RAID 5

[avamar]

Which is better using 3D+1P Raid 5 or using 7D+1P Raid 5 .Which gives better performance ,resillience etc ?
How are the two different ,even though they are RAID 5 .

[Nigel Poulton]

Avamar,

RAID5 in a 7D+1P formation is the better of the two options.

It will perform better than 3+1 because there are more disks brought into play each time you access LUNs on them raid group.

You also have the advantage that 7+1 is more space efficient yeilding 87.5% usable space as opposed to only 75% useable space with 3+1.

You can also use interleaved/striped raid groups with 7+1 which is where you can stripe a LUN across as many as 32 spindles (4 x 7+1 RAID groups) This only applies to recent firmwareon the RAID500 series. Striping across 4 x 7+1 gives you the best performance.

The potential drawback (and I personally do not worry about this) is that you are *slightly* more at risk of a double-disk failure.

Hope this helps,

Nigel

[Nigel Poulton]

Also, forgot to mention that a RAID5 7+1 can be spread across two ACPs (RAID processors) whereas RAID5 3+1 sits on a single ACP pair. Therefore 7+1 will give you twice the number of processors working on your I/O

[Peter Mattei]

I assume you are reffering to XP12000; right?

It very much depends on the used OS.
For OSes without a reliable Volume Manager to stripe across multiple LUNs/LDEVs you want to have as many disks under a single LUN as possible. (Windows, Netware, Linux etc)
With a unix system you usually use the Volume Manager (LVM, LSM, VxVM etc) to build volume groups that stripe across dozens or hundreds of LUNs to gain performance.

My experience show that 90+% of all XP12000 are installed with Raid5 7D+1D. It gives best price / performance and as stated by Nigel it can be used to stripe XP internally to build 28D+4P LDEVs.

Looking at max performance:
The highest writeperformance for a whole XP array can be achieved with RAID1 2+2 or 4+4!!
Two RAID5 3D+1P LDEVs striped with LVM have a higer writeperformance than one RAID5 7D+1P LDEV.
But if you use Windows and can only assign one LDEV as a Driveletter one 7+1 LDEV of course has higher performance than on 3+1.

See my attached slides.

Cheers
XP-Pete

[Peter Mattei]

And by the way; there is a new interesting whitepaper availablea bout XP12000 performance.
See http://h71028.www7.hp.com/ERC/downloads/4AA0-7923ENW.pdf

Cheers
XP-Pete

[Amar_Joshi]

Hi Peter,

The "Slide 4" in your attachment which mentions about different microcode supports different level of concatenation. Where can I get it from the web? I have searched alot but failed to get any document which support this slide.

Thanks in advance.

[Peter Mattei]

What information are you exactly looking for?

What I can tell is when the respective support was introduced on the XP12000:

50-05-06 introduced 14+2 support
50-06-14 introduced 28+4 support

Is that OK for you?

Cheers
XP-Pete