Low Cache hit Rate

[DaveFL]

Hey folks,

I'm trying to better understand what could be causing this.

I have database like application and when I am running it hard the cache hit rate seems to be low.

The application is running inside a VM. It stores it's data on the VMs drive. All VM data resides in a data store which sits on top of ZFS.

What I did was create a new NFS mount and make the mount available to the VM. I configured my database application to use the files on the NFS mount. Started it up and started hitting it hard again.

Cache hit rates from arcstat seem to be high.

Is there a logical reason for this? I'm trying to understand why hit rates when this app is running and using data inside the VM is low but high when reading directly from the NFS mount.

Would appreciate any thoughts or suggestions on this.

 

[Robert Trevellyan]

Would appreciate any thoughts or suggestions on this.

My superficial take on this is that in the 1st case you're using block storage, which is much harder for ZFS to cache effectively. In the 2nd case you're using files, which ZFS can cache much more easily. My reasoning is based on what I've read in these forums about how to effectively resource a FreeNAS system for block storage.