The CRUSH map holds a variety of information necessary for Ceph to distribute and place objects. Sometimes it is useful to find out if this distribution isn't working properly because of a wrong value in the CRUSH map. The CRUSH map can be downloaded as a file in a binary format that we decompile locally for viewing. It is also possible to dump the CRUSH map and rules directly from the command line, which is expedient for a quick check.

We can use osd crush dump to display the entire CRUSH map.

root@ceph-client0:~# ceph osd crush dump
{
"devices": [
{
"id": 0,
"name": "osd.0"
},
...
"rules": [
{
"rule_id": 0,
"rule_name": "replicated_ruleset",
"ruleset": 0,
...
"tunables": {
"choose_local_tries": 0,
"choose_local_fallback_tries": 0,
"choose_total_tries": 50,
"chooseleaf_descend_once": 1,
...

This might not always be useful depending on the information sought. A better way to dump individual rules is by using the osd crush rule dump subcommand.

We first list the CRUSH rules currently defined within our cluster:

root@ceph-client0:~# ceph osd crush rule ls
[
"replicated_ruleset"
]

Here we only have one rule, and it is named replicated_ruleset. Let's see what it does.

root@ceph-client0:~# ceph osd crush rule dump replicated_ruleset
{
"rule_id": 0,
"rule_name": "replicated_ruleset",
"ruleset": 0,
"type": 1,
"min_size": 1,
"max_size": 10,
"steps": [
{
"op": "take",
"item": -1,
"item_name": "default"
},
{
"op": "chooseleaf_firstn",
"num": 0,
"type": "host"
},
{
"op": "emit"
}
]
}

This rule is applied to replicated pools and distributes one copy of all objects to separate hosts.