Re: Btrfs occupies more space than du reports...

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2018-02-23 06:21, Shyam Prasad N wrote:
> Hi,
> 
> Can someone explain me why there is a difference in the number of
> blocks reported by df and du commands below?
> 
> =====================
> # df -h /dc
> Filesystem      Size  Used Avail Use% Mounted on
> /dev/drbd1      746G  519G  225G  70% /dc
> 
> # btrfs filesystem df -h /dc/
> Data, single: total=518.01GiB, used=516.58GiB
> System, DUP: total=8.00MiB, used=80.00KiB
> Metadata, DUP: total=2.00GiB, used=1019.72MiB
> GlobalReserve, single: total=352.00MiB, used=0.00B
> 
> # du -sh /dc
> 467G    /dc
> =====================
> 
> df shows 519G is used. While recursive check using du shows only 467G.
> The filesystem doesn't contain any snapshots/extra subvolumes.
> Neither does it contain any mounted filesystem under /dc.
> I also considered that it could be a void left behind by one of the
> open FDs held by a process. So I rebooted the system. Still no
> changes.
> 
> The situation is even worse on a few other systems with similar configuration.
> 

At least part of this is a difference in how each tool computes space usage.

* `df` calls `statvfs` to get it's data, which tries to count physical 
allocation accounting for replication profiles.  In other words, data in 
chunks with the dup, raid1, and raid10 profiles gets counted twice, data 
in raid5 and raid6 chunks gets counted with a bit of extra space for the 
parity, etc.

* `btrfs fi df` looks directly at the filesystem itself and counts how 
much space is available to each chunk type in the `total` values and how 
much space is used in each chunk type in the `used` values, after 
replication.  If you add together the data used value and twice the 
system and metadata used values, you get the used value reported by 
regular `df` (well, close to it that is, `df` rounds at a lower 
precision than `btrfs fi df` does).

* `du` scans the directory tree and looks at the file allocation values 
returned form `stat` calls (or just looks at file sizes if you pass the 
`--apparent-size` flag to it).  Like `btrfs fi df`, it reports values 
after replication, it has a couple of nasty caveats on BTRFS, namely 
that it will report sizes for natively compressed files _before_ 
compression, and will count reflinked blocks once for each link.

Now, this doesn't explain the entirety of the discrepancy with `du`, but 
it should cover the whole difference between `df` and `btrfs fi df`.