btrfs send does not work from readonly device

btrfs send does not work from readonly device

[GEO]

Let's say we have a snapshot called "snapshot" on our device "/dev/sdb".
Now we boot a qemu machine and attatch the disk read only (since we do not
want to put our data at risk in the vm, and read only should be enough to
get our snapshot data).

I used

qemu-system-x86_64 -drive -file=/dev/sdb,if=scsi,readonly -hda
<path_to_guest_operating_system>

When we are in the qemu instance (suppose our device  /dev/sdb is mounted
on /mnt/import), we want to import our data now using btrfs send/receive:

btrfs send /mnt/import/snapshot | btrfs receive  /

we get the following problems in syslog due to the fact that our device is
read only:

delayed_refs has NO entry
BTRFS warning (device sdb): btrfs_update_root:151: Aborting unsused
transaction(No space left)
BTRFS warning (device sdb): Skipping commit of aborted transaction.
BTRFS warning (device sdb): cleanup_transaction:1547: Aborting unused
transaction (No space left)

Apart from the snapshot folder being created in / nothing happens, the
process simply hangs forever, with no actual data getting imported.

Is that supposed to be that way? Why is readonly not enough to import data
using btrfs send?

Re: btrfs send does not work from readonly device

[Duncan]

GEO posted on Tue, 23 Sep 2014 14:58:06 +0200 as excerpted:

> Is that supposed to be that way? Why is readonly not enough to import
> data using btrfs send?

This is a known issue.  The subvolume itself needs to be set read-only, 
and of course that can't be done when the whole filesystem is set read-
only.

But a read-only filesystem should be sufficient, provided it can be 
forced to remain read-only thru the entire send.  I think that's the 
worry, that someone will switch their system from ro to rw in the middle 
of a send, thus screwing it up.

But they're aware of the problem and should eventually have a fix.  But 
I'm not sure how long as there's other work going on too, of course.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

Re: btrfs send does not work from readonly device

[G EO]

The snapshots underneath are set read only anyway in my case, since
the they landed there by btrfs receive or I am thinking wrong here?
Duncan, do you remember the bug number?


Thanks!

Re: btrfs send does not work from readonly device

[Duncan]

On Thu, 25 Sep 2014 21:39:41 +0200
G EO <1g2e3o4@gmail.com> wrote:

> The snapshots underneath are set read only anyway in my case, since
> the they landed there by btrfs receive or I am thinking wrong here?
> Duncan, do you remember the bug number?

I don't.  I remember seeing the discussion on the list.  Without
googling my local list archive it seems to me it might have been in one
of Marc Merlin's threads.

-- 
Duncan - No HTML messages please, as they are filtered as spam.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman