Questions on CRUSH map

[Cody <codeology.lab@xxxxxxxxx>]

Hi everyone,

I am new to Ceph and trying to test out my understanding on the CRUSH
map. Attached is a hypothetical cluster diagram with 3 racks. On each
rack, the first host runs 3 SSD-based OSDs and the second 3 HDD-based.

My goal is to create two rules that separate SSD and HDD performance
domains (by using device class) and both rules should use a*rack*
level failure domain.

Here is my CRUSH map:

# Devices

device 0 osd.0 class ssd
device 1 osd.1 class ssd
device 2 osd.2 class ssd
device 3 osd.3 class hdd
device 4 osd.4 class hdd
device 5 osd.5 class hdd
device 6 osd.6 class ssd
device 7 osd.7 class ssd
device 8 osd.8 class ssd
device 9 osd.9 class hdd
device 10 osd.10 class hdd
device 11 osd.11 class hdd
device 12 osd.12 class ssd
device 13 osd.13 class ssd
device 14 osd.14 class ssd
device 15 osd.15 class hdd
device 16 osd.17 class hdd
device 17 osd.17 class hdd

# Buckets

  host a1-1 {
      id -1
      alg straw
      hash 0
      item osd.0 weight 1.00
      item osd.1 weight 1.00
      item osd.2 weight 1.00
  }

  host a1-2 {
      id -2
      alg straw
      hash 0
      item osd.3 weight 1.00
      item osd.4 weight 1.00
      item osd.5 weight 1.00
  }

  host a2-1 {
      id -3
      alg straw
      hash 0
      item osd.6 weight 1.00
      item osd.7 weight 1.00
      item osd.8 weight 1.00
  }

  host a2-2 {
      id -4
      alg straw
      hash 0
      item osd.9 weight 1.00
      item osd.10 weight 1.00
      item osd.11 weight 1.00
  }

  host a3-1 {
      id -5
      alg straw
      hash 0
      item osd.12 weight 1.00
      item osd.13 weight 1.00
      item osd.14 weight 1.00
  }

  host a3-2 {
      id -6
      alg straw
      hash 0
      item osd.15 weight 1.00
      item osd.16 weight 1.00
      item osd.17 weight 1.00
  }

  rack a1 {
      id -7
      alg straw
      hash 0
      item a1-1 weight 3.0
      item a1-2 weight 3.0
  }

  rack a2 {
      id -8
      alg straw
      hash 0
      item a2-1 weight 3.0
  }

  rack a3 {
      id -9
      alg straw
      hash 0
      item a3-1 weight 3.0
  }

  row a {
      id -10
      alg straw
      hash 0
      item a1 6.0
      item a2 6.0
      item a3 6.0
  }

# Rules

  rule ssd {
      id 1
      type replicated
      min_size 2
      max_size 11
      step take a class ssd
      step chooseleaf firstn 0 type row
      step emit
  }

  rule hdd {
      id 2
      type replicated
      min_size 2
      max_size 11
      step take a class ssd
      step chooseleaf firstn 0 type row
      step emit
  }


Q1: Is the above CRUSH map correct for my goal?

Q2: What does the "step choose" and "step chooseleaf" mean? I read its
definitions in the documentation several times, but still cannot get
it.

Any helps would be greatly appreciated!

Regards,
Cody

Attachment: ceph-crush-layout.jpg
Description: JPEG image

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