I'm starting a new thread based on the previous discussion in "bhyve uses all 
available memory during IO-intensive operations" relating to size inflation of 
bhyve data stored on zvols. I've done some experimenting with this, and I think 
it will be useful for others.

The zvols listed here were created with this command:

        zfs create -o volmode=dev -o volblocksize=Xk -V 30g 

The zvols were created on a raidz1 pool of four disks. For each zvol, I created 
a basic zfs filesystem in the guest using all default tuning (128k recordsize, 
etc). I then copied the same 8.2GB dataset to each filesystem.

        volblocksize    size amplification

        512B            11.7x
        4k                      1.45x
        8k                      1.45x
        16k                     1.5x
        32k                     1.65x
        64k                     1x
        128k            1x

The worst case is with a 512B volblocksize, where the space used is more than 
11 times the size of the data stored within the guest. The size efficiency 
gains are non-linear as I continue from 4k and double the block sizes; 32k 
blocks being the second-worst. The amount of wasted space was minimized by 
using 64k and 128k blocks.

It would appear that 64k is a good choice for volblocksize if you are using a 
zvol to back your VM, and the VM is using the virtual device for a zpool. 
Incidentally, I believe this is the default when creating VMs in FreeNAS.

        - .Dustin

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