Chapter 8. CRUSH Weights

The CRUSH algorithm assigns a weight value per device with the objective of approximating a uniform probability distribution for I/O requests. As a best practice, we recommend creating pools with devices of the same type and size, and assigning the same relative weight. Since this is not always practical, you may incorporate devices of different size and use a relative weight so that Ceph will distribute more data to larger drives and less data to smaller drives.

To adjust an OSD’s crush weight in the CRUSH map of a running cluster, execute the following:

ceph osd crush reweight {name} {weight}

Where:

CRUSH buckets reflect the sum of the weights of the buckets or the devices they contain. For example, a rack containing a two hosts with two OSDs each, might have a weight of 4.0 and each host a weight of 2.0--the sum for each OSD, where the weight per OSD is 1.00. Generally, we recommend using 1.0 as the measure of 1TB of data.

CRUSH weight is a persistent setting, and it affects how CRUSH assigns data to OSDs. Ceph also has temporary reweight settings if the cluster gets out of balance. For example, whereas a Ceph Block Device will shard a block device image into a series of smaller objects and stripe them across the cluster, using librados to store data without normalizing the size of objects can lead to imbalanced clusters (e.g., storing 100 1MB objects and 10 4MB objects will make a few OSDs have more data than the others).

You can temporarily increase or decrease the weight of particular OSDs by executing:

ceph osd reweight {id} {weight}

Where:

You can also temporarily reweight OSDs by utilization.

ceph osd reweight-by-utilization {threshold}

Where: