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Add a section in glossary to explain what "logical size" means. (#2306)

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Heikki Linnakangas
2022-08-19 21:57:00 +03:00
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docs/glossary.md
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@@ -92,6 +92,7 @@ The layer map tracks what layers exist in a timeline.
### Layered repository ### Layered repository
Neon repository implementation that keeps data in layers. Neon repository implementation that keeps data in layers.
### LSN ### LSN
The Log Sequence Number (LSN) is a unique identifier of the WAL record[] in the WAL log. The Log Sequence Number (LSN) is a unique identifier of the WAL record[] in the WAL log.
@@ -125,6 +126,26 @@ TODO: use this name consistently in remote storage code. Now `disk_consistent_ls
* `ancestor_lsn` - LSN of the branch point (the LSN at which this branch was created) * `ancestor_lsn` - LSN of the branch point (the LSN at which this branch was created)
TODO: add table that describes mapping between PostgreSQL (compute), safekeeper and pageserver LSNs. TODO: add table that describes mapping between PostgreSQL (compute), safekeeper and pageserver LSNs.
### Logical size
The pageserver tracks the "logical size" of a timeline. It is the
total size of all relations in all Postgres databases on the
timeline. It includes all user and system tables, including their FSM
and VM forks. But it does not include SLRUs, twophase files or any
other such data or metadata that lives outside relations.
The logical size is calculated by the pageserver, and is sent to
PostgreSQL via feedback messages to the safekeepers. PostgreSQL uses
the logical size to enforce the size limit in the free tier. The
logical size is also shown to users in the web console.
The logical size is not affected by branches or the physical layout of
layer files in the pageserver. If you have a database with 1 GB
logical size and you create a branch of it, both branches will have 1
GB logical size, even though the branch is copy-on-write and won't
consume any extra physical disk space until you make changes to it.
### Page (block) ### Page (block)
The basic structure used to store relation data. All pages are of the same size. The basic structure used to store relation data. All pages are of the same size.