P9000Watch and P9000Sketch Reference guide for HP StorageWorks P9000 Performance Advisor v5.3 (T1789-96325, August 2011)

B Understanding write ratio in P9000Watch
The following screen shows the performance of the entire RAID group 01-04 being collected with the
performance of LDEVs 01:6c, 01:6d, 02:a3, 02:a4, and 02:a5.
The LDEVs in RAID group 01-04 are exposed only through port 1A. There are no I/O activities or
LDEVs reported on this port because there is no I/O activity in port 1A and these LDEVs are not visible
through any other port.
RAID group 01-04 shows some activity because it has another LDEV(01:f4) exposed through some
other ports (these LDEVs were not available for selection in P9000Watch as we selected only port
1A), which have I/Os in it.
The average write percentage (Wr%) for the above output can be inferred by the following calculations:
Calculate the I/O per second:
I/O per second (total front-end writes + total front-end reads) = 2
r_H% (cache read hit %)=0 and Be (back-end I/O per second)=0
Because both of the above values are 0, it implies that there are no back-end reads or back-end writes
or front-end reads.
• Total front-end reads = 0
• Total front-end writes = 2
Therefore, Wr% can be calculated as:
Wr% = 100*(total front-end writes / (I/O per second))
= 100*(2/2)
= 100*1
= 100
The Wr% is 100% because all the front-end I/Os on that RAID group are writes.
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