ZFS Zvols containing disk images, nested mounting

M

martin_b

I have a server with a zpool containing zvols containing disk images which are typically used in VMs. Sometimes it would be helpful to manipulate such an image directly from the server. With the "old" ZFS implementation, this was possible - the various partitions in such a zvol would be exposed at the appropriate places in /dev (/dev/zvol/<volume name>pN, if label exist then also /dev/gpt/<gpt label>pN, /dev/label/<ufs label>pN, etc.), and they could be mounted, manipulated, etc. directly on the server.

With the OpenZFS implementation this is no longer possible. Some sysctl variables which might allow this are normally off, and if one turns them on and tries an operation, a deadlock + reboot/panic follows.

What are the plans for FreeBSD regarding this feature?

-- Martin
 
I know how to do this in Linux using loop devices but I don't (unless I'm sure the disk image is not currently mounted in a VM) Therein lies data inconsistencies...lots of inconsistencies. For safety sake stick with NFS mounting the disk image with the VM as the server and host as the client.
 
[0]# sysctl -d vfs.zfs.vol.recursive
vfs.zfs.vol.recursive: Allow zpools to use zvols as vdevs (DANGEROUS)
[0]#
 
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