mounting/exporting/importing a zfs volume

Victor Sudakov vas at
Tue Feb 12 01:46:49 UTC 2019

Frank Leonhardt wrote:
> >
> > vm-bhyve keeps virtual machines on zfs volumes with volmode=dev. How can
> > I access/mount the filesystems within the volume when the virtual host
> > is offline?
> >
> > If I kept virtual disks in raw files, I could access them as devices
> > with mdconfig. But:
> >
> > root at newserv:~ # mdconfig -a -f /dev/zvol/zroot/vm/mail/disk0
> > mdconfig: /dev/zvol/zroot/vm/mail/disk0 is not a regular file
> > root at newserv:~ #
> >
> > Also, how can I exchange those zfs volumes for use with other
> > hypervisors? They are not real raw disk files so I cannot use
> > sysutils/vmdktool etc.
> >
> I don't know this, but I'll guess(!)
> If you've set volmode to dev then you get a cdev device in devfs, and 
> you'll never get it to mount. Try using geom instead (which IIRC is the 
> default).

The default in vm-bhyve is volmode=dev, and I think this is reasonable.
Do you know if I can clone an existing volmode=dev volume into a
volmode=geom volume and then work with the clone?

> HOWEVER, I suspect you're doing this because you're hoping that a ZFS 
> volume is faster than a file. 

Well, not actually.

> I went through this, in the hope it wouldn't do CoW and would
> therefore be a lot better for databases. I was disappointed!
> Bascially, it's no better than a ZFS file. If that was your plan, use
> a UFS partition. 

A UFS partition? Where?

> I don't use ZFS volumes any more; I 
> think they're more useful on Solaris. A md mapped on to a ZFS file seems 
> to be the BSD way, and for VMs just use a file in its own dataset. You 
> can then clone the dataset. Just what you need for nearly identical VMs.

I've preferred disk0_dev="zvol" VMs for aesthetical reasons since
vm-bhyve started supporting them. Those file-based VMs get in the way
while backing up $vm_dir, and their disks are not visible in 
"zfs list -t volume"

Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
2:5005/49 at fidonet
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