garage/doc/book/operations/layout.md

+++ title = "Cluster layout management" weight = 20 +++

The cluster layout in Garage is a table that assigns to each node a role in the cluster. The role of a node in Garage can either be a storage node with a certain capacity, or a gateway node that does not store data and is only used as an API entry point for faster cluster access. An introduction to building cluster layouts can be found in the production deployment page.

In Garage, all of the data that can be stored in a given cluster is divided into slices which we call partitions. Each partition is stored by one or several nodes in the cluster (see replication_mode). The layout determines the correspondence between these partition, which exist on a logical level, and actual storage nodes.

How cluster layouts work in Garage

A cluster layout is composed of the following components:

• a table of roles assigned to nodes, defined by the user
• an optimal assignation of partitions to nodes, computed by an algorithm that is ran once when calling garage layout apply or the ApplyClusterLayout API endpoint
• a version number

Garage nodes will always use the cluster layout with the highest version number.

Garage nodes also maintain and synchronize between them a set of proposed role changes that haven't yet been applied. These changes will be applied (or canceled) in the next version of the layout.

All operations on the layout can be realized using the garage CLI or using the administration API endpoint. We give here a description of CLI commands, the admin API semantics are very similar.

The following commands insert modifications to the set of proposed role changes for the next layout version (but they do not create the new layout immediately):

garage layout assign [...]
garage layout remove [...]

The following command can be used to inspect the layout that is currently set in the cluster and the changes proposed for the next layout version, if any:

garage layout show

The following commands create a new layout with the specified version number, that either takes into account the proposed changes or cancels them:

garage layout apply --version <new_version_number>
garage layout revert --version <new_version_number>

The version number of the new layout to create must be 1 + the version number of the previous layout that existed in the cluster. The apply and revert commands will fail otherwise.

Warnings about Garage cluster layout management

⚠️ Never make several calls to garage layout apply or garage layout revert with the same value of the --version flag. Doing so can lead to the creation of several different layouts with the same version number, in which case your Garage cluster will become inconsistent until fixed. If a call to garage layout apply or garage layout revert has failed and garage layout show indicates that a new layout with the given version number has not been set in the cluster, then it is fine to call the command again with the same version number.

If you are using the garage CLI by typing individual commands in your shell, you shouldn't have much issues as long as you run commands one after the other and take care of checking the output of garage layout show before applying any changes.

If you are using the garage CLI or the admin API to script layout changes, follow the following recommendations:

• If using the CLI, make all of your garage CLI calls to the same RPC host. If using the admin API, make all of your API calls to the same Garage node. Do not connect to individual nodes to send them each a piece of the layout changes you are making, as the changes propagate asynchronously between nodes and might not all be taken into account at the time when the new layout is applied.

• Only call garage layout apply/ApplyClusterLayout once, and call it strictly after all of the layout assign and layout remove commands/UpdateClusterLayout API calls have returned.

Understanding unexpected layout calculations

Example 1

$ garage layout show
==== CURRENT CLUSTER LAYOUT ====
ID                Tags   Zone  Capacity   Usable capacity
b10c110e4e854e5a  node1  dc1   1000.0 MB  1000.0 MB (100.0%)
a235ac7695e0c54d  node2  dc2   1000.0 MB  1000.0 MB (100.0%)
62b218d848e86a64  node3  dc3   1000.0 MB  1000.0 MB (100.0%)

Zone redundancy: maximum

Current cluster layout version: 6

==== STAGED ROLE CHANGES ====
ID                Tags   Zone  Capacity
a11c7cf18af29737  node4  dc1   1000.0 MB


==== NEW CLUSTER LAYOUT AFTER APPLYING CHANGES ====
ID                Tags   Zone  Capacity   Usable capacity
b10c110e4e854e5a  node1  dc1   1000.0 MB  1000.0 MB (100.0%)
a11c7cf18af29737  node4  dc1   1000.0 MB  0 B (0.0%)
a235ac7695e0c54d  node2  dc2   1000.0 MB  1000.0 MB (100.0%)
62b218d848e86a64  node3  dc3   1000.0 MB  1000.0 MB (100.0%)

Zone redundancy: maximum

==== COMPUTATION OF A NEW PARTITION ASSIGNATION ====

Partitions are replicated 3 times on at least 3 distinct zones.

Optimal partition size:                     3.9 MB (3.9 MB in previous layout)
Usable capacity / total cluster capacity:   3.0 GB / 4.0 GB (75.0 %)
Effective capacity (replication factor 3):  1000.0 MB

A total of 0 new copies of partitions need to be transferred.

dc1                 Tags   Partitions        Capacity   Usable capacity
  b10c110e4e854e5a  node1  256 (0 new)       1000.0 MB  1000.0 MB (100.0%)
  a11c7cf18af29737  node4  0 (0 new)         1000.0 MB  0 B (0.0%)
  TOTAL                    256 (256 unique)  2.0 GB     1000.0 MB (50.0%)

dc2                 Tags   Partitions        Capacity   Usable capacity
  a235ac7695e0c54d  node2  256 (0 new)       1000.0 MB  1000.0 MB (100.0%)
  TOTAL                    256 (256 unique)  1000.0 MB  1000.0 MB (100.0%)

dc3                 Tags   Partitions        Capacity   Usable capacity
  62b218d848e86a64  node3  256 (0 new)       1000.0 MB  1000.0 MB (100.0%)
  TOTAL                    256 (256 unique)  1000.0 MB  1000.0 MB (100.0%)

Example 2

==== CURRENT CLUSTER LAYOUT ====
ID                Tags   Zone  Capacity   Usable capacity
b10c110e4e854e5a  node1  dc1   1000.0 MB  500.0 MB (50.0%)
a11c7cf18af29737  node4  dc1   1000.0 MB  500.0 MB (50.0%)
a235ac7695e0c54d  node2  dc2   1000.0 MB  1000.0 MB (100.0%)
62b218d848e86a64  node3  dc3   1000.0 MB  1000.0 MB (100.0%)

Zone redundancy: maximum

Current cluster layout version: 8

==== STAGED ROLE CHANGES ====
ID                Tags   Zone  Capacity
a11c7cf18af29737  node4  dc3   1000.0 MB


==== NEW CLUSTER LAYOUT AFTER APPLYING CHANGES ====
ID                Tags   Zone  Capacity   Usable capacity
b10c110e4e854e5a  node1  dc1   1000.0 MB  1000.0 MB (100.0%)
a235ac7695e0c54d  node2  dc2   1000.0 MB  1000.0 MB (100.0%)
62b218d848e86a64  node3  dc3   1000.0 MB  753.9 MB (75.4%)
a11c7cf18af29737  node4  dc3   1000.0 MB  246.1 MB (24.6%)

Zone redundancy: maximum

==== COMPUTATION OF A NEW PARTITION ASSIGNATION ====

Partitions are replicated 3 times on at least 3 distinct zones.

Optimal partition size:                     3.9 MB (3.9 MB in previous layout)
Usable capacity / total cluster capacity:   3.0 GB / 4.0 GB (75.0 %)
Effective capacity (replication factor 3):  1000.0 MB

A total of 128 new copies of partitions need to be transferred.

dc1                 Tags   Partitions        Capacity   Usable capacity
  b10c110e4e854e5a  node1  256 (128 new)     1000.0 MB  1000.0 MB (100.0%)
  TOTAL                    256 (256 unique)  1000.0 MB  1000.0 MB (100.0%)

dc2                 Tags   Partitions        Capacity   Usable capacity
  a235ac7695e0c54d  node2  256 (0 new)       1000.0 MB  1000.0 MB (100.0%)
  TOTAL                    256 (256 unique)  1000.0 MB  1000.0 MB (100.0%)

dc3                 Tags   Partitions        Capacity   Usable capacity
  62b218d848e86a64  node3  193 (0 new)       1000.0 MB  753.9 MB (75.4%)
  a11c7cf18af29737  node4  63 (0 new)        1000.0 MB  246.1 MB (24.6%)
  TOTAL                    256 (256 unique)  2.0 GB     1000.0 MB (50.0%)