pair and keep the odd one as the hot spare. That's sacrificing a lot of capacity, but you're getting effectively two out of three drive failures before you lose the data.

You could do RAID 0 across all the drives for capacity but you'd have no resilience at all.

IIRC there is one form of RAID which is 'stripe across two with parity on another', but that would only get you the same as RAID-5 effectively anyway. So with three drives, those are your choices

RAID 0 - Maximum capacity, minimum protection
RAID 5 - Some capacity, some protection
RAID 1 - minimum capacity, maximum protection

And, obviously (I hope), having RAID is not an excuse not to take backups.
Author:  big_D [ Sat Jan 28, 2012 8:56 am ]
Post subject:  Re: RAID on a Dell server
Raid is scaled from 1 through 7, "0" is NOT RAID it is an AID (Array of Inexpensive Disks), but the Redundant part is not met.

1 is the easiest to set up, but the least efficient. You also need 2 drives, using 1+0 or 10, you can stretch across more disks by using a stripe and then mirroring the stirpe. You need an even number of drives.

RAID 2 through 4 aren't really used much these days, people generally use RAID 5 or RAID 7. If you have a good RAID controller fitted to the Dell, it will be quick and redundant. The more disks you have in the array, the better.

A hot spare isn't a bad idea, but with only 3 drives, that means you are left with a capacity of 180GB, out of a total of 540GB. RAID 5 gives you double that, whilst still giving redundancy and still gives you time to get a replacement disk, if one goes tits up. If 2 goes tits up, you are screwed, but the same goes for RAID 1, if both drives die, before you can remove the broken disk and mirror the good disk to the spare (assuming it doesn't mirror the spare to the good disk! Which is more common than people like to think! :shock: ), you are still screwed.

Personally, I'd go with RAID 5 and look at getting some extra drives as your budget allows.

Also remember: RAID is not a replacement for doing backups!
Author:  AlunD [ Sat Jan 28, 2012 9:19 am ]
Post subject:  Re: RAID on a Dell server
big_D wrote:
Also remember: RAID is not a replacement for doing backups!

One of the best bits of advice I've seen fro a long time. :D
Author:  big_D [ Sat Jan 28, 2012 9:52 am ]
Post subject:  Re: RAID on a Dell server
I have an iMac and a Sony Vaio laptop, the iMac uses a 1TB USB drive for Timemachine, the Sony has a 1TB with Acronis TrueImage NonStop running, the machines get mirrored, the data is mirrored onto the house NAS and the iMac is also backed up to Carbonite in the 'Net.

3-2-1 Rule

No file is backed up until:
There are at least 3 copies of it
On a least 2 different types of media
And one of those copies is off site.
Author:  davrosG5 [ Sat Jan 28, 2012 12:08 pm ]
Post subject:  Re: RAID on a Dell server
Out of curiosity, what is RAID 7? I've heard of RAID 6 (like RAID 5 but with double parity instead of single)
Author:  big_D [ Sat Jan 28, 2012 3:37 pm ]
Post subject:  Re: RAID on a Dell server
RAID 7 is an extension to RAID 6. It goes further and passes all inbound and outbound data transfers through a central cache. The data are striped across several disks, using sector striping. More reserve disk take over in the event of a disk in the array. Instead of just a single parity disk, the set can have several parity disks. Read operations are very fast. Obviously, the central cache could be considered a weak spot and it needs to be covered by redundant power supplies and a batter backup.

RAID 7 is especially flexible in large storage systems. It is usually used in external RAID system (SAN or similar) and it can be extended on the fly by adding new drives to the fly.
Author:  DizietSma [ Tue Feb 07, 2012 12:22 pm ]
Post subject:  Re: RAID on a Dell server
Thanks for your incredibly usefull and detailed replies, everyone.
Will implement forthwith.

Thanks again,
Diz
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