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Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices

to how tightly the variance of sequential or random data access is contained

within the volume. This can be purely random across an entire virtual disk, or

random within some bounds, such as a large file stored within a virtual disk

compared to large non-contiguous bursts of sequential data access randomly

distributed within some bounds. Each of these is a different I/O pattern, and has

a discrete case to be applied when tuning the storage.

The data from the stateCaptureData.txt file can be helpful in determining these

characteristics. Sequential read percentage can be determined based on the

percentage of total cache hits. If the cache hits and read percentage are high,

then first assume the I/O pattern tends to be more sequential I/O. However,

since cache hits are not broken out statistically by read and write, some variable

experimentation may have to be performed with the representative data set if the

pattern is unknown. For single threaded I/O host streams, this behavior can be

confirmed by comparing the magnitude of reads to read pre-fetch statistics.

In cases where many sequential read operations are expected, enabling read

pre-fetch in cache is recommended. If the cache hits percentage is low, the

application tends to be more random and read-ahead should be disabled. Mid-

range percentages possibly indicate bursts of sequential I/O but do not

necessarily denote their affiliation to read or write I/O. Again, testing with read-

ahead on/off would be required.

In the second generation firmware, the segment, stripe, and pre-fetch statistics

have been reorganized as seen in Figure 4 from the lower half of Figure 2.

4.7.4 Stripe Size

For the best performance, stripe size should always be larger than the maximum

I/O size performed by the host. As identified previously, stripes should be sized

as even powers of two. The average block size can be identified from the

collected data. Additionally, I/Os over 2MiB are considered large and broken out

separately from smaller I/Os in the statistics. While all RAID level's benefit from

careful tuning of stripe and segment size, RAID 5 and 6 with their parity

calculations are the most dependant.

December 2008 – Revision A01

Figure 4: Second Generation Firmware - Performance

Statistics Broken Out. File: stateCaptureData.txt

*** Performance stats ***

Cluster Reads

6252626

Stripe Writes

2040493

RPA Requests

982036

Full Writes

653386

No Parity Writes

Cluster Writes

3015009

Cache Hits

Cache Hit Blks

4685032

RPA Width

3932113

Partial Writes

Fast Writes

Full Stripe WT

Stripe Reads

5334257

737770040

RPA Depth

418860162

RMW Writes

328612

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