Using the vSphere flash read cache feature to speed up sequential reads

Duncan Epping gave a great overview of vSphere’s flash read cache feature and I wanted to take it for a ride. This feature reminds me of the ZFS level-2 ARC which allows SSD drives to be used as read and write caches. The vSphere vcache only provides read caching but that is still super useful for read-intensive workloads. To see how it performed I broke out my trusty old sequential read script to get a baseline:

$ fsexercise mongo.dbf2 1M
43 Gb read, 111 MB/s

Not too shabby for my old rusty NFS datastore. Once the initial test was complete I connected a Crucial MX300 SSD to my server and added it as a flash cache though Hosts and clusters -> Settings -> Virtual Flash Resource Management. Next I added 50GB of flash cache space to the disk I was testing and ran a read test to “prime the cache”. Once this completed I ran a second test which provided significantly different results:

$ fsexercise mongo.dbf2 1M
43 Gb read, 443 MB/s

The addition of one SSD sped up single threaded sequential reads by 4X which was significantly more than I was expecting. I’m planning to run some random read tests this weekend and suspect they will fair FAR better. This is a useful feature and 100% free if you have the right licenses in place. Definitely something to keep in your tool belt if you manage VMware infrastructure.

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