garage/doc/book/operations/layout.md at 8d07888fa2da8f72aa1e7e63bfd9ac9eb3b1e6ab

Alex Auvolat 8d07888fa2 layout doc: write explanations for bizarre scenarios

2023-09-22 16:07:46 +02:00

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+++ 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

When adding, removing or modifying nodes in a cluster layout, sometimes unexpected assigntations of partitions to node can occure. These assignations are in fact normal and logical, given the objectives of the algorihtm. Indeed, the layout algorithm prioritizes moving less data between nodes over the fact of achieving equal distribution of load. This section presents two examples and illustrates how one can control Garage's behavior to obtain the desired results.

Example 1

In this example, a cluster is originally composed of 3 nodes in 3 different zones (data centers). The three nodes are of equal capacity, therefore they are all fully exploited and all store a copy of all of the data in the cluster.

Then, a fourth node of the same size is added in the datacenter dc1. As illustrated by the following, Garage will by default not store any data on the new node:

$ 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%)

While unexpected, this is logical because of the following facts:

storing some data on the new node does not help increase the total quantity of data that can be stored on the cluster, as the two other zones (dc2 and dc3) still need to store a full copy of everything, and their capacity is still the same;
there is therefore no need to move any data on the new node as this would be pointless;
moving data to the new node has a cost which the algorithm decides to not pay if not necessary.

This distribution of data can however not be what the administrator wanted: if they added a new node to dc1, it might be because the existing node is too slow, and they wish to divide its load by half. In that case, what they need to do to force Garage to distribute the data between the two nodes is to attribute only half of the capacity to each node in dc1 (in our example, 500M instead of 1G). In that case, Garage would determine that to be able to store 1G in total, it would need to store 500M on the old node and 500M on the added one.

Example 2

The following example is a slightly different scenario, where dc1 had two nodes that were used at 50%, and dc2 and dc3 each have one node that is 100% used. All node capacities are the same.

Then, a node from dc1 is moved into dc3. One could expect that the roles of dc1 and dc3 would simply be swapped: the remaining node in dc1 would be used at 100%, and the two nodes now in dc3 would be used at 50%. Instead, this happens:

==== 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%)

As we can see, the node that was moved to dc3 (node4) is only used at 25% (approximatively), whereas the node that was already in dc3 (node3) is used at 75%.

This can be explained by the following:

node1 will now be the only node remaining in dc1, thus it has to store all of the data in the cluster. Since it was storing only half of it before, it has to retrieve the other half from other nodes in the cluster.
The data which it does not have is entirely stored by the other node that was in dc1 and that is now in dc3 (node4). There is also a copy of it on node2 and node3 since both these nodes have a copy of everything.
node3 and node4 are the two nodes that will now be in a datacenter that is under-utilized (dc3), this means that those are the two candidates from which data can be removed to be moved to node1.
Garage will move data in equal proportions from all possible sources, in this case it means that it will tranfer 25% of the entire data set from node3 to node1 and another 25% from node4 to node1.

This explains why node3 ends with 75% utilization (100% from before minus 25% that is moved to node1), and node4 ends with 25% (50% from before minus 25% that is moved to node1).

This illustrates another principle of the layout computation: if there is a choice in moving data out of some nodes, then all links between pairs of nodes are used in equal proportions (this is approximately true, there is randomness in the algorihtm to achieve this so there might be some small fluctuations, as we see above).

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