[ceph-users] Erasure coded calculation

Hello everyone! I'm trying to calculate the theoretical usable storage of a ceph cluster with erasure coded pools. I have 8 nodes and the profile for all data pools will be k=6 m=2. If every node has 6 x 1TB wouldn't the calculation be like this: RAW capacity: 8Nodes x 6Disks x 1TB = 48TB Loss to m=2: 48TB / 8Nodes x 2m = 12TB EC capacity: 48TB - 12TB = 36TB At the moment I have one cluster with 8 nodes and different disks than the sample (but every node has the same amount of disks and the same sized disks). The output of ceph df detail is: --- RAW STORAGE --- CLASS SIZE AVAIL USED RAW USED %RAW USED hdd 109 TiB 103 TiB 5.8 TiB 5.9 TiB 5.41 TOTAL 109 TiB 103 TiB 5.8 TiB 5.9 TiB 5.41 --- POOLS --- POOL ID PGS STORED OBJECTS %USED MAX AVAIL device_health_metrics 1 1 51 MiB 48 0 30 TiB rep_data_fs 2 32 14 KiB 3.41k 0 30 TiB rep_meta_fs 3 32 227 MiB 1.72k 0 30 TiB ec_bkp1 4 32 4.2 TiB 1.10M 6.11 67 TiB So ec_bkp1 uses 4.2TiB an there are 67TiB free usable Storage. This means total EC usable storage would be 71.2TiB. But calculating with the 109TiB RAW storage, shouldn't it be 81.75? Are the 10TiB just some overhead (that would be much overhead) or is the calculation not correct? And what If I want to expand the cluster in the first sample above by three nodes with 6 x 2TB, which means not the same sized disks as the others. Will the calculation with the same EC profile still be the same? RAW capacity: 8Nodes x 6Disks x 1TB + 3Nodes x 6Disks x 2TB = 84TB Loss to m=2: 84TB / 11Nodes x 2m = 15.27TB EC capacity: 84TB - 15.27TB = 68.72TB Thanks in advance, Simon