feat(lep): recurring filesystem trim design

Ref: 5186

Signed-off-by: Chin-Ya Huang <chin-ya.huang@suse.com>

enhancements/20230309-recurring-filesystem-trim.md

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+# Recurring Filesystem Trim
+## Summary
+
+Longhorn currently supports the [filesystem trim](./20221103-filesystem-trim.md) feature, which allows users to reclaim volume disk spaces of deleted files. However, this is a manual process, which can be time-consuming and inconvenient.
+
+To improve user experience, Longhorn could automate the process by implementing a new RecurringJob `filesystem-trim` type. This enhancement enables regularly freeing up unused volume spaces and reducing the need for manual interventions.
+
+### Related Issues
+
+https://github.com/longhorn/longhorn/issues/5186
+
+## Motivation
+
+### Goals
+
+Introduce a new recurring job type called `filesystem-trim` to periodically trim the volume filesystem to reclaim disk spaces.
+
+### Non-goals [optional]
+
+`None`
+
+## Proposal
+
+To extend the RecurringJob custom resource definition by adding new `RecurringJobType: filesystem-trim`.
+
+### User Stories
+
+To schedule regular volume filesystem trims, user can create a RecurringJob with `spec.task=filesystem-trim` and associating it with volumes.
+
+### User Experience In Detail
+
+#### Recurring Filesystem Trim
+1. The user sees workload volume size has increased over time.
+1. Create RecurringJob with the `filesystem-trim` task type and assign it to the volume.
+   ```yaml
+   apiVersion: longhorn.io/v1beta2
+   kind: RecurringJob
+   metadata:
+     name: recurring-fs-trim-per-min
+     namespace: longhorn-system
+   spec:
+     concurrency: 1
+     cron: '* * * * *'
+     groups: []
+     labels: {}
+     name: recurring-fs-trim-per-min
+     retain: 0
+     task: filesystem-trim
+   ```
+1. The RecurringJob runs and relaims some volume spaces.
+
+### API changes
+
+`None`
+
+## Design
+
+### Implementation Overview
+
+#### The RecurringJob `filesystem-trim` Task Type
+
+1. Call Volume API `ActionTrimFilesystem` when the RecurringJob type is `filesystem-trim`.
+
+### RecurringJob Mutate
+
+1. Mutate the `Recurringjob.Spec.Retain` to 0 when the task type is `filesystem-trim` as it is not effective for this type of task.
+
+### Test plan
+
+#### Test Recurring Filesystem Trim
+1. Create workload.
+1. Create a file with some data in the workload.
+1. Volume actual size should increase.
+1. Delete the file.
+1. Volume actual size should not decrease.
+1. Create RecurringJob with type `filesystem-trim` and assign to the workload volume.
+1. Wait for RecurringJob to complete.
+1. Volume actual size should decrease.
+
+### Upgrade strategy
+
+`None`
+
+## Note [optional]
+
+`None`