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diff --git a/doc/book/operations/layout.md b/doc/book/operations/layout.md index 93d502a7..0eb0e215 100644 --- a/doc/book/operations/layout.md +++ b/doc/book/operations/layout.md @@ -93,9 +93,23 @@ follow the following recommendations: ## 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 ==== @@ -146,8 +160,37 @@ dc3 Tags Partitions Capacity Usable capacity 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 @@ -197,3 +240,34 @@ dc3 Tags Partitions Capacity Usable capacity 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). |