The Schrodinger Ceph cluster

Working with asymetric Ceph nodes in small clusters

Posted by Maxime on April 5, 2017 in ceph

Inspired by Schrodinger’s famous thought experiment, this is the the story of a Ceph cluster that was both full and empty until reality kicked in.

The cluster

Let’s imagine a small 3 servers cluster. All servers are identical and contain 10x 2 TB hard drives. All Ceph pools are triple replicated on three different hosts.

The total capacity of the cluster is 60 TB raw and 20 TB usable. All is well.

The increase

After some time you need more capacity and decide to add a new node, but this time with 10x 6 TB drives. The anticipated capacity is then 120 TB raw and 40 TB usable.

After the installation ceph status reports the expected raw capacity of 120 TB.

 health HEALTH_OK
[...]
 osdmap e10: 40 osds: 40 up, 40 in
  pgmap v20: 256 pgs, 2 pools, 30 TB data, 1000 objects
        30 TB used, 90 TB / 120 TB avail

The problem

The problem with this setup is that the usable capacity is lower than expected. If you would try to fill the pool with data you would notice that the maximum usable capacity of this cluster is 30TB, that’s 10 TB lower than anticipated: simply because of triple replication.

The table below shows the space usage when you try to to fill this cluster.

Server Disks % of Total Used  Available
ceph1 10x2TB 16.7% 20 TB 0 TB
ceph2 10x2TB 16.7% 20 TB 0 TB
ceph3 10x2TB 16.7% 20 TB 0 TB
ceph4 10x6TB 50% 30 TB 30 TB

In this situation you see that the last node (ceph4) still has 30 TB raw available: that’s the missing 10 TB usable. Since all other nodes (ceph1-3) are full, there is nowhere to store the additional copies required by the storage policy (3 copies on 3 different hosts), so that space is not usable for a triple replication pool.

The solutions

This is happening when using replication with N copies (N=3 in this example) and a single node is responsible for more than 1/N (33% in this example) of the overall cluster capacity.

Theoretical solution

Some would say:

Just use N=1, then you would be guaranteed that no node will be responsible for more than 100% of the cluster capacity

That’s right, but in practice that would disable the data protection (1 copy = no replication), so definitly not what we want.

Equilibrate the cluster

The most practical way to address this is to equilibrate the servers. We can swap half of the 2TB drives in ceph3 with half of the 6 TB drives in ceph4.

Server 2 TB drive  6 TB drive % of Total
ceph1 10 0 16.7%
ceph2 10 0 16.7%
ceph3 5 5 33%
ceph4 5 5 33%

I recommend you to first update the CRUSHmap, this will start the data re-balancing and will make the new space available quickly, but will relax your storage policy (3 copies on 2 different servers) until you also swap the disks physically.

# Move 5x2TB OSDs to ceph4
for i in {20..25}
do
  ceph osd crush set osd.$i 2.0 root=default host=ceph4
done
# Move 5x6TB OSDs to ceph3
for i in {30..35}
do
  ceph osd crush set osd.$i 6.0 root=default host=ceph3
done