Date: Mon, 4 Dec 2017 21:37:43 -0600 From: Adam Vande More <amvandemore@gmail.com> To: Dustin Wenz <dustinwenz@ebureau.com> Cc: FreeBSD virtualization <freebsd-virtualization@freebsd.org> Subject: Re: Storage overhead on zvols Message-ID: <CA%2BtpaK3GpzcwvRFGoX5xdmwGnGWay0z_kqgW6Tg7hX5UBbz4og@mail.gmail.com> In-Reply-To: <CC62E200-A749-4406-AC56-2FC7A104D353@ebureau.com> References: <CC62E200-A749-4406-AC56-2FC7A104D353@ebureau.com>
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On Mon, Dec 4, 2017 at 5:19 PM, Dustin Wenz <dustinwenz@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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