AP09 – Fast and easy disk workload characterization on ESX

AP09 – Fast and easy disk workload characterization on ESX

(Richard McDougall)

 

When moving an I/O insensitive workload onto ESX, you’re always asking yourself: “Will it hold? Will it perform? Is there something I can tweak to make it perform better?”. In this session Richard showed some things to look at.

 

The key questions before moving to virtual are:

–         What is the I/O blocksize?

–         What is the spatial locality?

–         What is the I/O interarrival period?

–         What is the Queue Depth needed?

–         How much latency is allowed?

–         What are the read / write rations?

 

When knowing these figures for you physical environment, you can start testing in your virtual environment to see if your configuration can meet the demand. Richard is an SUN and Oracle guy, so a lot of examples are Oracle based, but that is no problem for understanding where its all about.

 

The tools he uses frequently to test performance are Filebench OLTP, OSDL dbt-2 and Windows large file copies.  In the virtual environment you can monitor what your vSCSI adapter of your VM is doing by looking at /usr/lib/vmware/bin/vscsistats -h. It can show you metrics like I/O size, R/W ratios and Outstanding I/Os. Very important data to have when discussing performance with your SAN admin.

 

There are some limitations in using vscsistats, it only shows histograms per virtual disk and it doesn’t give a complete I/O picture because you will have other VMs interfering and traffic from the ESX host itself. But, nevertheless still a great tool to start with when diving into performance characteristics. Be sure to check out these sites, on which more information can be found about performance:

http://www.virtualscoop.org and the VROOM blog on the VMware site.

 

Great session, although Richard sometimes went pretty fast and left no time for me to take notes. Well, there is always the online pdf in a couple of weeks.