CRDT Disk-space requirements

CRDT Disk-space requirements#

An important consideration is the overhead associated with CRDT types, particularly the on-disk size.

Operation-based CRDT disk-space reqs#

For operation-based types , this is trivial because the types are merely domains on top of other types. They have the same disk space requirements no matter how many nodes are there:

  • crdt_delta_counter - Same as bigint (8 bytes)

  • crdt_delta_sum - Same as numeric (variable, depending on precision and scale)

There’s no dependency on the number of nodes because operation-based CRDT types don’t store any per-node information.

State-based CRDT disk-space reqs#

For state-based types , the situation is more complicated. All the types are variable length (stored essentially as a bytea column) and consist of a header and a certain amount of per-node information for each node that modified the value.

For the bigint variants, formulas computing approximate size are:

  • crdt_gcounter - 32B (header) + N * 12B (per-node)

  • crdt_pncounter –48B (header) + N * 20B (per-node)

N denotes the number of nodes that modified this value.

For the numeric variants, there’s no exact formula because both the header and per-node parts include numeric variable-length values. To give you an idea of how many such values you need to keep:

  • crdt_gsum

  • fixed: 20B (header) + N * 4B (per-node)

  • variable: (2 + N) numeric values

  • crdt_pnsum

  • fixed: 20B (header) + N * 4B (per-node)

  • variable: (4 + 2 * N) numeric values

Note

It doesn't matter how many nodes are in the cluster if the values are never updated on multiple nodes. It also doesn't matter whether the updates were concurrent (causing a conflict).

In addition, it doesn’t matter how many of those nodes were already removed from the cluster. There’s no way to compact the state yet.

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