Re: "relaxed" `-hda file.qcow2` equivalent ?

[lacsaP Patatetom]

Le jeu. 14 nov. 2024 à 17:00, Frantisek Rysanek <Frantisek.Rysanek@post.cz> a écrit :

> my image `fie.qcow2` is mounted elsewhere (nbd) and running
> `qemu-system-x86_64 -snapshot -hda file.qcow2` fails with the error
> "qemu-system-x86_64: -hda usb1.disk: Failed to get shared “write”
> lock / Is another process using the image [usb1.disk] ?".

Oh...

So you want concurrent access to the block device.
1) have it mounted on the host=HV using qemu-nbd
2) while at the same time, you want to give it to a guest VM?

Unless you use a filesystem such as GFS2 or OCFS2 inside the
partitions that exist inside that QCOW2 black box, or mount it
read-only at both ends, you are asking for trouble :-)
That is, if QEMU permits unlocked access by a VM, parallel to the
qemu-nbd loop-mount.

Note that this is just a tangential pointer on my part. Just a start
of a further journey. Further reading and study.

If you really want concurrent access to a directory tree, consider
using something like Samba or NFS as the simpler options.

Then again, concurrent access is definitely fun to play with and grow
:-)

Frank

my problem is exactly this, the link between `device`, `drive` and `blockdev` : while `device` and `drive` link quite easily, I can't link `blockdev` to the other two...

setting aside the fact that the image is used elsewhere, my aim is to mimic `-hda file.qcow2` but without taking the lock into account (`locking=off`).

I currently do this:

```

# qemu-nbd -c /dev/nbd0 file.qcow2

# mount /dev/nbd0p1 /mnt -o uid=me

$ # do some changes in /mnt/...

# umount /mnt

# qemu-nbd -d /dev/nbd0

$ qemu-system-x86_64 -snapshot -hda file.qcow2

# qemu-nbd -c /dev/nbd0 file.qcow2

# mount /dev/nbd0p1 /mnt -o uid=me

$ # do some changes in /mnt/...

# umount /mnt

# qemu-nbd -d /dev/nbd0

$ qemu-system-x86_64 -snapshot -hda file.qcow2

# qemu-nbd -c /dev/nbd0 file.qcow2

# mount /dev/nbd0p1 /mnt -o uid=me

$ # do some changes in /mnt/...

etc...

```

and I'd like to be able to do this :

```

# qemu-nbd -c /dev/nbd0 file.qcow2

# mount /dev/nbd0p1 /mnt -o uid=me

$ # do some changes in /mnt/...

$ qemu-system-x86_64 -snapshot -hda file.qcow2 # with locking=off

$ # do some changes in /mnt/...

$ qemu-system-x86_64 -snapshot -hda file.qcow2 # with locking=off

$ # do some changes in /mnt/...

etc...

```