Latency - "RAID 5" vs "RAID 0+1"

[Adam Stahl]

I have an MSA1000 with no additional storage enclosures. Would I achieve much lower latency if I created all RAID 0+1 luns vs all RAID 5 luns?

[Víctor Cespón]

You mean with all disks on a single RAID, right?
RAID 5 has a write penalty due to the need to recalculate parity.
The difference is then proportional to the % of writes.

For example, 14 drives, 10000 rpm, with 40% writes give 740 I/Os in RAID 5 and 1170 in RAID 0.

You can find a lot of papers about this on the Internet. Here's one from HP: http://h50146.www5.hp.com/products/storage/whitepaper/pdfs/c00386950.pdf

[Adam Stahl]

Correct. I created one array which contains 13 drives, I have the 14th as a spare. I am experiencing high latency which is affecting my servers. I have 512MB cache (50% read/50% write) which is supposed to help with latency due to writes. I was just wondering if it would be a big difference in latency if I changed all of my luns from RAID 5 to RAID 0+1.

[IBaltay]

Hi,
if you have e.g. the RAID5 and RAID01 logical volume to compare, you could measure the perf counters yourself, see this treads pls:

http://forums11.itrc.hp.com/service/forums/questionanswer.do?threadId=1275913

http://forums11.itrc.hp.com/service/forums/questionanswer.do?threadId=1282347

[Víctor Cespón]

There's a I/O limit for 13 disks, between 370 (10K drives, RAID 5, 100% writes) and 2100 (15K drives, RAID 1, 100% reads).

A detailed analysis would require to connect to the controller serial port and collect performance data (I/Os per port, per logical unit, etc)

We are seeing more performance complains since people started using virtualization and hosting 15 - 30 servers on a single array with 6 - 14 disks.
Just remember that until we switch to flash-based storage, the mechanical disks have a limit of 100 I/Os per second or so.

[Adam Stahl]

Well this MSA1000 is in fact used for virtualization. I have 4 ESX servers attached to it with about 65 vms running.

I infact thought that we would get better performance if we added more disks to the MSA so that we could stripe the luns across more disks. Do you think that would help a lot?

And if I add a shelf of disks does the MSA have the ability to span an array (and therefore lun) to the 2nd shelf?

[IBaltay]

the more disks in the array (group) the more spindles in the logical volumes

if you expand the array (group) after the second disk enclosure physical addition, then you can have even more spindles in the array/logical disks

[Adam Stahl]

So then the answer is "yes", you can span the array across shelves of disks?

[IBaltay]

yes,
and the verticality (across the 2 enclosures) is giving you also more performance because of the load distribution across more disk enclosures controllers...

[Adam Stahl]

Thank you everyone.