ZFS: Mount partition from ZVOL with volmode=dev

[Steven Hartland]

On 27/12/2014 13:18, Paul Chakravarti wrote:
>> On 26/12/2014 23:54, Paul Chakravarti wrote:
>>> Hello,
>>>
>>> I am using a ZVOL configured with 'volmode=dev' as the virtio disk device for
>>> a bhyve instance (which works fine) but was trying to workout whether there
>>> was any way of mounting the underlying partitions on the host system - the
>>> partitions don’t show up under /dev/zvol as separate devices with
>>> 'volmode=dev' so was wondering is there is any other way of getting at these
>>> other than mounting in a bhyve instance?
>>>
>>> Thanks, Paul
>> I cant reproduce this on HEAD r276067
>>
>> zfs create -V 8192 -o volmode=dev tank/tvol
>> root at head:src> ls -l /dev/zvol/tank/
>> total 0
>> crw-r-----  1 root  operator  0x85 Dec 27 00:08 tvol
>>
>>       Regards
>>       Steve
> Hi,
>
> Sorry - I should have been clearer. The zvol shows up on the host
> system but the partitions aren’t exposed to geom.

That's exactly what volmode=dev does, if you want geom ones don't 
specify volmode.

> On the host system:
>
>      # zfs create -V10G -o volmode=dev tank/vps/vm0
>
>      # zfs list -o name,used,volmode tank/vps/vm0
>      NAME           USED  VOLMODE
>      tank/vps/vm0  10.3G      dev
>
>      # ls -l /dev/zvol/tank/vps/
>      total 0
>      crw-r-----  1 root  operator  0x7b Dec 27 13:30 vm0
>
> The zvol mounted on the bhyve guest:
>
>      # bhyveload -c /dev/nmdm0A -m 512M  -d /dev/zvol/tank/vps/vm0 vm0
>      # bhyve -c 2 -m 512M -A -H -P -s 0:0,hostbridge -s 1:0,virtio-net,tap0 -s 2:0,lpc -s 3:0,virtio-blk,/dev/zvol/tank/vps/vm0 -l com1,/dev/nmdm0A vm0
>
> On the bhyve guest this shows up as a geom device:
>
>      root at vm0:~ # geom disk list
>      Geom name: vtbd0
>      Providers:
>      1. Name: vtbd0
>         Mediasize: 10737418240 (10G)
>         Sectorsize: 512
>         Mode: r2w2e3
>         descr: (null)
>         ident: BHYVE-747A-2A76-FAC
>         fwsectors: 0
>         fwheads: 0
>
> And is partitioned in the guest as follows:
>
>      root at vm0:~ # gpart show -p
>      =>      34  20971453    vtbd0  GPT  (10G)
>              34      1024  vtbd0p1  freebsd-boot  (512K)
>            1058  19919872  vtbd0p2  freebsd-ufs  (9.5G)
>        19920930   1048576  vtbd0p3  freebsd-swap  (512M)
>        20969506      1981           - free -  (991K)
>
> What I am trying to work out is whether there is any way I can mount the guest
> UFS partition on the host with volmode=guest - with volmode=default (ie. geom
There is no volmode=guest, where did you get that from?
> when vfs.zfs.vol.mode=1) the device does show up as a geom provider and you can
> just mount the partition from /dev/vol directly. The ZFS man page suggests that
> you can’t but given that you can clearly mount from within a VM was wondering
> is there is any way round this on the host (I am trying to clone a disk device
> to run multiple bhyve instances but want to mount and modify some of the
> rc.conf parameters before passing to bhyve)
>
>       volmode=default | geom | dev | none
>           This property specifies how volumes should be exposed to the OS.
>           Setting it to geom exposes volumes as geom(4) providers, providing
>           maximal functionality.  Setting it to dev exposes volumes only as
>           cdev device in devfs.  Such volumes can be accessed only as raw disk
>           device files, i.e. they can not be partitioned, mounted, participate
>           in RAIDs, etc, but they are faster, and in some use scenarios with
>           untrusted consumer, such as NAS or VM storage, can be more safe.
>           Volumes with property set to none are not exposed outside ZFS, but
>           can be snapshoted, cloned, replicated, etc, that can be suitable for
>           backup purposes.  Value default means that volumes exposition is con-
>           trolled by system-wide sysctl/tunable vfs.zfs.vol.mode, where geom,
>           dev and none are encoded as 1, 2 and 3 respectively.  The default
>           values is geom.  This property can be changed any time, but so far it
>           is processed only during volume creation and pool import.
>
> Using volmode=default (geom - vfs.zfs.vol.mode=1) causes the installer to fail
> when you try to create a UFS filesystem under bhyve - it is possible to get
> round this by creating the partitions manually but my preference would be to
> use volmode=dev.
What error do you get?

     Regards
     Steve