Seed device questions

Hi,
I am trying the 'seed device' functionality, trying to get a read-only block-device to act as a seed for multiple other filesystems.

The following sequence works for setting up the first filesystem.

#-------------

#First initialize 'seed device'
mkfs.btrfs /dev/vdb
mount /dev/vdb /mnt
btrfs -S subvol /mnt
echo hello > /mnt/subvol/xxx
umount /mnt

#Make block device a "seed device" (read-only)
btrfstune -S 1 /dev/vdb

#Mount the seed device... it will be mounted read-only. mount /dev/vdb /mnt

#Add the writeable block device
btrfs-vol -a /dev/vdc /mnt

#Now remount the filesystem read-write.
mount -oremount,rw /mnt

#Now all changes to /mnt should go to /dev/vdc.

#--------------

After this, however, it's not possible to mount /dev/vdb again in order to set up a second filesystem as seed. I want to do:

mount /dev/vdb /othermnt
btrfs-vol -a /dev/vdd /othermnt
mount -oremount,rw /othermnt

So my questions (comments):

i) Is the seed-device _really_ read-only... no metadata updates or anything? (i.e. I can have my seed device on read-only media)
ii) Should it be possible to have a device as a 'seed' for multiple other filesystems.
iii) When the seeded filesystem is created, the size of the filesystem is the aggregate of all devices, whereas it should be the aggregate size of only the 'writable' devices.

/Jonas

yes

it's possible, but only one filesystem can be mounted at a time. I will add 'mounting multiple filesystems at the same time' support later.

Since btrfs mirrors metadata and creates chunk dynamically, how to compute the size of filesystem isn't clear. I'm afraid I can't answer this question.

Regards
Yan Zheng