Imagine that you have a tenant with a single branch like this:
---------------==========>
^
gc horizon
where:
---- is the portion of the branch that is older than retention period
==== is the portion of the branch that is newer than retention period.
Before this commit, the sizing model included the logical size at the
GC horizon, but not the WAL after that. In particular, that meant that
on a newly created tenant with just one timeline, where the retention
period covered the whole history of the timeline, i.e. gc_cutoff was 0,
the calculated tenant size was always zero.
We now include the WAL after the GC horizon in the size. So in the
above example, the calculated tenant size would be the logical size
of the database the GC horizon, plus all the WAL after it (marked with
===).
This adds a new `insert_point` function to the sizing model, alongside
`modify_branch`, and changes the code in size.rs to use the new
function. The new function takes an absolute lsn and logical size as
argument, so we no longer need to calculate the difference to the
previous point. Also, the end-size is now optional, because we now
need to add a point to represent the end of each branch to the model,
but we don't want to or need to calculate the logical size at that
point.
Tenant size information is gathered by using existing parts of
`Tenant::gc_iteration` which are now separated as
`Tenant::refresh_gc_info`. `Tenant::refresh_gc_info` collects branch
points, and invokes `Timeline::update_gc_info`; nothing was supposed to
be changed there. The gathered branch points (through Timeline's
`GcInfo::retain_lsns`), `GcInfo::horizon_cutoff`, and
`GcInfo::pitr_cutoff` are used to build up a Vec of updates fed into the
`libs/tenant_size_model` to calculate the history size.
The gathered information is now exposed using `GET
/v1/tenant/{tenant_id}/size`, which which will respond with the actual
calculated size. Initially the idea was to have this delivered as tenant
background task and exported via metric, but it might be too
computationally expensive to run it periodically as we don't yet know if
the returned values are any good.
Adds one new metric:
- pageserver_storage_operations_seconds with label `logical_size`
- separating from original `init_logical_size`
Adds a pageserver wide configuration variable:
- `concurrent_tenant_size_logical_size_queries` with default 1
This leaves a lot of TODO's, tracked on issue #2748.
