Activating space_cache after read-only snapshots without space_cache have been taken

[Ochi <ochi@arcor.de>]

Hello everyone,

I've ran into problems with _very_ slow unmounting of my btrfs-formatted 
"backup" volume. I have a suspicion what might be the cause but maybe 
someone with more experience with the btrfs code could enlighten me 
whether it is actually correct.

The situation is the following: I have created a backup-volume to which 
I regularly rsync a backup of my system into a subvolume. After 
rsync'ing, I take a _read-only_ snapshot of that subvolume with a 
timestamp added to its name.

Now at the time I started using this backup volume, I was _not_ using 
the space_cache mount option and two read-only snapshots were taken 
during this time. Then I started using the space_cache option and 
continued doing snapshots.

A bit later, I started having very long lags when unmounting the backup 
volume (both during shutdown and when unmounting manually). I scrubbed 
and fsck'd the volume but this didn't show any errors. Defragmenting the 
root and subvolumes took a long time but didn't improve the situation much.

Now I started having the suspicion that maybe the space cache possibly 
couldn't be written to disk for the readonly subvolumes/snapshots that 
were created during the time when I wasn't using the space_cache option, 
forcing the cache to be rebuilt every time.

Clearing the cache didn't help. But when I deleted the two snapshots 
that I think were taken during the time without the mount option, the 
unmounting time seems to have improved considerably.

I will have to observe whether unmounting stays quick now but my 
question is whether it is possible that the read-only snapshots taken 
during the time when I wasn't using space_cache might actually have been 
the culprits.

Best,
Sebastian