Question about RAID?

Is it possible to set up RAID within a disk?

For example Imagion I had two Hard Drives

Disk 1 (160Gb)                  Disk 2 (160Gb)
Parition 1 (80Gb)               Parition 2 (80Gb)

Is it possible to set up some kind of RAID array where by the data is spread like so .... So that I/O to either partition would be much faster as they could be read from either disk and the data would also be mirrored and so safe if one disk died?

Disk 1 (160Gb)                  Disk 2 (160Gb)
Parition 1 (80Gb)               Parition 2 (80Gb)
Parition 2 (80Gb)               Parition 1 (80Gb)

From what I have read this would be RAID 1+0 wouldnt it?

David
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DavidBirch2dotComAsked:
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Lee W, MVPTechnology and Business Process AdvisorCommented:
No, RAID MUST be spread over multiple disks.  Two disks will only allow RAID 1 or RAID 0, not both.  To do RAID 0+1 (or 10, or 1+0) you NEED 4 drives).  
DavidBirch2dotComAuthor Commented:
So there is no way of setting something like the above up with two disks?
Lee W, MVPTechnology and Business Process AdvisorCommented:
Windows software RAID won't let you do it and I've never heard or seen hardware RAID that did it.  The problem is when you create a RAID 0, it has to use both drives in the RAID to create a RAID 0 - that's the concept of RAID 0.  So now the system only sees ONE hard drive, that is in your case, 320GB in size.  So where do you mirror it to?

If you try the reverse, create a RAID 1 first (mirror), your system looks at both drives as a SINGLE drive, 160GB in size.  So how do you create a RAID 0 when the OS only sees 1 drive.

Your best hope, if you have a compatible RAID controller, is to do part RAID 1, part RAID 0.  Some controllers can make different raids out of partitions, some can't.  So you could break up the disks into, say two 120 GB drives that use RAID 1 (120GB usable space), and 40 GB RAID 0 from each disk (80 GB usable space).
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Duncan MeyersCommented:
You could potentially do this with Unix/Linux Logical Volume Management. However, using a setup like this would result in poorer performance than a mirrored pair (and quite possibly even a single disc) due to file system locality. As the logical volume is read the disc heads will jump from the outside tracks to the inside tracks as data is read, taking up heaps of time in multiple seek operations. It potentially gets even worse if, say, the mirrored half of a partition is on the same disc and the system tries to get clever and read from both halves of the mirror at the same time - seek seek seek seek seek seek seek seek seek seek seek seek. Now *that* would be slow!

So performance-wise, you'd be better off with a simple mirrored pair or even a single disc. For performance and redundnacy, go with a mirrored pair and you'll be fine.
DavidBirch2dotComAuthor Commented:
I am slightly confused... am I right in thinking:

RAID 1 - mirroring
allows me to safe guard my data from disk failure but at a cost in terms of performance - "you'd be better off with a simple mirrored pair or even a single disc" mirroring is slower than a single disk?

RAID 0
does this result in a boost in performance?

When using RAID's I can still partition up an 160Gb disk into say 4 40Gb paritions ?
Duncan MeyersCommented:
>allows me to safe guard my data from disk failure but at a cost in terms of performance - "you'd be better off with a simple mirrored pair or even a single disc" mirroring is slower than a single disk?

Typically RAID 1 has better than single disc performance for reads (up to 2x - the system can read from two different discs at the same time) and the same as single disc performance for write operations. What I meant was that your proposed design would result in extremely poor performance, and that you would be better off with a single disc because of that. Since you have two discs you're best off witha mirrored pair - for redundancy and performance.

RAID 0 is not RAID!!!! RAID is Redundant Array of Industry standard Discs (or Inexpensive if you prefer). RAID 0 has no redundancy therefore it is not RAID. Do not use RAID 0 where data integrity is important, because if you lose one disc, you kiss your data goodbye. Do a search on this site on RAID 0... Having said that, if you combine RAID 0 and RAID 1 to give you RAID 1/0, you get excellent performance and excellent redundnancy.

>When using RAID's I can still partition up an 160Gb disk into say 4 40Gb paritions ?
Yep, no problemo. If you are using a quality hardware RAID controller, then the OS is simply presented with what it thinks is a single disc. You can then slice it and dice it any way you like. Any OS restrictions still apply, of course.
 

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DavidBirch2dotComAuthor Commented:
Thanks meyersd  thats an excellent explaination I will go for RAID 1 when I get my new machine
Duncan MeyersCommented:
Glad to be of assistance. :-)
DavidBirch2dotComAuthor Commented:
Hi meyersd

Sorry to ask a question after accepting an answer, but I feel this is revelvent to what I asked,

please look at http://www.tomshardware.com/motherboard/20050629/intel_955-03.html

it appears to show what I was talking about is possible

"One interesting aspect of these various features (as shown in the figure at the head of this section) is the combination of a fast array (for example, RAID 0) with a redundant array (such as RAID 1 or 5). In deploying pairs of drives, a RAID 0 array handles the operating system and data storage operations. A follow-on RAID 1 array can preserve transactions, and even capture a system image for fast recovery. "

or am I reading it wrong? Which is best for performance raid 0 (ok its not a real raid ;) ) or raid 1 ?
Duncan MeyersCommented:
Yuk. That is truly doolally.

>"One interesting aspect of these various features (as shown in the figure at the head of this section) is the combination of a fast >array (for example, RAID 0) with a redundant array (such as RAID 1 or 5). In deploying pairs of drives, a RAID 0 array handles >the operating system and data storage operations. A follow-on RAID 1 array can preserve transactions, and even capture a >system image for fast recovery. "

From my previous post: It potentially gets even worse if, say, the mirrored half of a partition is on the same disc and the system tries to get clever and read from both halves of the mirror at the same time - seek seek seek seek seek seek seek seek seek seek seek seek. Now *that* would be slow!

So if you had one half of the pair of discs mirrored and one half as a RAID 0 set (say with the page file on it), then you have set up the conditions for the above. It would be seriously slow because every time the OS wants to write to the page file it has to seek half way across the disc surface, wait for settling time, write/read whatever necessary then seek back halway across the disc to continue whatever it was doing.  Baaaaaaaaad.

But to address your other question now that I'm off my high horse, RAID 0 is damn quick, no question. In a two disk set it would be about 1.5 time faster than the mirrored set for writing, but read performance will be almost identical althiough the mirrored pair will have a slight edge. The picture changes as you add more discs. a 4 disc RAID 0 set is really quick and easily outperforms the mirrored pair on reads and writes. In a perfect world where you have lots and lots of money, you set up a RAID 1/0 set which is a RADI 0 set striped across lots of mirrored pair. Lots of redundancy and lots of speed. And lots of squids too.
DavidBirch2dotComAuthor Commented:
Thanks meyersd

So its possible with that motherboard but not recommended.  I think I will as you recommend go for  a mirrored pair unless i can find some cheap 60Gig hard drives & get four lol
Duncan MeyersCommented:
My pleasure.
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