Re: Understanding "btrfs filesystem usage"

[Hugo Mills <hugo@carfax.org.uk>]

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On Mon, Oct 29, 2018 at 05:57:10PM -0400, Remi Gauvin wrote:
> 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.

   Opinions are divided. I find it almost impossible to read, and
always use btrfs fi df and btrfs fi show together.

   There's short tutorials of how to read the output in both cases in
the FAQ, which is where I start out by directing people in this
instance.

   Hugo.

> 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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