How to remove a host from a Ceph cluster?


I’m still studying Ceph, and recently faced a scenario in which one of my Ceph nodes went down due to hardware failure. Even though my data was safe due to the replication factor, I was not able to remove the node from the cluster.

I could remove the OSDs on the node, but I didn’t find a way to remove the node being listed in ‘ceph osd tree’. I ended up editing the CRUSH map by hand, to remove the host, and uploaded it back. This worked as expected. Following are the steps I did to achieve this.

a) This was the state just after the node went down:


# ceph osd tree

# id     weight    type     name                up/down        reweight
 -10        .08997    root     default
 -20        .01999            host hp-m300-5
 00        .009995            osd.0                up             1
 40        .009995            osd.4                up             1
 -30        .009995            host hp-m300-9
 10        .009995            osd.1                 down         0
 -40        .05998            host hp-m300-4
 20        .04999            osd.2                up             1
 30        .009995            osd.3                up             1


# ceph -w

    cluster 62a6a880-fb65-490c-bc98-d689b4d1a3cb
     health HEALTH_WARN 64 pgs degraded; 64 pgs stuck unclean; recovery 261/785 objects degraded (33.248%)
     monmap e1: 1 mons at {hp-m300-4=10.65.200.88:6789/0}, election epoch 1, quorum 0 hp-m300-4
     osdmap e130: 5 osds: 4 up, 4 in
     pgmap v8465: 196 pgs, 4 pools, 1001 MB data, 262 objects
         7672 MB used, 74192 MB / 81865 MB avail
         261/785 objects degraded (33.248%)
         64 active+degraded
         132 active+clean
 

I started with marking the OSDs on the node out, and removing them. Note that I don’t need to stop the OSD (osd.1) since the node carrying osd.1 is down and not accessible.

b) If not, you would’ve to stop the OSD using:

 # sudo service osd stop osd.1
 

c) Mark the OSD out, this is not ideally needed in this case since the node is already out.

 # ceph osd out osd.1
 

d) Remove the OSD from the CRUSH map, so that it does not receive any data. You can also get the crushmap, de-compile it, remove the OSD, re-compile, and upload it back.

Remove item id 1 with the name ‘osd.1’ from the CRUSH map.

 # ceph osd crush remove osd.1
 

e) Remove the OSD authentication key

 # ceph auth del osd.1
 

f) At this stage, I had to remove the OSD host from the listing but was not able to find a way to do so. The ‘ceph-deploy’ didn’t have any tools to do this, other than ‘purge’, and ‘uninstall’. Since the node was not f) accessible, these won’t work anyways. A ‘ceph-deploy purge’ failed with the following errors, which is expected since the node is not accessible.

 # ceph-deploy purge hp-m300-9

[ceph_deploy.conf][DEBUG ] found configuration file at: /root/.cephdeploy.conf
 [ceph_deploy.cli][INFO  ] Invoked (1.5.22-rc1): /usr/bin/ceph-deploy purge hp-m300-9
 [ceph_deploy.install][INFO  ] note that some dependencies *will not* be removed because they can cause issues with qemu-kvm
 [ceph_deploy.install][INFO  ] like: librbd1 and librados2
 [ceph_deploy.install][DEBUG ] Purging from cluster ceph hosts hp-m300-9
 [ceph_deploy.install][DEBUG ] Detecting platform for host hp-m300-9 ...
 ssh: connect to host hp-m300-9 port 22: No route to host
 [ceph_deploy][ERROR ] RuntimeError: connecting to host: hp-m300-9 resulted in errors: HostNotFound hp-m300-9

I ended up fetching the CRUSH map, removing the OSD host from it, and uploading it back.

g) Get the CRUSH map

 # ceph osd getcrushmap -o /tmp/crushmap
 

h) De-compile the CRUSH map

 # crushtool -d /tmp/crushmap -o crush_map
 

i) I had to remove the entries pertaining to the host-to-be-removed from the following sections:

a) devices
b) types
c) And from the ‘root’ default section as well.

j) Once I had the entries removed, I went ahead compiling the map, and inserted it back.

 # crushtool -c crush_map -o /tmp/crushmap
 # ceph osd setcrushmap -i /tmp/crushmap
 

k) A ‘ceph osd tree’ looks much cleaner now 🙂

 # ceph osd tree

# id         weight             type         name                up/down        reweight
 -1             0.07999            root         default
 -2            0.01999                        host hp-m300-5
 0            0.009995                    osd.0                down        0
 4            0.009995                    osd.4                 down         0
 -4            0.06                        host hp-m300-4
 2            0.04999                        osd.2                 up             1
 3            0.009995                    osd.3                 up             1
 

There may be a more direct method to remove the OSD host from the listing. I’m not aware of anything relevant, based on my limited knowledge. Perhaps I’ll come across something as I progress with Ceph. Comments welcome.

5 thoughts on “How to remove a host from a Ceph cluster?

    1. Zeus,
      This was a typo when I wrote down the steps I tested. Sorry for that, and thank you for pointing it out.

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