HOWTO test a Ceph crush rule [feedly]

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HOWTO test a Ceph crush rule
// Ceph

The crushtool utility can be used to test Ceph crush rules before applying them to a cluster.

$ crushtool --outfn crushmap --build --num_osds 10 \     host straw 2 rack straw 2 default straw 0  # id	weight	type name	reweight  -9	10	default default  -6	4		rack rack0  -1	2			host host0  0	1				osd.0	1  1	1				osd.1	1  -2	2			host host1  2	1				osd.2	1  3	1				osd.3	1  -7	4		rack rack1  -3	2			host host2  4	1				osd.4	1  5	1				osd.5	1  -4	2			host host3  6	1				osd.6	1  7	1				osd.7	1  -8	2		rack rack2  -5	2			host host4  8	1				osd.8	1  9	1				osd.9	1	  

Creates a crushmap from scratch (–build). It assumes there is a total of 10 OSDs available ( –num_osds 10 ). It then places two OSDs in each host ( host straw 2 ). The resulting hosts (five of them) are then placed in racks, at most two per racks ( rack straw 2 ). All racks are placed in the default root (that's what the zero stands for : all of them) ( default straw 0 ). The last rack only has one host because there is an odd number of hosts available.
The crush rule to be tested can be injected in the crushmap with

crushtool --outfn crushmap --build --num_osds 10 host straw 2 rack straw 2 default straw 0  crushtool -d crushmap -o crushmap.txt  cat >> crushmap.txt <<EOF  rule myrule {  	ruleset 1  	type replicated  	min_size 1  	max_size 10  	step take default  	step choose firstn 2 type rack  	step chooseleaf firstn 2 type host  	step emit  }  EOF  crushtool -c crushmap.txt -o crushmap  

This crushmap should be able to provide two OSDs ( for placement groups for instance ) and it can be verified with the –test option.

$ crushtool -i crushmap --test --show-statistics --rule 1 --min-x 1 --max-x 2 --num-rep 2  rule 1 (myrule), x = 1..2, numrep = 2..2  CRUSH rule 1 x 1 [0,2]  CRUSH rule 1 x 2 [7,4]  rule 1 (myrule) num_rep 2 result size == 2:	2/2  

The –rule 1 designates the rule that was injected. The –rule 0 is the default rule that is created by default. The x can be thought of as the unique name of the placement group for which OSDs are reclaimed. The –min-x 1 –max-x 2 varies the value of x from 1 to 2 therefore trying the rule only twice. –min-x 1 –max-x 2048 would create 2048 lines. Each line shows the value of x after the rule number. In rule 1 x 2 the 1 is the rule number and the 2 is the value of x. The last line shows that for all values of x (2/2 i.e. 2 values of x out of 2), when asked to provide 2 OSDs (num_rep 2) the crush rule was able to provide 2 (result size == 2).

If asked for 4 OSDs, the same crush rule may fail because it has barely enough resources to satisfy the requirements.

$ crushtool -i crushmap --test --show-statistics --rule 1 --min-x 1 --max-x 2 --num-rep 4  rule 1 (myrule), x = 1..2, numrep = 4..4  CRUSH rule 1 x 1 [0,2,9]  CRUSH rule 1 x 2 [7,4,1,3]  rule 1 (myrule) num_rep 4 result size == 3:	1/2  rule 1 (myrule) num_rep 4 result size == 4:	1/2  

The statistics at the end shows that one of the two mappings failed: the result size == 3 is lower than the required number num_rep 4. If asked for more OSDs than the rule can provide, the rule will always fail.

crushtool -i crushmap --test --show-statistics --rule 1 --min-x 1 --max-x 2 --num-rep 5  rule 1 (myrule), x = 1..2, numrep = 5..5  CRUSH rule 1 x 1 [0,2,9]  CRUSH rule 1 x 2 [7,4,1,3]  rule 1 (myrule) num_rep 5 result size == 3:	1/2  rule 1 (myrule) num_rep 5 result size == 4:	1/2  

More examples of crushtool usage can be found in the crushtool directory of the Ceph sources.

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