SSD disk tier usage

  • 24 March 2020
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You may have noticed when adding disks to the node or replacing disks with larger capacity ones, utilisation distribution between the disks does not occur immediately. You check on the cluster sometime later and notice that newly added disks still show minimal usage, much lower than expected.

 

By default, the aim is to bring disks utilisation within +/-7.5% spread of the tier utilisation. There are some things to consider when expecting a certain outcome:

  1. Disk balancing is not triggered unless the tier usage is at least 35%. 

  2. Only 1 GB of data is moved during a Curator scan per node. 

  3. Even if the tier usage is below 35% should any disk usage across the cluster reach 70%, disk balancing takes place.

 

Disk Balancing: 

Disk balancing ensures data is evenly distributed across all disks in a cluster. In disk balancing, data is moved within the same tier to balance out the disk utilization. This is different from ILM (Information Lifecycle Management), where data is moved between different tiers, preventing the SSD or high tier from getting full.

Disk balancing is done in each Partial Curator Scan and Full Curator Scan.

 

Curator:

Curator is a process that is responsible for managing and distributing tasks throughout the cluster, including disk balancing, proactive scrubbing, and many more items. Curator runs on every node and is controlled by an elected Curator Master who is responsible for the task and job delegation. There are two scan types for Curator, a full scan which occurs around every 6 hours and a partial scan which occurs every hour.

 

More about disk balancing consult these two resources:

Nutanix Bible: Disk Balancing

Video Tech TopX video Disk Balancing

 

More about Curator:

Under what conditions do Curator scans run

Nutanix University video on Curator


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