Hi,

I've hit a problem with a BTRFS filesystem I have on my main machine. It's gone 
into read-only mode, I suspect because it ran out of disk space when doing a 
rebalance (it's got v little space left). Looking at the logs I see the 
following for BTRFS:

Nov 15 09:09:35 selket kernel: BTRFS: Transaction aborted (error -28)
Nov 15 09:09:35 selket kernel: BTRFS: error (device dm-1: state A) in 
btrfs_finish_ordered_io:3315: errno=-28 No space left
Nov 15 09:09:35 selket kernel: BTRFS info (device dm-1: state EA): forced 
readonly

Ironically I was going to shift a large chunk of data off this filesystem onto 
an external backup disk but hadn't gotten around to doing it.

Digging around on the web I've seen suggestions involving unmounting the 
filesystem, remounting it R/W, removing any unwanted data, then continuing the 
rebalance. They generally seem to want to add another disk temporarily as well.

I don't really want to go to the trouble of temporarily adding a secondary disk 
only to then remove it again. As I was already planning on moving a bunch of 
old data off to external backup, I'm wondering if the following would be safe 
to do:

1) Unmount the filesystem.
2) Remount it as R/W
3) Move data to the external disk
4) Resume the rebalance operation (is this required?)

Does that look reasonable? Is there likely to be any problem with moving files 
off the filesystem when the rebalance operation failed?

Thanks for you attention,

Spencer

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