Storage overhead on zvols

[Dustin Wenz]

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 vm00/chyves/guests/myguest/diskY

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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