Calculate used precentage of multi device btrfs /

Hello,

I have some checks for btrfs usage that are failing. The problem lies in the / filesystem configuration which on those machines have 2 devices.

For example, the btrfs output for one is:

myhost $ btrfs filesystem sh --raw /
Label: none  uuid: 0cd6d969-fddf-4eba-92fd-8826a9a73376
        Total devices 2 FS bytes used 13946572800
        devid    1 size 15025045504 used 11143217152 path /dev/sdu2
        devid    2 size 11811160064 used 5939134464 path /dev/sdu3

The check uses this line for the calculation:

ocupatpercent=`sudo /usr/sbin/btrfs filesystem sh --raw $volum | awk '$1=="devid" {print int( 100-((($4-$6)/$4)*100) )}'`

which calculates de used percentage for each device so instead of one value:

myhost $ echo $ocupatpercent
74 50

My quick solution, replacing that one line in the script is this:

# saves total space of both devices ($4) in an array
total=( `sudo /usr/sbin/btrfs filesystem sh --raw $volum | awk '$1=="devid" {print $4}'` )
# saves used space of both devices  ($6) in an array
usage=( `sudo /usr/sbin/btrfs filesystem sh --raw $volum | awk '$1=="devid" {print $6}'` )
# sums each device total space
total_space=$(( ${total[0]}+${total[1]} ))
# sums each device used space
total_usage=$(( ${usage[0]}+${usage[1]} ))      
# calculates used percentage from the sum of both total and used space
ocupatpercent=$(( $total_usage*100/$total_space ))      

So basically, I sum the total space and the total used space for both devices and use that to calculated the used percentage of /.

So my first question is, is this the correct way to calculate used space for this setup of btrfs?

And, how can I improve the calculation (making it simpler and clearer)?

Regards,

Enrique