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
