Access FFS partition on GPT on ZVOL

[Matthias Petermann <mp%petermann-it.de@localhost>]

Hello all,

here is a ZVOL that is used by a VM as backing storage. Consequently, 
this is provided with a GPT in which, among other things, an FFS 
partition is defined. I want to access this partition from the host side 
(i.e. from outside the VM).

The listing of the GPT works already :

```
saturn$ doas gpt show /dev/zvol/rdsk/tank/backup/vhost/vol/net-export
       start        size  index  contents
           0           1         PMBR
           1           1         Pri GPT header
           2          32         Pri GPT table
          34  2147483581      1  GPT part - NetBSD FFSv1/FFSv2
  2147483615          32         Sec GPT table
  2147483647           1         Sec GPT header`
```

My thought now was that I could create a wedge out of it, but no go:

```
saturn$ doas dkctl /dev/zvol/rdsk/tank/backup/vhost/vol/net-export 
addwedge myexport 34 2147483581 ffs
dkctl: /dev/zvol/rdsk/tank/backup/vhost/vol/net-export: addwedge: 
Inappropriate ioctl for device
```

I then read under [1] that ccd might help here, however I get a very 
similar error message:

```
saturn$ doas ccdconfig -cv ccd0 0 
/dev/zvol/rdsk/tank/backup/vhost/vol/net-export
ccdconfig: ioctl (CCDIOCSET): /dev/rccd0d: Block device required
saturn$ doas ccdconfig -cv ccd0 0 
/dev/zvol/dsk/tank/backup/vhost/vol/net-export
ccdconfig: ioctl (CCDIOCSET): /dev/rccd0d: Inappropriate ioctl for device
```

Am I doing something fundamentally wrong here? Or does this require a 
kernel newer than 9.3?

Kind regards
Matthias


[1] https://wiki.netbsd.org/zfs/

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