[pbs-devel] [PATCH v2 proxmox-backup 1/2] docs: add security implications of prune and change detection mode

[Christian Ebner]

Users should be made aware that the data stored in chunks outlives
the backup snapshots on pruning and that backups created using the
change-detection-mode set to metadata might reference chunks
containing files which have vanished since the previous backup, but
might still be accessible when access to the chunks raw data is
possible (client or server side).

Signed-off-by: Christian Ebner <c.ebner at proxmox.com>
---
changes since version 1:
- explicitly mention steps to get rid of chunks for both, regular and
  file-level backups with change detection mode metadata
- reworded and restructured according to feedback

diff --git a/docs/maintenance.rst b/docs/maintenance.rst
index 4bb135e4e..e8a26d69c 100644
--- a/docs/maintenance.rst
+++ b/docs/maintenance.rst
@@ -6,8 +6,34 @@ Maintenance Tasks
 Pruning
 -------
 
-Prune lets you specify which backup snapshots you want to keep.
-The following retention options are available:
+Prune lets you specify which backup snapshots you want to keep, removing others.
+When pruning a snapshot, only the snapshot metadata (manifest, indices, blobs,
+log and notes) is removed. The chunks containing the actual backup data and
+previously referenced by the pruned snapshot, have to be removed by a garbage
+collection run.
+
+.. Caution:: Take into consideration that sensitive information stored in a
+   given data chunk will outlive pruned snapshots and remain present in the
+   datastore as long as referenced by at least one backup snapshot. Further,
+   *even* if no snapshot references a given chunk, it will remain present until
+   removed by the garbage collection.
+
+   Further, file-level backups created using the change detection mode
+   `metadata` can reference backup chunks containing files which have vanished
+   since the previous backup, but might still be accessible when reading the
+   chunks raw data is possible (client or server side).
+
+   To remove chunks containing sensitive data, prune any snapshot made while the
+   data was part of the backup input and run a garbage collection. Further, if
+   using file-based backups with change detection mode `metadata`, additionally
+   prune all snapshots since the sensitive data was no longer part of the backup
+   input and run a garbage collection.
+
+   The no longer referenced chunks will then be marked for deletion on the next
+   garbage collection run and removed by a subsequent run after the grace
+   period.
+
+The following retention options are available for pruning:
 
 ``keep-last <N>``
   Keep the last ``<N>`` backup snapshots.
-- 
2.39.5