Information Storage and Management Book ISM Page 60

Mark..

The images are correct.

If you take a look at RAID 1+0 : Imcomming data is Mirrored and then Striped which the way it is

RAID 0+1 : imcomming data is first Striped and then Mirrored.

Guys please correct me if I am wrong.

Cheers!!

Hi Mark,

I've attached a diagram which is another way of looking at this. Please bear with me, it is 4 pages and is just hand-drawn.

To quickly explain the diagram:

• 0,1,2,3,4,5,6,7 are 'strips'  (this is the ISM terminology. You may be familiar with this as the 'stripe-unit' - Veritas, 'interlace' - Disk Suite / Solaris Volume Manager, 'element' - Clariion etc etc. I drew the diagram a long time ago, before ISM, and I used the term 'stripe-unit').
• The first column is physical - hard drives.
• The second column is the first layer of logical abstraction.
• The third column is the second layer of logical abstraction.
• As a small exercise for the reader, there is a tiny mistake - a digit 2 that should be a 3.

Please let me know if you have any questions. I hope the diagram helps.

Thanks, Richard.

Well here is one source that is confusing me. Look at the Six disk 0+1 example. I must be looking at something wrong. It is the same picture as (a) RAID 1+0 figure on page 60.

http://en.wikipedia.org/wiki/Nested_RAID_levels

Hello Mark,

The wiki diagram is really the same as my hand-drawn diagram (mine is constructed left to right, whilst the wiki diagram is constructed bottom to top; also, the wiki diagram uses 6 disks for 0+1, whilst my diagram uses just 4. Apart from these minor differences, the diagrams are logically identical). If you find the page 60 diagram confusing, then I would just forget about it and refer to one of the other two diagrams for your understanding. If you understand either my diagram or the wiki diagram, then you understand the distinction between 1+0 and 0+1.

Best regards.

By the way, I reckon RAID 1+0 vs RAID 0+1 has probably been one of the hottest/most popular discussion topics in storage management over the last 15 years, or maybe longer. There has also been confusion in the field about which one was striped mirrors, and which one was mirrored stripes - (mirroring + striping) was often called striped mirrors, and (striping + mirroring) was often called mirrored stripes.

However, RAID 0+1 is now often considered to be legacy/historical. In the last 15 years the only product that I have personally encountered (and there will be others that I am unaware of) where you could only implement 0+1, and had no choice to implement 1+0, was with Veritas Volume Manager version 2.x. When Veritas Volume Manager 3.x shipped (many years ago), then Veritas introduced a new layered volume architecture. This new architecture allowed the implementation of 1+0, along with other nice features such as dynamically migrating from one RAID level to another (eg. 5 -> 1+0).

There can be 1+0 subtle implementation differences between different vendors and different products. As one example, some vendors allow 1+0 using an odd number of disks, whilst many vendors mandate an even number of disks.

Cheers.

Thank you very much for your responses. They are helpful. I think the main thing that will help me understand which is which is this: which Nested RAID 1+0 or 0+1 can sustain only a two disk failure. So if I have 10 disks and I take 5 disks and stripe them and then take the other 5 disks and stripe them and the data is mirrored between the two stripes I can only have 2 disks fail from the whole group and not from the same striped set. So which Nested RAID is this?

thanks,

Mark

DING DING DING! That is what I expected, but just needed it clear. Thanks for all your responses. I completely understand now.

Mark