latest PERC firmware == slow

[John Vasileff]

For anyone using PERC raid that is concerned about IO performance, read on...

The latest firmware adds a "Patrol Read" feature that is supposed to
detect disk failures before they occur by reading from each disk that
is part of an array.  By default, patrol read automatically runs every
4 hours (actually, 4 hours between each run.)  On my test system,
patrol read takes about 20-25 minutes to complete.

According to the docs, patrol read is supposed to scale back during
periods of high IO, thereby minimizing impact.  But, my tests show
severe impact to random IO.  The first iozone test below is with
patrol read not running.  The second is the same benchmark with patrol
read running.  The entire first run was completed in about 10 minutes.
 The second test didn't even finish the first random read test before
patrol read was complete - over 20 minutes.

### IOZONE when Patrol Read is NOT running
### 5 Disk RAID5
### (PERC 4e/Di, 2850, 2x 3.0GHz, kernel mem=512m)
# iozone -Ra -n 1G -g 1G -i 0 -i 2 -b output.wks

                                    random  random
      KB  reclen   write rewrite      read   write
 1048576      64   87756   86538     16940   30001
 1048576     128   78120   87021     24289   42940
 1048576     256   75545   89749     26922   48614
 1048576     512   75519   91079     44636   66116
 1048576    1024   78189   84996     70692   74730
 1048576    2048   77202   84699    103527   71719
 1048576    4096   61583   78150    130275   73023
 1048576    8192   71793   78952    162446   70795
 1048576   16384   71136   82325    185971   76474


### IOZONE when Patrol Read IS running
### The first "random read" test took over 20 minutes, Patrol Read was
### finished prior to the tests competion.
### (PERC 4e/Di, 2850, 2x 3.0GHz, kernel mem=512m)
### 5 Disk RAID5
# iozone -Ra -n 1G -g 1G -i 0 -i 2 -b output-pr.wks

                                    random  random
      KB  reclen   write rewrite      read   write
 1048576      64   71802   86163       715   25764
 1048576     128   77812   94691     23645   43401
 1048576     256
iozone: interrupted


John