OSD weights and space usage

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Ceph is a cool software but from time to time I am getting gray hairs
with it. And I hope that's because of a misunderstanding. This time I
want to balance the load between three osd's evenly (same usage %). Two
OSD are 2GB, one is 4GB (test environment). By the way: The pool is
erasure coded (k=2, m=1) and has a cache tier on top.

My osd tree is this:
# id    weight  type name       up/down reweight
-1      8       root default
-4      8               room serverroom
-2      8                       host test1
0       2                               osd.0   up      1
1       2                               osd.1   up      1
2       4                               osd.2   up      1

resulting in this strange usage (where testdisk3 is the only usage of
the pool):
Filesystem 1K-blocks   Used Available Use% Mounted on
/dev/sda1   2085868 1209344    876524  58% /mnt/osd.0
/dev/sdb1   2085868 1208256    877612  58% /mnt/osd.1
/dev/sdc1   4183020 1211712   2971308  29% /mnt/osd.2
/dev/rbd0   3562916 2044692   1317520  61% /mnt/testdisk3

First thing: My weights are silently ignored.

Second thing: The used space is ~2045MB (see above). Multiplicated with
(k+m=3) and divided by (k=2) it should use 3068MB on the cluster. But at
the moment it uses 3629MB. Beside the metadata of xfs (say 50MB for each
drive) and the cache pool (max. 100MB) there is a difference of 300MB.
Where does this come from?

Anybody an explanation for this?
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