Seed device questions

Seed device questions

[Jonas Bonn]

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

Re: Seed device questions

[Yan Zheng]

Jonas Bonn wrote:
> 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)
yes

> ii)  Should it be possible to have a device as a 'seed' for multiple
> other filesystems.
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.

> 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.
>
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