Ceph cluster shows following weird behavior with ceph df output:
--- RAW STORAGE ---
CLASS SIZE AVAIL USED RAW USED %RAW USED
hdd 817 TiB 399 TiB 418 TiB 418 TiB 51.21
ssd 1.4 TiB 1.2 TiB 22 GiB 174 GiB 12.17
TOTAL 818 TiB 400 TiB 418 TiB 419 TiB 51.15
--- POOLS ---
POOL ID PGS STORED OBJECTS USED %USED MAX AVAIL
pool1 45 300 21 TiB 6.95M 65 TiB 20.23 85 TiB
pool2 50 50 72 GiB 289.15k 357 GiB 0.14 85 TiB
pool3 53 64 2.9 TiB 754.06k 8.6 TiB 3.24 85 TiB
erasurepool_data 57 1024 138 TiB 50.81M 241 TiB 48.49 154 TiB
erasurepool_metadata 58 8 9.1 GiB 1.68M 27 GiB 2.46 362 GiB
device_health_metrics 59 1 22 MiB 163 66 MiB 0 85 TiB
.rgw.root 60 8 5.6 KiB 17 3.5 MiB 0 85 TiB
.rgw.log 61 8 70 MiB 2.56k 254 MiB 0 85 TiB
.rgw.control 62 8 0 B 8 0 B 0 85 TiB
.rgw.meta 63 8 7.6 MiB 52 32 MiB 0 85 TiB
.rgw.buckets.index 64 8 11 GiB 1.69k 34 GiB 3.01 362 GiB
.rgw.buckets.data 65 512 23 TiB 33.87M 72 TiB 21.94 85 TiB
As seen above available storage 399TiB, and max avail in pool list shows 85TiB. I use 3 replicas for each pool replicated pool and 3+2 erasure code for the erasurepool_data.
As far as I know Max Avail segment shows max raw available capacity according to replica size. So it comes up to 85*3=255TiB. Meanwhile cluster shows almost 400 available.
Which to trust? Is this only a bug?
MAX AVAIL column represents the amount of data that can be used before the first OSD becomes full. It takes into account the projected distribution of data across disks from the CRUSH map and uses the 'first OSD to fill up' as the target. it does not seem to be a bug. If MAX AVAIL is not what you expect it to be, look at the data distribution using ceph osd tree and make sure you have a uniform distribution.
You can also check some helpful posts here that explains some of the miscalculations:
As you have Erasure Coding involved please check this SO post: