On 2018-10-29 02:11 PM, Ulli Horlacher wrote:
> I want to know how many free space is left and have problems in
> interpreting the output of: 
> 
> btrfs filesystem usage
> btrfs filesystem df
> btrfs filesystem show
> 
>

In my not so humble opinion, the filesystem usage command has the
easiest to understand output.  It' lays out all the pertinent information.

You can clearly see 825GiB is allocated, with 494GiB used, therefore,
filesystem show is actually using the "Allocated" value as "Used".
Allocated can be thought of "Reserved For".  As the output of the Usage
command and df command clearly show, you have almost 400GiB space available.

Note that the btrfs commands are clearly and explicitly displaying
values in Binary units, (Mi, and Gi prefix, respectively).  If you want
df command to match, use -h instead of -H (see man df)

An observation:

The disparity between 498GiB used and 823Gib is pretty high.  This is
probably the result of using an SSD with an older kernel.  If your
kernel is not very recent, (sorry, I forget where this was fixed,
somewhere around 4.14 or 4.15), then consider mounting with the nossd
option.  You can improve this by running a balance.

Something like:
btrfs balance start -dusage=55

You do *not* want to end up with all your space allocated to Data, but
not actually used by data.  Bad things can happen if you run out of
Unallocated space for more metadata. (not catastrophic, but awkward and
unexpected downtime that can be a little tricky to sort out.)


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