Ceph - ceph-mgr dashboard does not match 'ceph df' output

Solution Verified - Updated -

Environment

  • Red Hat Ceph Storage 3.x

Issue

  • There is a difference in "Usage" for the ceph-mgr dashboard when compared to the "Used" & "MAX AVAIL" columns from the 'ceph df' output.

  • Information From ceph-mgr Dashboard:

Pools
Name    PG status   Usage   Activity
volumes 128 active+clean    0 / 32.1G   0 rd, 0 wr
images  128 active+clean    0 / 32.1G   0 rd, 0 wr
backups 128 active+clean    0 / 32.1G   0 rd, 0 wr
vms 128 active+clean    0 / 32.1G   0 rd, 0 wr
rbd 32 active+clean       2.76k / 32.1G 204 rd, 0 wr
  • From 'ceph df' output:
user@hostname ~]# ceph df
GLOBAL:
    SIZE       AVAIL      RAW USED     %RAW USED 
    98705M     97489M        1216M          1.23 
POOLS:
    NAME        ID     USED     %USED     MAX AVAIL     OBJECTS 
    volumes     19        0         0        30673M           0 
    images      20        0         0        30673M           0 
    backups     21        0         0        30673M           0 
    vms         22        0         0        30673M           0 
    rbd         24     2760         0        30673M          12

Resolution

  • This is currently being tracked by Red Hat downstream BZ 1611669.
  • If you hitting this bug and need more information please contact Red Hat support.

Root Cause

  • The difference in the 'ceph df' output and the ceph-mgr dashboard is a difference in rounding & calculation method used.
  • In summary, 'ceph df' uses binary math, where as the dashboard is using decimal math. The more accurate of the two is the 'ceph df' output because of the binary math.
  • The larger the cluster size, the more inflated this difference will be.

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