The Notes

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====== BTRFS snapshot size ======
  * First you need to enable quota
<code bash>
sudo btrfs quota enable /home
</code>
  * You need to wait for BTRFS so scan the filesystem for quota usage, then
<code bash>
sudo btrfs qgroup show /home
</code>
  * You get output which looks like this:
<code>
qgroupid         rfer         excl
--------         ----         ----
0/5         142.21GiB     89.70MiB
0/410        96.00KiB     96.00KiB
0/2191       17.62GiB        0.00B
0/3237      137.90GiB      4.05GiB
</code>
''qgroupid'' is the snapshot ID, ''rfer'' is the total size of that snapshot (the size of the parent subvolumes are included), ''excl'' is the exclusive space that particular snapshot occupies. 

===== Deleting snapshots =====


  * You can list all the subvolume by running this: 
<code bash>
sudo btrfs subvolume list /home
</code>
Note that this will display the link between the ''BTRFS_SNAPSHOT_ID'' and ''SNAPPER_SNAPSHOT_ID''.

  * Note that if you are using snapper, you should delete snapshots using:
<code bash>
snapper -c home delete $SNAPPER_ID
</code>
You shouldn't delete the snapshot using ''btrfs subvolume delete'', as this will confuse Snapper. 

===== Re-initialise qgroups =====
If your filesystem has just suffered an episode of catastrophic failure, you might need to re-initialise things. A Snapper rollback can cause it. 
<code bash>
for i in $(btrfs qgroup show /home | tail -n+3 | cut -d ' ' -f 1); do btrfs qgroup destroy $i /home; done
btrfs quota disable /home
btrfs quota enable /home
</code>