Eviction

Write out in-memory layer to disk, into a delta layer.

To release memory
To make it possible to advance disk_consistent_lsn and allow the WAL service to release some WAL.
Triggered if we are short on memory
Or if the oldest in-memory layer is so old that it's holding back the WAL service from removing old WAL

Materialization

Create a new image layer of a segment, by performing WAL redo

To reduce the amount of WAL that needs to be replayed on a GetPage request.
To allow garbage collection of old layers
Triggered by distance to last full image of a page

Coalescing

Replace N consecutive layers of a segment with one larger layer.

To reduce the number of small files that needs to be uploaded to S3

Bundling

Zip together multiple small files belonging to different segments.

To reduce the number of small files that needs to be uploaded to S3

Garbage collection

Remove a layer that's older than the GC horizon, and isn't needed anymore.