Confusion with RAID

[Neilfud]

Hello,

I've just bought a new ML350 G5 configured with 4 x 250GB SATA drives.

I had originally intended to configure these as two mirrored drives Drives 0 and 1 making up the first mirrored set and drives 2 and 3 the second mirror set giving me two logical drives of 250GB.

When I entered the RAID Configuration utility at startup I was only offered RAID 1+0. Whilst I didn't expect it to work, I selected the drives 0 and 1 and configured one array and then drives 2 and 3 and configured the second.

So, I ended up with two RAID 1+0 drives, which is pretty much what I wanted, but I am confused as I always thought that RAID 1+0 required a minimum of 4 drives. Now whilst I have four drives it appears the the RAID controller as created the arrays from two pairs of drives.

Part on me says 'so what, it's what I wanted' but I would like to understand more about how this works, and also whether I really do have 2 x 1+0 RAID arrays!

Any help in putting me straight would be appreciated.

[David Claypool]

With 2 drives RAID 1 mirrors the data from the first drive to the second. By default a SmartArray controller gives you RAID 1+0 which provides the protection of mirroring but alternates tracks for better performance for sequential reads/writes. In other words, if one drive were designated A and the other B and you were to read the first 10 tracks of the set, it would be

A0 B1 A2 B3 A4 B5 A6 B7 A8 B9

...where A0 means that track 0 is being read from drive A. If one drive is removed or fails, all of the data is preserved on the remaining drive and the controller works with the tracks on that drive independently.

[Taifur]

Hi Neilfud,

The difference between RAID10 and RAID01 as follows;

RAID 10 and RAID 01 are not the same thing and it does matter. RAID 01 is a mirrored configuration of two striped sets. RAID 10 is a stripe across a number of mirrored sets.

RAID 10 provides better fault resilience and "rebuild" performance than RAID 01. Both array types provide very good to excellent overall performance by combining the speed of RAID 0 with the redundancy of RAID 1 without requiring parity calculations.

For more details you can follow below link,

http://searchstorage.techtarget.com/expert/KnowledgebaseAnswer/0,289625,sid5_gci1135899,00.html

Rgds//
Taifur

[Jimmy Vance]

The Smart Array controller offers RAID 1+0.

When there are only two physical drives in the array, this fault-tolerance method is
often referred to as RAID 1.

So yes, you've created two RAID 1 arrays

Refer to page 82 in the Configuring Arrays on HP Smart Array Controllers Reference Guide

http://bizsupport2.austin.hp.com/bc/docs/support/SupportManual/c00729544/c00729544.pdf

[Neilfud]

That's great, thanks for all your replies. I was confused as external RAID references (e.g. Wikipedia) suggest that RAID 1+0 require a minimum of four drives.

Thanks again.