自己搭的3个OSD节点的集群的健康状态经常处在”WARN”状态，replicas设置为3，OSD节点数量大于3，存放的data数量也不多，ceph -s 不是期待的health ok，而是active+undersized+degraded。被这个问题困扰有段时间，因为对Ceph不太了解而一直没有找到解决方案，直到最近发邮件到社区才得到解决[1]。
PG状态的含义
PG的非正常状态说明可以参考[2]，undersized与degraded的含义记录于此：
undersized
The placement group has fewer copies than the configured pool replication level.
degraded
Ceph has not replicated some objects in the placement group the correct number of times yet.
这两种状态一般同时出现，大概的意思就是有些PG没有满足设定的replicas数量要求，PG中的部分objects亦如此。看下PG的详细信息：
ceph health detail
HEALTH_WARN 2 pgs degraded; 2 pgs stuck degraded; 2 pgs stuck unclean; 2 pgs
stuck undersized; 2 pgs undersized
pg 17.58 is stuck unclean for 61033.947719, current state
active+undersized+degraded, last acting [2,0]
pg 17.16 is stuck unclean for 61033.948201, current state
active+undersized+degraded, last acting [0,2]
pg 17.58 is stuck undersized for 61033.343824, current state
active+undersized+degraded, last acting [2,0]
pg 17.16 is stuck undersized for 61033.327566, current state
active+undersized+degraded, last acting [0,2]
pg 17.58 is stuck degraded for 61033.343835, current state
active+undersized+degraded, last acting [2,0]
pg 17.16 is stuck degraded for 61033.327576, current state
active+undersized+degraded, last acting [0,2]
pg 17.16 is active+undersized+degraded, acting [0,2]
pg 17.58 is active+undersized+degraded, acting [2,0]
解决办法
虽然设定的拷贝数量是3，但是PG 17.58与17.58却只有两个拷贝，分别存放在OSD 0与OSD 2上。
而究其原因则是我们的OSD所在的磁盘不是同质的，从而每个OSD的weight不同，而Ceph对异质OSD的支持不是很好。从而导致部分PG无法满足我们设定的备份数量限制。
OSD状态树：
ceph osd tree
ID WEIGHT TYPE NAME UP/DOWN REWEIGHT PRIMARY-AFFINITY
-1 5.89049 root default
-2 1.81360 host ceph3
2 1.81360 osd.2 up 1.00000 1.00000
-3 0.44969 host ceph4
3 0.44969 osd.3 up 1.00000 1.00000
-4 3.62720 host ceph1
0 1.81360 osd.0 up 1.00000 1.00000
1 1.81360 osd.1 up 1.00000 1.00000
解决办法是另外构建一个OSD，使其容量大小和其它节点相同，是否可以有偏差？猜测应该有一个可以接受的偏差范围，重构后的OSD节点树看起来像这样：
$ ceph osd tree
ID WEIGHT TYPE NAME UP/DOWN REWEIGHT PRIMARY-AFFINITY
-1 7.25439 root default
-2 1.81360 host ceph3
2 1.81360 osd.2 up 1.00000 1.00000
-3 0 host ceph4
-4 3.62720 host ceph1
0 1.81360 osd.0 up 1.00000 1.00000
1 1.81360 osd.1 up 1.00000 1.00000
-5 1.81360 host ceph2
3 1.81360 osd.3 up 1.00000 1.00000
ceph4节点被删除，重新加入了另一个OSD节点ceph2。
$ ceph -s
cluster 20ab1119-a072-4bdf-9402-9d0ce8c256f4
health HEALTH_OK
monmap e2: 2 mons at {ceph2=192.168.17.21:6789/0,ceph4=192.168.17.23:6789/0}
election epoch 26, quorum 0,1 ceph2,ceph4
osdmap e599: 4 osds: 4 up, 4 in
flags sortbitwise,require_jewel_osds
pgmap v155011: 100 pgs, 1 pools, 18628 bytes data, 1 objects
1129 MB used, 7427 GB / 7428 GB avail
100 active+clean
另外，为了满足HA的要求，OSD需要分散在不同的节点上，这里拷贝数量为3，则需要有三个OSD节点来承载这些OSD，如果三个OSD分布在两个OSD节点上，则依然可能会出现”active+undersized+degraded”的状态。
官方是这样说的：
This, combined with the default CRUSH failure domain, ensures that replicas or erasure code shards are separated across hosts and a single host failure will not affect availability.
理解如有错误还望能点醒。
[1] https://www.mail-archive.com/[email protected]/msg47070.html
[2] http://docs.ceph.com/docs/master/rados/operations/pg-states/
转： https://blog.csdn.net/chenwei8280/article/details/80785595