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Don't use term 'snapshot' to describe layers

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anastasia
2021-09-02 14:45:58 +03:00
committed by lubennikovaav
parent c6678c5dea
commit fabf5ec664
11 changed files with 85 additions and 78 deletions
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19
docs/glossary.md
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@@ -26,7 +26,7 @@ A checkpoint record in the WAL marks a point in the WAL sequence at which it is
NOTE: This is an overloaded term.
Whenever enough WAL has been accumulated in memory, the page server []
writes out the changes in memory into new layer files[]. This process
writes out the changes from in-memory layers into new layer files[]. This process
is called "checkpointing". The page server only creates layer files for
relations that have been modified since the last checkpoint.
@@ -41,17 +41,28 @@ Stateless Postgres node that stores data in pageserver.
Each of the separate segmented file sets in which a relation is stored. The main fork is where the actual data resides. There also exist two secondary forks for metadata: the free space map and the visibility map.
Each PostgreSQL fork is considered a separate relish.
### Layer file
### Layer
Each layer corresponds to one RELISH_SEG_SIZE slice of a relish in a range of LSNs.
There are two kinds of layers, in-memory and on-disk layers. In-memory
layers are used to ingest incoming WAL, and provide fast access
to the recent page versions. On-disk layers are stored as files on disk, and
are immutable.
### Layer file (on-disk layer)
Layered repository on-disk format is based on immutable files. The
files are called "layer files". Each file corresponds to one 10 MB
files are called "layer files". Each file corresponds to one RELISH_SEG_SIZE
segment of a PostgreSQL relation fork. There are two kinds of layer
files: image files and delta files. An image file contains a
"snapshot" of the segment at a particular LSN, and a delta file
contains WAL records applicable to the segment, in a range of LSNs.
### Layer map
The layer map tracks what layers exist for all the relishes in a timeline.
### Layered repository
Zenith repository implementation that keeps data in layers.
### LSN
@@ -121,7 +132,7 @@ Each SLRU segment is considered a separate relish[].
### Tenant (Multitenancy)
Tenant represents a single customer, interacting with Zenith.
Wal redo[] activity, timelines[], snapshots[] are managed for each tenant independently.
Wal redo[] activity, timelines[], layers[] are managed for each tenant independently.
One pageserver[] can serve multiple tenants at once.
One safekeeper