WIP: draft snapmgmt

draft_snapmgmt.md

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+# Tiered Snapshotting, Pruning & Replication
+
+## Use Case
+
+* Differently paced snapshotting & replication
+    * 20 x 10min-spaced snapshots for rollback in case of adminstrative mistakes.
+        * Should never be replicated
+    * Daily Snapshots for off-site replication
+        * at 3:00 AM to avoid impacting production workload
+
+* Sometimes, out-of-routine snapshot for off-site replication
+    * e.g. before system update
+
+
+## Config Draft
+
+```yaml
+- type: push
+  connect:
+    type: tcp
+    address: "backup-server.foo.bar:8888"
+  filesystems: {
+    "<": true,
+    "tmp": false
+  }
+  send:
+    snap_filters:
+    - type: snapmgmt # auto-add this snap filter if snapmgmt != {} 
+      # every snapmgmt method produces a list of snaps_filters which this snap_filter type represents
+    - type: regex
+      regex: "manual_.*" # admin may create snapshots named like `manual_pre_upgrade` and have those replicated as well
+      # TODO: how does a manual `zfs snapshot` trigger replication? `zrepl signal wakeup ...` good enough for now
+  snapmgmt:
+    type: tiered
+    prefix: zrepl_
+    tiers:
+    - name: local
+      cron: every 10 minutes
+      keep: { local: 10, remote: 0 }
+    - name: daily
+      cron: every day at 3 AM
+      keep: { local: 14, remote: 30 }
+    - name: weekly
+      cron: every sunday at 3 AM
+      keep: { local: 0, remote: 20 }
+    - name: monthly
+      cron: every last sunday of the month at 3 AM
+      keep: { local: 0, remote: 12 }
+```
+
+TODO pull - likely breaks existing split of responsibilities about pruning between `pull` and `source`
+
+## Implementation
+
+A `snapmgmt` type implements both snapshotting and *local* pruning in one service (`RunSnapshotterAndLocalPruner()`)
+The `snapmgmt` type `tiered` works as follows:
+
+* Snapshots are created per tier:
+  * snapshot name = `${PREFIX}_${JOBNAME}_TIERED_${thistier.name}_${NOW}`
+  * property `zrepl:JOBNAME:tier=${thistier.name}`
+  * property `zrepl:JOBNAME:destroy_at=${thistier.destroy_at(NOW)}`
+* Sender-side pruning by
+  * filtering all snaps by userprop `zrepl:JOBNAME:tier=${thistier.name}`
+  * destroying all snaps in the filtered list with `(zrepl:JOBNAME:destroy_at).Before(NOW)`
+  * sleeping until MIN of
+    * `MIN(filtered_list.zrepl:JOBNAME:destroy_at)`
+    * `(<-newSnapshots).DestroyAt)`
+
+The replication enigne remains orthogonal to `snapmgmt`, but the protocol allows a sender to present a filtered view of snapshots in the `ListFilesystemVersions` RPC response.
+The filters are specified in a list, and have OR semantics, i.e., if one filter matches, the snapshot is presented tothe replication engine.
+The filter type supporting the feature proposed in this issue is the `tiered` filter type:
+it includes snapshots that are
+* named by the snapshotter above: `${PREFIX}_${JOBNAME}_${snapmgmt.tiers|.name}_${NOW}`
+* and the respective tier has a non-zero `keep.remote`
+* TODO: deriving the tier from the snapshot name is unclean.
+  We could instead allow that a replication filter is not pure (i.e. may use zfs.ZFSGet).
+  Would work for both `push` and `source` jobs.
+
+Receiver-side pruning is still unsolved (TODO):
+
+* We could replicate the `destroy_at` property, and have independent pruning on the receive-side.
+    * While there are cases where this is desirable (use case "guarantee that replicated data is destroyed after X months")
+    * it might be undesirable in cases where the receive-side is a backup (which should not self-destroy)
+* We could require each `snapmgmt` method to have a `PruneReceiver(receiver)` method, and require that `receiver` provides an RPC endpoint to get the `destroy_at` infromation required to evaluate the prune policy (e.g. a flag in the request, like `GetDestroyAt=true`)
+
+## Other future `snapmgmt` methods
+
+```yaml
+snapmgmt:
+  # zrepl signal ondemand JOBNAME SNAPNAME EXPIRE_AT
+  # => creates snapshot @SNAPNAME for all filesystems matched by `$JOB.filesystems` with expiration date `EXPIRE_AT`
+  # => snapshot has user prop `zrepl:JOBNAME:ondemand=on`
+  # replication filter `snapmgmt` filters by `ondemand` property
+  # pruning filter uses expieration data encoded in property
+  type: ondemand
+
+snapmgmt:
+  # no snapshots, never-matching filter
+  type: manual
+```
+
+## Existing Use-Cases
+
+### Existing Configurations & Migration to New Config
+
+`snapmgmt` replaces the previous `snapshotting` and `pruning` fields.
+Proposed migration for existing configs:
+
+```yaml
+snapmgmt:
+  type: pre-snapmgmt
+  snapshotting:
+    type: periodic
+    interval: 10m
+    prefix: zrepl_
+  pruning:
+    keep_sender:
+      - type: not_replicated
+      - type: last_n
+        count: 10
+      - type: grid
+        grid: 1x1h(keep=all) | 24x1h | 14x1d
+        regex: "^zrepl_.*"
+    keep_receiver:
+      - type: grid
+        grid: 1x1h(keep=all) | 24x1h | 35x1d | 6x30d
+        regex: "^zrepl_.*"
+```
+