Storage overhead on zvols

[Dustin Wenz]

Thanks for linking that resource. The purpose of my posting was to increase the body of knowledge available to people who are running bhyve on zfs. It's a versatile way to deploy guests, but I haven't seen much practical advise about doing it efficiently. 

Allan's explanation yesterday of how allocations are padded is exactly the sort of breakdown I could have used when I first started provisioning VMs. I'm sure other people will find this conversation useful as well.

	- .Dustin

> On Dec 4, 2017, at 9:37 PM, Adam Vande More <amvandemore at gmail.com> wrote:
> 
> On Mon, Dec 4, 2017 at 5:19 PM, Dustin Wenz <dustinwenz at ebureau.com> wrote:
> 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.
> 
> I'm not sure what your purpose is behind the posting, but if its simply a "why this behavior" you can find more detail here as well as some calculation leg work:
> 
> https://www.delphix.com/blog/delphix-engineering/zfs-raidz-stripe-width-or-how-i-learned-stop-worrying-and-love-raidz
> 
> -- 
> Adam

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