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.