Planet Ceph

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April 29, 2017

Ceph hybrid storage tiers

In a previous post I showed you how to deploy storage tiering for Ceph, today I will explain how to setup hybrid storage tiers.

What is hybrid storage?

Hybrid storage is a combination of two different storage tiers like SSD and HDD. In Ceph terms that means that the copies of each objects are located in different tiers – maybe 1 copy on SSD and 2 copies on HDDs.

The idea is to keep 1 copy of the data on a high performance tier (usually SSD or NVMe) and 2 additional copies on a lower cost tier (usually HDDs) in order to improve the read performance at a lower cost.

The following diagram explains the difference between read and write I/O, when using a hybrid storage tier:

wdm usage

How to set it up?

To get this to work in Ceph, we create a two step storage policy:

  • First step: choose the primary OSD (firstn 1) in the high performance tier, “root-ssd” in the example
  • Second step: choose the rest of the OSDs (firstn -1) in the low performance tier, “root-hdd” in the example

Assuming a replication factor of 3, the following Ceph ruleset will place 1 copy of each object on SSD and 2 copies on HDDs.

# Hybrid storage policy
rule hybrid {
  ruleset 2
  type replicated
  min_size 1
  max_size 10
  step take root-ssd
  step chooseleaf firstn 1 type host
  step emit
  step take root-hdd
  step chooseleaf firstn -1 type host
  step emit
}

Now that you know how to set it up, it’s up to you to combine all the storage tiers.

Source: Maxime Guyot (Ceph hybrid storage tiers)

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