Predicting which Ceph OSD will fill up first

When a device is added to Ceph, it is assigned a weight that reflects its capacity. For instance if osd.1 is a 1TB disk, its weight will be 1.0 and if osd.2 is a 4TB disk, its weight will be 4.0. It is expected that osd.1 will receive exactly four times more objects than osd.2. So that when osd.1 is 80% full, osd.2 is also 80% full.

But running a simulation on a crushmap with four 4TB disks and one 1TB disk, shows something different:

         WEIGHT     %USED
osd.4       1.0       86%
osd.3       4.0       81%
osd.2       4.0       79%
osd.1       4.0       79%
osd.0       4.0       78%

It happens when these devices are used in a two replica pool because the distribution of the second replica depends on the distribution of the first replica. If the pool only has one copy of each object, the distribution is as expected (there is a variation but it is around 0.2% in this case):

         WEIGHT     %USED
osd.4       1.0       80%
osd.3       4.0       80%
osd.2       4.0       80%
osd.1       4.0       80%
osd.0       4.0       80%

This variation is not new but there was no way to conveniently show it from the crushmap. It can now be displayed with crush analyze command. For instance:

    $ ceph osd crush dump > crushmap-ceph.json
    $ crush ceph --convert crushmap-ceph.json > crushmap.json
    $ crush analyze --rule replicated --crushmap crushmap.json
            ~id~  ~weight~  ~over/under used~
    g9       -22  2.299988     10.400604
    g3        -4  1.500000     10.126750
    g12      -28  4.000000      4.573330
    g10      -24  4.980988      1.955702
    g2        -3  5.199982      1.903230
    n7        -9  5.484985      1.259041
    g1        -2  5.880997      0.502741
    g11      -25  6.225967     -0.957755
    g8       -20  6.679993     -1.730727
    g5       -15  8.799988     -7.884220

shows that g9 will be ~90% full when g1 is ~80% full (i.e. 10.40 – 0.50 ~= 10% difference) and g5 is ~74% full.

By monitoring disk usage on g9 and adding more disk space to the cluster when the disks on g9 reach a reasonable threshold (like 85% or 90%), one can ensure that the cluster will never fill up, since it is known that g9 will always be the first node to become overfull. Another possibility is to run the ceph osd reweight-by-utilization command from time to time and try to even the distribution.
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