On 2018-02-23 06:21, Shyam Prasad N wrote:

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

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`.
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