DL380G4 RAID 1+0: Really?

[PDMeat]

A question came up in an internal meeting re standards and an intellectual discussion broke out re what RAID 1+0, 0+1 actually were and why HP uses this term "loosely". I say "loosely" because apparently one can make RAID 1+0 labelled RAID arrays (and I could swear RAID 0+1 in past HP/compaw servers) with only 2 physical SCSI disks in a DL server for example. The point being "real" RAID 1+0 or 0+1 supposedly requires a minimum of 4 disks. http://en.wikipedia.org/wiki/Redundant_array_of_independent_disks
What gives? Any comments?

[David Claypool]

RAID 0 is striped across 2 or more disks and provides no redundancy information.

RAID 1 is mirrored across 2 or more pairs of disks for redundancy.

RAID 1+0 is mirrored and striped which provides for performance advantages (i.e. a sequential read of more than 1 track means that alternatively each drive provides it, raising the utilization of the SCSI bus)

[Marek Nelec]

Yes, I think I also saw the term "RAID 0+1" in the past. RAID 1 is a mirror of two drives. And although for example in ORCA you have only options for: RAID6 (ADG)(not in all controllers), RAID 5, RAID 1+0 and RAID 0. That does not mean that you cannot create RAID 1. It only means that the array controller will create RAID 1, if only two drives are available/selected, and it will create RAID 1+0 (or RAID 10) if there is even numbers of drives available/selected. RAID 1+0 and RAID 0+1 are different. In case of RAID 1+0 pairs of drives are mirrored, and then stripped, so when HDD fails, you only loses one HDD from a mirrored pair and theoretically you can have half of HDDs dead in case if RAID 1+0 and still you won't lose data. In case of RAID 0+1 controller creates two RAID 0's (stripping) and then these strips are mirrored. In that case if you lose one drive from the RAID 0, you lose all RAID 0. So you can lose your data if 2 drives will simultaneously fail in both RAID 0's.

[PDMeat]

Thanks for the replies which are correct and appreciated but I'm realizing I phrased my question improperly too.

What I really wanted to know was not a textbook explanation of RAID levels but why HP considers it ok to describe and label an array of 2 SCSI disks "RAID 1+0" when this is in fact not possible. Two disks can either be RAID0 or RAID1; 4 disks are required to reach RAID 1+0.

I'm sure HP knows RAID levels quite well but they've been doing this for years re two disks. They don't call it RAID1 or RAID0 like Dell and other server OEMs do. The question then is, are they just using the term improperly or is there some sort of technology in play that HP thinks they can use the term RAID1+0 to describe two disks and what they can do with them? If not, they really shouldn't be using the term in this manner as it's confusing and misrepresenting acutal RAID levels.

[Rob Leadbeater]

Hi,

If you read David's reply again, I think he has already answered the question...

To put it another way (if I've understood correctly) with RAID 1 there are two disks. All of the I/O will occur on one of these, with the second drive being just a mirror.

With RAID 1+0 with two disks, read I/O could come from either disk, thus giving better performance.

That sounds right, but feel free to correct me...

Cheers,

Rob

[PDMeat]

Rob, you're right and I'd interpret David's reply as suggesting that RAID1+0 can be done across two drives. Though not stated in the way I'd like (so I can go back to my disbeleiving coworkers and show them something concrete), David's reply indicates to me that a 2-disk RAID1 set implemented by HP smart Array controllers can share some or all of the advantages of RAID0 by performing some or all operations using either drive to increase performance.

However, if I present this to my coworkers, I'm not certain they're going to believe it.

Is there any technically meaty paper that explains how this works? I'd be quite interested to read up on it.

Thanks you all for answering my questions!

[Marek Nelec]

Rob, even with RAID 1 read I/O can come from either disk, enhancing disk read operations performance. RAID 1+0 is just a strip of RAID 1's.
PDMeat, the fact that HP Array Controllers states that even with two drives it is RAID 1+0 is just some kind of simplification done by HP. But after all they could just call it "RAID 1/RAID 1+0.
Here is HP summary of RAID methods:
http://docs.hp.com/en/J6369-90036/ch01s04.html?jumpid=reg_R1002_USEN

And here is a bit more technical document (look at table one):
http://h20000.www2.hp.com/bc/docs/support/SupportManual/c00386950/c00386950.pdf?jumpid=reg_R1002_USEN

I hope this will help you to convince your coworkwers.

[Marek Nelec]

If the second link does not work, paste it in notepad and remove line break. I don't know why but sometimes when I post and URL it is broken because of line break.

[Marek Nelec]

One more try with the link:
http://h20000.www2.hp.com/bc/docs/support/SupportManual/c00386950/c00386950.pdf

[Marek Nelec]

Just out of curiosity I checked ran Array Diagnostics Utility on two servers. One has 2 drives with RAID 1 configuration, the other has 8 drives with RAID 1+0 configuration.
In both cases ADU showed:
Logical Drive 1:
Fault Tolerance Mode: Mirroring