Performance hit going from RAID 0/1 to RAID 5DP

[Eric Sauve]

Our VA7100 is currently set in AutoRAID mode and we use a little less than half of all available raw space for the LUNs so if I'm not mistaken it is currently operating in RAID 0+1 mode. If I start to use more space and the VA is forced to switch to RAID5DP mode how much will the performance be affected. I don't need solid numbers just a ballpark figure. We use our VA7100 primarily for Oracle 9i data. Thanks.

[Vincent Fleming · Accepted solution]

How much your performance will be affected depends on a few factors, including how much raw space you have, how much has been allocated to hosts in the form of LUNs, how much of that is actually being used, etc.

How it works is when you begin to pass that 50% mark (50% of total raw storage allocated as LUNs), the VA begins to "make room" for the LUN space over 50% by converting some of the RAID 0/1 to RAID 5DP. How much it converts depends on the above factors.

Since RAID 5DP can be quite a bit slower than RAID 0/1 in some cases (such as small block random writes), the more data it converts to RAID 5DP, the more you will see effects in your performance.

In theory, you should not see a large difference in performance if you do not go very far above 50%, as the RAID-5DP portion of the data will be relatively small, and the VA picks data blocks to be RAID-5DP based on access patterns - the least-used data blocks will be converted to RAID-5DP first.

It's a performance-for-capacity trade-off.

Good luck!

Vince

[Eric Sauve]

Thanks for the clarification. I didn't know that a VA7100 could do this, I never knew exactly why the HP AutoRAID was considered such a cool technolog, now I begin to understand why... I thought it was all or nothing, either the VA was running the entire array in RAID 0+1 or in RAID 5DP. I didn't know it could run portions of it in both modes at the same time. Thanks again...

[Vladimir Vybiral]

Hello,
I would continue a bit where Vincent finished a correct explanation of AutoRAID. The most important thing is that the least used data is moved to R5DP only, as Vincent says. So, it is data that has not been touched for the longest time. R5DP has a very similar performance for reading as R0+1 has, so You should not notice a performance hit (and, You have a cache there for quick rewrites), if You are up to some 75% used capacity. Quite clever. Vladimir

[Roger_22]

The best performance is with: HP18 firmware, Normal Resiliency, Pre-fetch enabled, Optimize Enabled.

Is yours so?

[Roger_22]

Next. You should verify the array is not broken ??? like a failed disk or controller. Next would be to review the controller event logs, sometimes intermittent backend fibre problems can present themselves as performance problems.

Once you???ve sure there is no hardware problem, then most likely cause of performance problems in the AutoRAID setting is due to the workload. If the workload is consistently high demand and the workload contains a signification write content (~>50%) then (typically) the AutoRAID polices can???t keep up. This can be verified by reviewing the internal performance statistics. See the armperf OPAQUE and RAID 5DP to RAID 5DP activity. (the AutoRAID policies are background tasks??? if there is no background time, then they can???t run).

This would be an indicator that your workload is a candidate for RAID 1+0 operation. If you reply with the exact disk configuration (number and sizes), I???ll reply with the maximum LUN capacity to create a RAID 1+0 only array ??? it???s not exactly 50% of anything!