AIX > Administrator > Performance

AIX Flash Cache

Flash Cache

The issue with the “monitor get” command is that it reports by disk. This is fine if you have three or four target disks, but when you have 88 of them it is a lot of data to look at. It would be great if they also provided an overall average at the end. Here is an example of what it showed for one of the disks while I was running the iotest.sh script from SAS:

ETS Device I/O Statistics -- hdisk12
Start time of Statistics -- Wed Feb  1 12:02:30 2017
-----------------------------------------------------
 Read Count:                           	      63299
 Write Count:                         	        69692
 Read Hit Count:                               6547
 Partial Read Hit Count:                         17
 Read Bytes Xfer:                     18253348864
 Write Bytes Xfer:                     18253611008
 Read Hit Bytes Xfer:                 1676115968
 Partial Read Hit Bytes Xfer:          10100736
 Promote Read Count:               7488929792
 Promote Read Bytes Xfer:                    7142

As you can see the read hits were low but this is most likely because the script was only reading the data one time so there was no real benefit from caching.

Testing

Testing flash cache turned out to be a challenge. First you have to remember that it takes time to select the blocks to be cached. This means that short simple tests such as a dd are of little to no value. I did some testing with ndisk64 and also with the SAS iotest.sh program. Unfortunately, iotest.sh uses dd under the covers so the results were very patchy. I got a little carried away with the ndisk64 testing and managed to run myself out of memory, surprising considering I had 512GB. These tests are being rerun in the next couple of weeks. Flash cache was very easy to set up and I was able to show that it was working and speeding up disk access very quickly. Note that you need to let it run for a while (at least 5 minutes) in order to select the disk blocks that would benefit the most from caching. This is why you should always have a warm up time whenever you do any benchmarks and is normal.

Summary

Flash cache has great potential for heavy read workloads for improving performance. It is still in its first version so there is still room to improve some of the functionality, but overall it is well worth exploring if you have a workload that can benefit from read caching.

NOTE: Thanks to Nigel Griffiths for his edits and the information he provided to me.

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



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