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layout doc: write explanations for bizarre scenarios
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@ -93,9 +93,23 @@ follow the following recommendations:
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## Understanding unexpected layout calculations
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When adding, removing or modifying nodes in a cluster layout, sometimes
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unexpected assigntations of partitions to node can occure. These assignations
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are in fact normal and logical, given the objectives of the algorihtm. Indeed,
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**the layout algorithm prioritizes moving less data between nodes over the fact
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of achieving equal distribution of load**. This section presents two examples
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and illustrates how one can control Garage's behavior to obtain the desired
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results.
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### Example 1
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In this example, a cluster is originally composed of 3 nodes in 3 different
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zones (data centers). The three nodes are of equal capacity, therefore they
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are all fully exploited and all store a copy of all of the data in the cluster.
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Then, a fourth node of the same size is added in the datacenter `dc1`.
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As illustrated by the following, **Garage will by default not store any data on the new node**:
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```
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$ garage layout show
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==== CURRENT CLUSTER LAYOUT ====
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@ -146,8 +160,37 @@ dc3 Tags Partitions Capacity Usable capacity
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TOTAL 256 (256 unique) 1000.0 MB 1000.0 MB (100.0%)
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```
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While unexpected, this is logical because of the following facts:
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- storing some data on the new node does not help increase the total quantity
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of data that can be stored on the cluster, as the two other zones (`dc2` and
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`dc3`) still need to store a full copy of everything, and their capacity is
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still the same;
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- there is therefore no need to move any data on the new node as this would be pointless;
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- moving data to the new node has a cost which the algorithm decides to not pay if not necessary.
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This distribution of data can however not be what the administrator wanted: if
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they added a new node to `dc1`, it might be because the existing node is too
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slow, and they wish to divide its load by half. In that case, what they need to
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do to force Garage to distribute the data between the two nodes is to attribute
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only half of the capacity to each node in `dc1` (in our example, 500M instead of 1G).
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In that case, Garage would determine that to be able to store 1G in total, it
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would need to store 500M on the old node and 500M on the added one.
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### Example 2
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The following example is a slightly different scenario, where `dc1` had two
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nodes that were used at 50%, and `dc2` and `dc3` each have one node that is
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100% used. All node capacities are the same.
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Then, a node from `dc1` is moved into `dc3`. One could expect that the roles of
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`dc1` and `dc3` would simply be swapped: the remaining node in `dc1` would be
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used at 100%, and the two nodes now in `dc3` would be used at 50%. Instead,
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this happens:
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```
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==== CURRENT CLUSTER LAYOUT ====
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ID Tags Zone Capacity Usable capacity
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@ -197,3 +240,34 @@ dc3 Tags Partitions Capacity Usable capacity
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a11c7cf18af29737 node4 63 (0 new) 1000.0 MB 246.1 MB (24.6%)
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TOTAL 256 (256 unique) 2.0 GB 1000.0 MB (50.0%)
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```
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As we can see, the node that was moved to `dc3` (node4) is only used at 25% (approximatively),
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whereas the node that was already in `dc3` (node3) is used at 75%.
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This can be explained by the following:
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- node1 will now be the only node remaining in `dc1`, thus it has to store all
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of the data in the cluster. Since it was storing only half of it before, it has
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to retrieve the other half from other nodes in the cluster.
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- The data which it does not have is entirely stored by the other node that was
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in `dc1` and that is now in `dc3` (node4). There is also a copy of it on node2
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and node3 since both these nodes have a copy of everything.
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- node3 and node4 are the two nodes that will now be in a datacenter that is
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under-utilized (`dc3`), this means that those are the two candidates from which
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data can be removed to be moved to node1.
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- Garage will move data in equal proportions from all possible sources, in this
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case it means that it will tranfer 25% of the entire data set from node3 to
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node1 and another 25% from node4 to node1.
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This explains why node3 ends with 75% utilization (100% from before minus 25%
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that is moved to node1), and node4 ends with 25% (50% from before minus 25%
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that is moved to node1).
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This illustrates another principle of the layout computation: **if there is a
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choice in moving data out of some nodes, then all links between pairs of nodes
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are used in equal proportions** (this is approximately true, there is
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randomness in the algorihtm to achieve this so there might be some small
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fluctuations, as we see above).
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