RAID confusion on Proliant LM350

[lew hunt]

I'm confused (and also new to all this server stuff!)
With 3 drives installed SmartStart offers either RAID10 on 2 drives(i thought 4 was a minumum) with one spare drive.

Alternatively if offers RAID 5 using all three drives. But MS TechNet says you can't put boot or system volume on a RAID 5 volume so where do they go?

Or am I stupidly confusing my volumes, drives, partitions etc etc?

[Kevin Harlan]

RAID 10 on two drives is the same thing as RAID 1.

The MS TechNet is referring to Windows LVM RAID (Software RAID). If you are configuring your RAID arrays on a hardware RAID controller, then these warnings do not apply.

So, if you have a 5i controller in your ProLiant server, you can use the controller to configure a 3 or 4 drive RAID 5 array. You could then put an OS partition on there (and boot from it), and put a second partition for your data.

But if you are using Windows LVM to do software RAID, then you would *not* be able to boot off that RAID 5 volume you created.

[lew hunt]

So if RAID 10 on 2 drives is effectively RAID 1 presumably I get no read/write speed benefits?

As to boot/system not working if using S/W RAID I'd been groping towards that answer on the assumption that the BIOS (or equivalent?) wouldn't have a hope in hell of guessing what Windows RAID was up to and thus couldn't find boot records etc - anything like close to the reason?

[Kevin Harlan]

Well, with RAID 10 on 2 drives (literally the same thing as RAID 1), you will be getting a read performance benefit but not a write performance benefit. Reads can come off either of the copies (i.e. one read could come off the primary copy, and the second read could simultaneously come off the mirrored copy), but writes will have to be mirrored. Obviously, with more drives, RAID 10 will have a better increase in read performance (but only due to the RAID 0 striping effect, and adding more spindles. You still only have two copies to read from).

Yes, you are correct with regards to why the software RAID 5 will not work on boot volumes. The BIOS, and even the OS boot loader code, will not understand the software RAID to be able to read the blocks off the disk correctly. It will not know to look for, say, the second 64KB stripe on another drive, and instead will read the second 64KB stripe off the same drive -- thereby skipping all the data in between, etc.

(As a side note, the reason the software RAID 1 will boot is because all the blocks are contiguous on both drives, thereby appearing logically consistent on a single drive -- and thus, all the boot loader code will work.)

Hardware RAID, on the other hand, abstracts all this out to the controller level. Any LUN presented up is logically consistent. A read request for the second 64KB stripe will be transparently handled by the controller, and thus, the LUN will appear as a single, logically contiguous disk. This is why hardware RAID will allow you to boot off the LUN, and all the boot loader code will work.

[Michael Schulte zur Sur]

Hi,

so far I have thought it an impossibility to have raid10 on two disks until I was convinced otherwise and it is not the same as raid 1. Read this thread:
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=389427

greetings,

Michael

[Kevin Harlan]

Err, no. RAID 10 and RAID 1 are the same thing (or, more accurately, RAID 10 is a superset of RAID 1).

I read that thread, and there is nothing in there to suggest that RAID 10 is not RAID 1. RAID 10 (RAID 1 + 0) is merely doing a striping set (RAID 0) of drives, then mirroring that RAID 0 set onto another RAID 0 set (thus doing RAID 1 mirroring).

I believe the confusion just comes from the way ACU labels things.

If you notice in ACU, if you want to use JBOD, or just a single stand-alone disk, each individual drive would be RAID 0. Even though it is not a raid array, it is considered a single drive RAID 0 array by ACU.

By the same token, they condensed out the RAID 1 option by re-labeling it as RAID 10 (or RAID 1 + 0). Thus, you can have a RAID 10 set on two disk drives, which is the same thing as having RAID 1 on those two drives. It consists of a single drive in a RAID 0 set, being mirrored to another single drive RAID 0 set.

[lew hunt]

Certainly true that the ACU help screens are less than crystal clear!

The statement that "RAID 1+0 first stripes your data across half of the disks, then mirrors this data to the other half" can be read in at least two ways:

1. half of (each) disk

2. half of the total number of separate disks

neither of which is obviously the right one unless you already know which is true!
A problem with plurals in english (and being from this side of the pond I can only apologise :)

[Michael Schulte zur Sur]

Hi,

I am still searching for the thread. I can't find it at the moment. The one I posted wasn't it. There was one, who insisted having raid10 on 2 disks and I think it is possible. You stripe on the first 50% of both disks and then mirror across on the the second 50% of the disks. However, when writing, you must have write back cache, otherwise it is desastrous for you performance. I think with 2 disks you should better stick to plain raid 1.

Michael

[Kevin Harlan]

Okay, let's back up a bit.

First of all, in the newer versions of the HP Array Configuration Utility, there IS NO OPTION FOR RAID 1!

The only option is for RAID 1 + 0 (or 0 + 1, or 10 or whatever you want to call it).

The reason for this is as follows:

A RAID 0 array can consist of 1 or more drives. 1 is the minimum.

A RAID 1 + 0 ( 0 + 1, 10 ) consists of 2 RAID 0 arrays.

Therefore, by definition, a RAID 1 + 0 array can consist of only 2 drives (in minimum configuration).

Also, by definition and implementation, RAID 1 is merely a special case of RAID 1 + 0.

The actual implementation on the hardware RAID controller works in this manner. And thus, so does the HP ACU utility.

The array controller does NOT mirror only 50% of one set of drives onto 50% of another set of drives, etc. It mirrors from one RAID 0 array to another RAID 0 array -- which in the above case, is from a single drive RAID 0 array to another single drive RAID 0 array.