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DL380G4 RAID 1+0: Really?

PDMeat
Occasional Visitor

DL380G4 RAID 1+0: Really?

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?
14 REPLIES
David Claypool
Honored Contributor

Re: DL380G4 RAID 1+0: Really?

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
Honored Contributor

Re: DL380G4 RAID 1+0: Really?

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
Occasional Visitor

Re: DL380G4 RAID 1+0: Really?

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
Honored Contributor

Re: DL380G4 RAID 1+0: Really?

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
Occasional Visitor

Re: DL380G4 RAID 1+0: Really?

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
Honored Contributor

Re: DL380G4 RAID 1+0: Really?

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
Honored Contributor

Re: DL380G4 RAID 1+0: Really?

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
Honored Contributor

Re: DL380G4 RAID 1+0: Really?

Marek Nelec
Honored Contributor

Re: DL380G4 RAID 1+0: Really?

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
Rob Leadbeater
Honored Contributor

Re: DL380G4 RAID 1+0: Really?

Hi,

I'd been doing some searching of my own, and came across the same document as Marek.

http://docs.hp.com/en/J6369-90036/ch01s03.html

Based on HP's own definition, I'd go back on my previous post, and say that the use of the term RAID 1+0 when referring to a mirror of two disks, is incorrect.

Cheers,

Rob
Adrian Clint
Honored Contributor

Re: DL380G4 RAID 1+0: Really?

My interpretation of this has always been:

If you have 2 drives then configuring RAID1 or RAID1+0 always results in a RAID1 config.

The change happens when you expand the array.

You can expand a RAID1 array a drive at a time and you just proceed to get one more mirrored copy of the orginal each drive you add.

You can expand the RAID1+0 array 2 drives at a time and when you do it starts to stripe on the new disks.

Never tried it but thats they way I assumed it works. The RAID1+0 option for 2 drives is there for those that only have 2 drives initially but want to go to 4+ drives later.

Does that make sense?
PDMeat
Occasional Visitor

Re: DL380G4 RAID 1+0: Really?

I think these replies are evidence of why HP's use of the term RAID1+0 is confusing re two drives. :) Thanks all!

What I find interesting too, is the fact that if RAID1 using 2 disks can read from either disk (performing like a 2 disk RAID0 array) and if you have significant cache memory (128mb+ BBWB) in the controller to buffer all writes, is there really any difference in performance between a RAID0 stripe set vs RAID1 set *with only 2 disks*?

Even more food for thought is that other OEMs to include Intel now offer multiple RAID types on the same set of physical disks. (not quite the same thing we're talking here...)
http://www.intel.com/design/chipsets/matrixstorage_sb.htm
http://www.ocforums.com/showthread.php?t=467848

It would nice to talk to an HP storage engineer and get their 2c on why the RAID1+0 designation is used for 2 disks.
HendrikFrans
Occasional Visitor

Re: DL380G4 RAID 1+0: Really?

I think..


RAID1+0 mirrors the data onto another disk first, and then stripes the data across more disks (obviously, only when more disks become available)

RAID0+1 stripes the data across multiple disks first, and then mirrors the stripe set.
B cannot happen before A has completed.
They are not the same.
shermaine_1
Advisor

Re: DL380G4 RAID 1+0: Really?

Dear Guys,
Thanks for so much information given.I went throught some of the document on RAID level configuration on HP, and finaly confirm that for a heavy load MS SQL database system, the best is implement RAID 1+0.

Currently my database is running on one logical drive with 2 HDD @ HP RAID 1+0 configuration, my server is Hp proliant G4 380 with Smart array 6404/128 controller card, deos any one have any idea whether the I/O performance will get improved if I migrate the RAID 1+0 from 2 HDD to 4 hdd?
If yes, how much difference?

Many thanks for any idea!
Shermaine