Comparing two Ceph CRUSH maps

Sometimes you want to test if changes you are about to make to a CRUSH map will cause data to move or not.

In this case I wanted to change a rule in CRUSH where it would use device classes, but I didn’t want any of the ~1PB of data in that cluster to move.

By swapping IDs I could prevent data to move:

root default {
id -50 # do not change unnecessarily
id -53 class hdd # do not change unnecessarily
id -122 class ssd # do not change unnecessarily
root default {
id -53 # do not change unnecessarily
id -50 class hdd # do not change unnecessarily
id -122 class ssd # do not change unnecessarily

Notice how I swapped the IDs. After this I updated the rule:

rule rgw {
id 6
type replicated
min_size 1
max_size 10
step take ams02-objects class hdd
step chooseleaf firstn 0 type host
step emit
}

I then compiled the CRUSHMap and ran crushtool to see if there were any differences:

root@mon01:~# crushtool -i crushmap --compare crushmap.new 
rule 0 had 0/10240 mismatched mappings (0)
rule 1 had 0/10240 mismatched mappings (0)
rule 2 had 0/10240 mismatched mappings (0)
rule 3 had 0/10240 mismatched mappings (0)
rule 4 had 0/10240 mismatched mappings (0)
rule 5 had 0/3072 mismatched mappings (0)
rule 6 had 0/10240 mismatched mappings (0)
maps appear equivalent
root@mon01:~#

No changes! So it was safe to inject this map:

root@mon01:~# ceph osd setcrushmap -i crushmap.new

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Wido den Hollander

I am the CTO at PCextreme B.V. and i will keep this blog just to post some interesting information about my daily work on Linux (and other) systems.