I assume most of you know that PernixData FVP provides a clustered solution to accelerate read and write I/O. In light of this I have received several questions around what the “Hit Rate” signifies in our UI. Since we commit every “write” to server-side flash then you obviously are going to have a 100% hit rate. This is one reason why I refrain calling our software a write caching solution!
However the hit rate graph in PernixData FVP as seen below is only referencing the read hit rate. In other words, every time we can reference a block of data on the server-side flash device it’s deemed a hit. If a read request cannot be acknowledged from the local flash device then it will need to be retrieved from the storage array. If a block needs to be retrieved from storage then it will not be registered in the hit rate graph. We do however copy that request into flash, so the next time that block of data is requested then it would then be seen as a hit.
Keep in mind that a low hit rate, doesn’t necessarily mean that you are not getting a performance increase. For example if you have a workload in “Write Back” mode and you have low hit rate, then this could mean that the workload has a heavy write I/O profile. So, even though you may have a low hit rate, all writes are still being accelerated because all the writes are served from the local flash device.