[Users] ploop and trim/ discard support

[Corin Langosch]

Hi,

it seems that when creating & deleting files inside a ploop backed 
container the backing image file doesn't shrink. So the image file is 
still consuming around 400 GB, while in the container df shows only 100 
GB in use.

ll on host: -rw------- 1 root root 384G Sep 12 21:06 root.hdd
df on container: /dev/ploop1           493G  103G  390G  21% /

I assume this is because ploop doesn't get informed by the ext4 fs when 
blocks are no longer in use. Should it help to to mount the ext4 on the 
ploop device with the "discard" option? I didn't test if it helps yet, 
but at least I was able to pass the mount option (thanks to kir's recent 
enhancements) and it started the container without any problems.

/dev/ploop1 on / type ext4 (rw,relatime,barrier=0,data=ordered,discard)

To free all "free" existing blocks I also tried to do a batch cleanup/ 
trim from inside the container using "fstrim /" but it doesn't work:

root at test:~# fstrim /
fstrim: /: FITRIM ioctl failed: Operation not permitted

Should this be working or is it not supported yet? Am I on the right 
path at all? Any better solutions? :)

Corin