Analyzing I/O Workloads for vSphere Flash Read Cache (vFRC)

A good understanding of the workload behavior and characteristics is the most important factor in deciding whether or not to enable vFRC because not all workloads will benefit from vFRC. vFRC caches data from both read and write I/Os, but write I/Os are always serviced by the underlying storage. Therefore, workloads that have a majority of reads can directly benefit from vFRC. Cache block size is the minimum granularity of cache fills and cache evictions. Having the optimal cache block size is critical to overall performance of vFRC. (Source: VMWare )

To set optimal cache block size we need to find a most frequent I/O size of the workload. VMWare ESXi has a tool named vscsistats for I/O size analyzing. Instead of vscsistats I have been using another free tool –  EMC XtremSW Cache Performance Predictor.

EMC XtremSW Cache Performance Predictor main function is to generate prediction reports based on host traces. Cache Reads Analysis report predicts benefits in IO workloads when using host based EMC XtremSF PCI Flash cards. But it also contains more valuable information – time window of trace, amount of I/O’s recorded, Workload Mix chart and I/O Size Distribution chart.

Workload mix chart shows read to write ratio. Big read percentage increases the chance for performance benefits when using vFRC.  I/O Size Distribution chart helps you to select the best vFRC cache block size.


I have not yet upgraded my VMWare environment to 5.5 but I have already found some candidates for vFRC. My suggestion is to start with database servers. They are most likely candidates for performance improvements.

EMC XtremSW Cache Performance Predictor tool can be downloaded from EMC Support site – link.
More information about EMC XtremSW Cache software – link and link.


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