This document is for a development version of Ceph.
In Luminous v12.2.z and later releases, there is a pg-upmap exception table in the OSDMap that allows the cluster to explicitly map specific PGs to specific OSDs. This allows the cluster to fine-tune the data distribution to, in most cases, uniformly distribute PGs across OSDs.
However, there is an important caveat when it comes to this new feature: it requires all clients to understand the new pg-upmap structure in the OSDMap.
In order to use
pg-upmap, the cluster cannot have any pre-Luminous clients.
By default, new clusters enable the balancer module, which makes use of
pg-upmap. If you want to use a different balancer or you want to make your
pg-upmap entries, you might want to turn off the balancer in
order to avoid conflict:
ceph balancer off
To allow use of the new feature on an existing cluster, you must restrict the cluster to supporting only Luminous (and newer) clients. To do so, run the following command:
ceph osd set-require-min-compat-client luminous
This command will fail if any pre-Luminous clients or daemons are connected to the monitors. To see which client versions are in use, run the following command:
The balancer module for
ceph-mgr will automatically balance the number of
PGs per OSD. See Balancer Module
Upmap entries are updated with an offline optimizer that is built into the osdmaptool -- ceph osd cluster map manipulation tool.
Grab the latest copy of your osdmap:
ceph osd getmap -o om
Run the optimizer:
osdmaptool om --upmap out.txt [--upmap-pool <pool>] \ [--upmap-max <max-optimizations>] \ [--upmap-deviation <max-deviation>] \ [--upmap-active]
It is highly recommended that optimization be done for each pool individually, or for sets of similarly utilized pools. You can specify the
--upmap-pooloption multiple times. “Similarly utilized pools” means pools that are mapped to the same devices and that store the same kind of data (for example, RBD image pools are considered to be similarly utilized; an RGW index pool and an RGW data pool are not considered to be similarly utilized).
max-optimizationsvalue determines the maximum number of upmap entries to identify. The default is 10 (as is the case with the
ceph-mgrbalancer module), but you should use a larger number if you are doing offline optimization. If it cannot find any additional changes to make (that is, if the pool distribution is perfect), it will stop early.
max-deviationvalue defaults to 5. If an OSD’s PG count varies from the computed target number by no more than this amount it will be considered perfect.
--upmap-activeoption simulates the behavior of the active balancer in upmap mode. It keeps cycling until the OSDs are balanced and reports how many rounds have occurred and how long each round takes. The elapsed time for rounds indicates the CPU load that
ceph-mgrconsumes when it computes the next optimization plan.
Apply the changes:
In the above example, the proposed changes are written to the output file
out.txt. The commands in this procedure are normal Ceph CLI commands that can be run in order to apply the changes to the cluster.
The above steps can be repeated as many times as necessary to achieve a perfect distribution of PGs for each set of pools.
To see some (gory) details about what the tool is doing, you can pass
--debug-osd 10 to
osdmaptool. To see even more details, pass
--debug-crush 10 to