ceph df (octopus) shows USED is 7 times higher than STORED in erasure coded pool

Question

The pool default.rgw.buckets.data has 501 GiB stored, but USED shows 3.5 TiB.

root@ceph-01:~# ceph df
--- RAW STORAGE ---
CLASS  SIZE     AVAIL    USED     RAW USED  %RAW USED
hdd    196 TiB  193 TiB  3.5 TiB   3.6 TiB       1.85
TOTAL  196 TiB  193 TiB  3.5 TiB   3.6 TiB       1.85

--- POOLS ---
POOL                       ID  PGS  STORED   OBJECTS  USED     %USED  MAX AVAIL
device_health_metrics       1    1   19 KiB       12   56 KiB      0     61 TiB
.rgw.root                   2   32  2.6 KiB        6  1.1 MiB      0     61 TiB
default.rgw.log             3   32  168 KiB      210   13 MiB      0     61 TiB
default.rgw.control         4   32      0 B        8      0 B      0     61 TiB
default.rgw.meta            5    8  4.8 KiB       11  1.9 MiB      0     61 TiB
default.rgw.buckets.index   6    8  1.6 GiB      211  4.7 GiB      0     61 TiB

default.rgw.buckets.data   10  128  501 GiB    5.36M  3.5 TiB   1.90    110 TiB

The default.rgw.buckets.data pool is using erasure coding:

root@ceph-01:~# ceph osd erasure-code-profile get EC_RGW_HOST
crush-device-class=hdd
crush-failure-domain=host
crush-root=default
jerasure-per-chunk-alignment=false
k=6
m=4
plugin=jerasure
technique=reed_sol_van
w=8

If anyone could help explain why it's using up 7 times more space, it would help a lot. Versioning is disabled. ceph version 15.2.13 (octopus stable).

Answer 1

This is related to bluestore_min_alloc_size_hdd=64K (default on Octopus).

If using Erasure Coding, data is broken up into smaller chunks, which each take 64K on disk.

One option would be to lower bluestore_min_alloc_size_hdd to 4K, which is good if your solution requires storing millions of tiny (16K) objects. In my case, I'm storing hundreds of millions of 3-4M photos, so I decide to skip Erasure Coding, stay on bluestore_min_alloc_size_hdd=64K and switch to replicated 3 (min 2). Which is much safer and faster in the long run.

Here is the reply from Josh Baergen on the mailing list:

Hey Arkadiy,

If the OSDs are on HDDs and were created with the default
bluestore_min_alloc_size_hdd, which is still 64KiB in Octopus, then in
effect data will be allocated from the pool in 640KiB chunks (64KiB *
(k+m)). 5.36M objects taking up 501GiB is an average object size of 98KiB
which results in a ratio of 6.53:1 allocated:stored, which is pretty close
to the 7:1 observed.

If my assumption about your configuration is correct, then the only way to
fix this is to adjust bluestore_min_alloc_size_hdd and recreate all your
OSDs, which will take a while...

Josh