CRUSH algorithm
The CRUSH algorithm enables the Ceph Storage Cluster to scale, rebalance, and recover dynamically.
Using the CRUSH algorithm：Ceph calculates which placement group should contain the object, and further calculates which Ceph OSD Daemon should store the placement group.
它保存的信息包括：集群设备列表、bucket 列表、故障域（failure domain）分层结构、保存数据时用到的为故障域定义的规则（rules）等；
# ceph osd crush dump
CRUSH Map 的内容
其包含很多内容，主要为如下四部分：Device、Bucket Type、Bucket、Rule；
Device（设备）
集群的所有设备列表，其位于 CRUSH Map 的开头部分：
# devices device 0 osd.0 class ssd device 1 osd.1 class ssd device 2 osd.2 class ssd device 3 osd.3 class ssd ...
要将 PG 映射到 OSD，CRUSH 需要 OSD 设备列表；
Bucket Type
... # types type 0 osd type 1 host type 2 chassis type 3 rack type 4 row type 5 pdu type 6 pod type 7 room type 8 datacenter type 9 zone type 10 region type 11 root ...
Bucket
host laptop-asus-k53sd {
id -7 # do not change unnecessarily
id -8 class ssd # do not change unnecessarily
# weight 1.819
alg straw2
hash 0 # rjenkins1
item osd.3 weight 1.819
}
host pc-amd64-100249 {
...
}
host pc-amd64-100254 {
...
}
root default {
id -1 # do not change unnecessarily
id -4 class ssd # do not change unnecessarily
# weight 2.529
alg straw2
hash 0 # rjenkins1
item pc-amd64-100254 weight 0.300
item pc-amd64-100249 weight 0.409
item laptop-asus-k53sd weight 1.819
}
...
Rules
...
rule replicated_rule {
id 0
type replicated
min_size 1
max_size 10
step take default
step chooseleaf firstn 0 type host
step emit
}
...
各部分间的关系
Device、Bucket Type、Bucket，这三者用以描述存储设备的结构。其为树形结构，使用节点与叶子两种层次。
Rule，通过引用 Device、Bucket Type、Bucket 来控制存储池内数据的存放方式：复制、放置、属性；