RAID storage question

Hi, all

I have a beginners' RAID question, that I'm sure a lot of you can answer, about advertised storage space vs. actual usable space.

Let's take, for example, the 3.5TB Xserve RAID advertised on the Apple Store pages. According to that ad, it has 7x500GB "Apple Drive Modules", which I am guessing are hard drive discs? Or, are those actual RAID pairs.

As I understand it, when you create a RAID 0, you take two same-size drives, and hook them together, and you get the total amount of storage space on one of those drives (2x500GB SATA=1x500GB RAID 0). So, would my 7x500GB then become 1.5TB, with one drive left over for some sort of archival data purpose?

And, if I were to set up a RAID 5, how much actual storage space would I have then?

Or, am I missing the concept entirely?

Thanks!

>As I understand it, when you create a RAID 0, you take two same-size drives, and hook them together, and you get the total amount of storage space on one of those drives (2x500GB SATA=1x500GB RAID 0).

No, you would have a 1TB raid 0.
My understanding is that Raid 0 just adds the total capacity of the 2 or more drives together. However Raid 5 needs at least 4 drives and one of them becomes the "safety" drive where back up info is stored in case of drive failure. So in a 4 x 500gb setup you would only have 750gb of storage.

Ummm, not quite. I did a quick Google on "raid" and got lots of good info. Here's a good discussion: [www.staff.uni-mainz.de].

Depending on the RAID level, you can get 100% of the combined drive capacities, 50% or other amounts in between. RAID 5 is most fault tolerant and pretty fast, but you "lose" the capacity of one of the drives in the process. But the benefit is that you could crash any one of those drives and not lose anything. Frank's math should say "in a 4 x 500gb setup you would have 1500gb of storage". The minimum number of drives for RAID 5 is 3.

Scott

OOPs! You're right, I used to be a lot better at math-

Actually, if you have ever installed a disk drive you notice that (for instance) a 120 GB drive only gives you 112 GB of usable space. That applies for all drives or RAID sets. A 2.0 TB (4 x 500GB) will actually give you about 1.85 TB of actual drive space.

RAID 0 is the fastest and you get the use of all the available drive space. Each drive is one of the stripes. It is also unsecure as one drive fails, you loose it all.

RAID 1 is slow and mirrors the same data from one drive to another.

RAID 3 is faster than RAID 5 but not as fast as RAID 0. One extra drive is required in the drive set for safety data ( 3 minumum)

RAID 5 allocates a part of each drive for storing safety code for that drive, so if it fails you can rebuild the RAID.

RAID 0+1 (RAID 10) is the best of both worlds. Since it uses all the available data for each drive and they are individual stripes, the speed is that of a simple RAID 0, but it also writes a copy of the RAID 0 to another complete set of identical disk drives as RAID 1 (mirror)



Post Edited (01-18-06 08:11)

So, let me get this straight:

If I buy a 3.5TB Xserve RAID, with 7x500GB drives, and set it up as a RAID 5, I would end up with, at best, a 2TB RAID 5, with one 500GB unit left over.

Or, I could set it up as one RAID 5, and one RAID 10, and have, again, a 2TB storage unit.

But, given the odd number of drives, it would never actually be possible to have 3.5TB of storage space, out of a 7x500GB drive system.

Or, alternatively, is it correct that by the term "Apple Drive Modules," Apple is actually referring to a whole RAID unit (two "drives" linked into one said "Module".

In that case, I could have all 3.5TB, but only in a RAID 0 configuration.

In that case, can I break up some of the "Modules" into individual drives? Then, I could set up a RAID 5 system that uses 2TB worth of whole "Modules," and 4 drives from the broken up Modules as the third drive in the RAID 5. I would be left with the seventh 500 GB "Apple Drive Module," which, in this case, would be its own RAID 0 (on the presumption that an "Apple Drive Module" is a RAID 0, and not an individual drive).

Or, in the "best of both worlds" scenario, I could set up 1.5TB worth of RAID 10, and again be left with a 500GB RAID 0 (on the previously stated presumption).

In other words, unless you are willing to live fast and loose with other people's media (assuming you are working for hire), you can only use 1.5TB or 2TB (depending on which of my scenarios is technologically feasible) of your 3.5TB Xserve RAID.

Is that correct?

I am, of course, ignoring the loss of space on any drive that occurs when you format it, in the interest of having easily calculable numbers.

Thank you!

>If I buy a 3.5TB Xserve RAID, with 7x500GB drives, and set it up as a RAID 5, I would end up with, at best, a 2TB RAID 5, with one 500GB unit left over.

No, you would end up with a bit less than 3 TB of storage.

>But, given the odd number of drives, it would never actually be possible to have 3.5TB of storage space, out of a 7x500GB drive system.

Yes it would. Just stripe them all together as a Raid 0.

I don't get it. You're saying that your 3TB of storage is backed up onto the remaining 500GB RAID 0?

Is it correct that each 500GB "Apple Drive Module" is a 500GB RAID 0 unit?

>Is it correct that each 500GB "Apple Drive Module" is a 500GB RAID 0 unit?

No! Best bet would be to go to some web pages on RAID (like the one in the link above) to get a better understanding. A drive is a drive is a drive. RAID is a Redundant Array of Disks. Disks (plural) being the key word. One drive or module on its own is not a raid. You need at least 2 drives to create a raid and in some cases, depending on the type of raid, more than two. You can take any types of drives and stripe them as a raid using software such as apple disk utility. In fact the "modules" as apple refers to them in their xserve raids are just 7200 rpm ultra ATA drives. You can also have a hardware based Raid system.

>I don't get it. You're saying that your 3TB of storage is backed up onto the remaining 500GB RAID 0?

No! If you have a 3.5 TB Raid 5. You would have about 3TB of storage space and the remaining drive, which is still part of the Raid 5, is used for parity information so if one of the drives was to go bad there would be enough information stored to "re-build" the information lost on the bad drive.

Actually a slight correction to my last sentence. In a Raid 5 configuration, parity information is stored across all of the drives, so you lose about 20-25% of your total storage.