AIX > Administrator > Performance

AIX Flash Cache Statistics

Flash Cache

This way I have both the longterm and shortterm data saved.

The pfcras command is an undocumented command that shows the last 60 seconds and the last 3600 seconds for every hdisk, but it also provides an average for that time as well. I find pfcras to be far more useful.

After running pfcras or “cache_mgt monitor get” you then get a report. The cache_mgt report provides the following information for each hdisk: start time of statistics, read count, write count, read hit count, partial read hit count, read bytes transferred, write bytes transferred, read hit bytes transferred, partial read hit bytes transferred, promote read count and promote read bytes transferred. These are all counts since boot or since monitoring was started, not percentages. The output from this command is described in the etcadmin -a iostat documentation online.

The pfcras report starts with an overall global cache stats report and then provides information on the cache operations, overall disk statistics and then provides per-lun cache operations statistics for the source disks. It provides statistics for the last 60 and 3600 seconds, which is more granular than what you get out of the monitor report. There is no documentation for this command but you can figure most of it out.

The global cache section indicates whether the cache is warm or not and provides information on the amount of allocated space and the size of the valid data in the space. As an example, my most recent report showed:

Sector size:                 512 bytes
Fragment size:           1048576 bytes
Extent size:          1073741824 bytes
Cache size:        6201932775424 bytes

Cache state: Cache is warm

Reporting average statistics for the last 60 and 3600 seconds.

* Global Cache Stats *
Allocated Space:    6134760996864  (98.92% of total cache space) 
Valid data:         6035699826688  (98.39% of allocated space) 

The second section of the report shows actual cache operations statistics. These are same as what you will see reported for each source hdisk, except this is the global average.

Cache operations               |           60 sec |         3600 sec |
Hit Rate                       |           98.50% |           96.46% |
Partial Hit Rate               |            0.00% |            0.00% |
Lookups                        |            15105 |          1431764 |
Promotes                       |                4 |            48555 |
Partial Promotes               |                0 |                0 |
Server Promotes                |                0 |                0 |
Invalidates                    |                1 |              428 |
Purges                         |                0 |                0 |

Jaqui Lynch is an independent consultant, focusing on enterprise architecture, performance and delivery on Power Systems with AIX and Linux.

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