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Dell Hardware RAID Performance Notes

Curious how much performance impact a rebuild/reconfigure will have on one of your Dell PERC/H700 arrays? How about how long it will take?

Drive copy speed is about 3-5 min per GB on a U320 10k SCSI drive. For something like a 15K SAS drive expect 1-2 min per GB. This of course depends on the normal disk load from things like the operating system, databases, etc.

A good formula is as follows:

G * M = T

G = total gigabytes of ONLY the addition drive you are expanding your volume to (not the total size of the array) M = estimated minutes per GB T = total time until completion

So for the 15K SAS drives I’m reconfiguring now I’m estimating about 1.5 minutes/GB:

146GB * 1.5min = 219min

After 45 minutes of running, I have completed 20% of the reconfigure. The remaining 80% should therefore take and additional 180 minutes for a grand total of 225 minutes – pretty darn close to the formulate estimate.

The default rebuilt rate and reconstruct rates are 30% which means that a maximum of 30% disk load will be used to expand or repair a volume. This is evident in the screenshot I attached where the logical drives C: and D: have a combined idle rate of 200% (each drive has a max idle of 100% but they share the same RAID array so their performance numbers are paired)

200% - (77.9% - 90.1%) = 32%

The extra 2% is the operating system load as is present before the reconfigure was initiated at 14:45.

On an extremely low load server (like the file/print server I’m using in my example here) you can get by with the default rate of 30% or even higher if you need to expand or rebuilt your volume faster. On something IO intense like a heavy hitting database server you might want to drop that rate much lower (at the expense of the operation taking longer).

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