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Chapter 13. CRUSH Storage Strategy Examples

Suppose you want to have most pools default to OSDs backed by large hard drives, but have some pools mapped to OSDs backed by fast solid-state drives (SSDs). It’s possible to have multiple independent CRUSH hierarchies within the same CRUSH map to reflect different performance domains. Cache-tiering is an example. Define two hierarchies with two different root nodes—​one for hard disks (e.g., "root platter") and one for SSDs (e.g., "root ssd") as shown below: 

device 0 osd.0
device 1 osd.1
device 2 osd.2
device 3 osd.3
device 4 osd.4
device 5 osd.5
device 6 osd.6
device 7 osd.7

  host ceph-osd-ssd-server-1 {
      id -1
      alg straw
      hash 0
      item osd.0 weight 1.00
      item osd.1 weight 1.00
  }

  host ceph-osd-ssd-server-2 {
      id -2
      alg straw
      hash 0
      item osd.2 weight 1.00
      item osd.3 weight 1.00
  }

  host ceph-osd-platter-server-1 {
      id -3
      alg straw
      hash 0
      item osd.4 weight 1.00
      item osd.5 weight 1.00
  }

  host ceph-osd-platter-server-2 {
      id -4
      alg straw
      hash 0
      item osd.6 weight 1.00
      item osd.7 weight 1.00
  }

  root platter {
      id -5
      alg straw
      hash 0
      item ceph-osd-platter-server-1 weight 2.00
      item ceph-osd-platter-server-2 weight 2.00
  }

  root ssd {
      id -6
      alg straw
      hash 0
      item ceph-osd-ssd-server-1 weight 2.00
      item ceph-osd-ssd-server-2 weight 2.00
  }

  rule data {
      ruleset 0
      type replicated
      min_size 2
      max_size 2
      step take platter
      step chooseleaf firstn 0 type host
      step emit
  }

  rule metadata {
      ruleset 1
      type replicated
      min_size 0
      max_size 10
      step take platter
      step chooseleaf firstn 0 type host
      step emit
  }

  rule rbd {
      ruleset 2
      type replicated
      min_size 0
      max_size 10
      step take platter
      step chooseleaf firstn 0 type host
      step emit
  }

  rule platter {
      ruleset 3
      type replicated
      min_size 0
      max_size 10
      step take platter
      step chooseleaf firstn 0 type host
      step emit
  }

  rule ssd {
      ruleset 4
      type replicated
      min_size 0
      max_size 4
      step take ssd
      step chooseleaf firstn 0 type host
      step emit
  }

  rule ssd-primary {
      ruleset 5
      type replicated
      min_size 5
      max_size 10
      step take ssd
      step chooseleaf firstn 1 type host
      step emit
      step take platter
      step chooseleaf firstn -1 type host
      step emit
  }
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You can then set a pool to use the SSD rule by executing: 

ceph osd pool set <poolname> crush_ruleset 4
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Your SSD pool can serve as the hot storage tier for cache tiering. Similarly, you could use the ssd-primary rule to cause each placement group in the pool to be placed with an SSD as the primary and platters as the replicas.

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