Date: Mon, 4 Dec 2017 17:22:09 -0600 From: Dustin Marquess <dmarquess@gmail.com> To: Dustin Wenz <dustinwenz@ebureau.com> Cc: FreeBSD virtualization <freebsd-virtualization@freebsd.org> Subject: Re: Storage overhead on zvols Message-ID: <CAJpsHY5SL101o_iRaO5UyH9H8mSFOmmT_SRPJGra%2BdKyq58%2Btw@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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I doubt it's best practice, and I'm sure I'm just crazy for doing it, but personally I try and match the ZVOL blocksize to whatever the underlaying filesystem's blocksize is. To me that just makes the most logical sense. -Dustin 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 infl= ation 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=3Ddev -o volblocksize=3DXk -V 30g vm00/chyv= es/guests/myguest/diskY > > The zvols were created on a raidz1 pool of four disks. For each zvol, I c= reated a basic zfs filesystem in the guest using all default tuning (128k r= ecordsize, 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 effici= ency 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 minimize= d by using 64k and 128k blocks. > > It would appear that 64k is a good choice for volblocksize if you are usi= ng a zvol to back your VM, and the VM is using the virtual device for a zpo= ol. Incidentally, I believe this is the default when creating VMs in FreeNA= S. > > - .Dustin >
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