Bezüglich Harry Schmalzbauer's Nachricht vom 15.10.2017 11:57 (localtime):
>  Bezüglich Harry Schmalzbauer's Nachricht vom 15.10.2017 11:33 (localtime):
>> 3.) Modify existing volmode=dev dataset and write new GPT
>> zfs set volmode=geom
>> hostPsys/bhyveVOL/sys/test                                                   
>> zfs get volmode
>> hostPsys/bhyveVOL/sys/test                                                   
>> NAME                         PROPERTY  VALUE   
>> SOURCE                                                                       
>> hostPsys/bhyveVOL/sys/test  volmode   geom     local
>> gpart create -s gpt
>> /dev/zvol/hostPsys/bhyveVOL/sys/test                                         
>> gpart: arg0 'zvol/hostPsys/bhyveVOL/sys/test': Invalid argument
>> (fails unexpected)
>> What can I do to let geom(4) know that there's a new device?
> Device should read provider.
> I found that last sentence in the zfs(8) man page, describing the
> volmode property:
> »This property can be changed any time, but
> so far it is processed only during volume creation and pool import.«
> So it seems to be a limitation by design.
> re-importing the pool is no option for me, so I'll keep in mind that
> changing volmode means outage.
> I'm aware that I can utilize ctl(8) to get access to a volmode=dev
> volume, also md(4) might help in that case, but for changing
> volmode=geom a reboot / re-import was required.

I'ts not correct that md(4) could help here since ms(4) can't use
character devices as vnode backend.
Just to correct myself.

> I guess the benefit of extending the implementation design is much to
> small to justify the effort.
> But I think making "volmode" a creation-only property (like utf8only)
> should be considered.

On the other hand, a reboot/re-import might be preferably over zfs
send|recv, which would be the only way in that case and which was a
regression; better to not consider making it creation-only...

I'd like to share two workarounds:
The first makes it possible to easily create backups from (bhyve(8))
guests which have ZVOLs as ahci/virtio-block backend.
Like described, changing "volmode" property for the corresponding volume
dataset from "dev" to "geom" doesn't help without reboot/re-import.
But you can clone:

zfs snapshot hostPsys/bhyveVOL/sys/guest@offline
zfs clone -o volmode=geom hostPsys/bhyveVOL/sys/guest@offline
glabel status
gpt/guestSWAP N/A zvol/hostPsys/bhyveVOL/sys/guest.gcp3
gpt/guestBOOT N/A zvol/hostPsys/bhyveVOL/sys/guest.gcp4
gpt/guestSAFE N/A zvol/hostPsys/bhyveVOL/sys/guest.gcp5
gpt/guestVAR N/A zvol/hostPsys/bhyveVOL/sys/guest.gcp6
gpt/guestLOCAL N/A zvol/hostPsys/bhyveVOL/sys/guest.gcp7
gpt/guestDATA N/A zvol/hostPsys/bhyveVOL/sys/guest.gcp8
gpt/guestJbase N/A zvol/hostPsys/bhyveVOL/sys/guest.gcp9
gpt/guestENTITIES N/A zvol/hostPsys/bhyveVOL/sys/guest.gcp10

After you did what ever you need, simply destroy the snapshot and it's

zfs destroy -R hostPsys/bhyveVOL/sys/guest@offline

This is just a way to work around the volmode limitation for one special
case (reading data from the guest's volume).

If you need to access the guest volume for other things than
non-destructive ones (writing data), you have to go the ctl(4) way.
Of course, also for read-only tasks ctl(4) would be appropriate, but
since I'm more used to `zfs` than to `ctladm`, the former is easier for me.
The latter has more dynamics.
Here's the ctl(4) example:

ctladm create -b block -d guest-zvol -o
ctladm port -p `ctladm port -l | grep "camsim.*naa" | cut -w -f 1` -o on
geom disk list
Geom name: da11
1. Name: da11
Mediasize: 8589934592 (8.0G)
Sectorsize: 512
Stripesize: 8192
Stripeoffset: 0
Mode: r0w0e0
lunname: FREEBSD guest-zvol
lunid: FREEBSD guest-zvol
ident: MYSERIAL 0
rotationrate: 0
fwsectors: 63
fwheads: 255
glabel status
gpt/guestSWAP N/A da11p3
gpt/guestBOOT N/A da11p4
gpt/guestSAFE N/A da11p5
gpt/guestVAR N/A da11p6
gpt/guestLOCAL N/A da11p7
gpt/guestDATA N/A da11p8
gpt/guestJbase N/A da11p9
gpt/guestENTITIES N/A da11p10

ctladm devlist and ctladm portlist give more info.

After you did what ever needed, switch camsim off and remove target:

ctladm port -p `ctladm port -l | grep "camsim.*naa" | cut -w -f 1` -o off
ctladm remove -b block -l `ctladm devl | grep "guest-zvol" | cut -w -f 2`


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