[pbs-devel] [PATCH v2 proxmox-backup 2/2] docs: deduplicate background details for garbage collection

[Christian Ebner]

Currently, common details regarding garbage collection are documented
in the backup client and the maintenance task. Deduplicate this
information by moving the details to the background section of the
maintenance task and reference that section in the backup client
part.

Signed-off-by: Christian Ebner <c.ebner at proxmox.com>
---
changes since version 1:
- refine phase 2 garbage collection description
- s/referred/referenced/
- reworded and restructured according to feedback

 docs/backup-client.rst | 28 ++++++++++++----------------
 docs/maintenance.rst   | 38 +++++++++++++++++++++++++++-----------
 2 files changed, 39 insertions(+), 27 deletions(-)

diff --git a/docs/backup-client.rst b/docs/backup-client.rst
index e56e0625b..892be11d9 100644
--- a/docs/backup-client.rst
+++ b/docs/backup-client.rst
@@ -789,29 +789,25 @@ Garbage Collection
 ------------------
 
 The ``prune`` command removes only the backup index files, not the data
-from the datastore. This task is left to the garbage collection
-command. It is recommended to carry out garbage collection on a regular basis.
+from the datastore. Deletion of unused backup data from the datastore is done by
+:ref:`garbage collection<_maintenance_gc>`. It is therefore recommended to
+schedule garbage collection tasks on a regular basis. The working principle of
+garbage collection is described in more details in the related :ref:`background
+section <gc_background>`.
 
-The garbage collection works in two phases. In the first phase, all
-data blocks that are still in use are marked. In the second phase,
-unused data blocks are removed.
+To start garbage collection from the client side, run the following command:
+
+.. code-block:: console
+
+  # proxmox-backup-client garbage-collect
 
 .. note:: This command needs to read all existing backup index files
   and touches the complete chunk-store. This can take a long time
   depending on the number of chunks and the speed of the underlying
   disks.
 
-.. note:: The garbage collection will only remove chunks that haven't been used
-   for at least one day (exactly 24h 5m). This grace period is necessary because
-   chunks in use are marked by touching the chunk which updates the ``atime``
-   (access time) property. Filesystems are mounted with the ``relatime`` option
-   by default. This results in a better performance by only updating the
-   ``atime`` property if the last access has been at least 24 hours ago. The
-   downside is that touching a chunk within these 24 hours will not always
-   update its ``atime`` property.
-
-   Chunks in the grace period will be logged at the end of the garbage
-   collection task as *Pending removals*.
+The progress of the garbage collection will be displayed as shown in the example
+below:
 
 .. code-block:: console
 
diff --git a/docs/maintenance.rst b/docs/maintenance.rst
index e8a26d69c..bba3feff4 100644
--- a/docs/maintenance.rst
+++ b/docs/maintenance.rst
@@ -197,6 +197,8 @@ It's recommended to setup a schedule to ensure that unused space is cleaned up
 periodically. For most setups a weekly schedule provides a good interval to
 start.
 
+.. _gc_background:
+
 GC Background
 ^^^^^^^^^^^^^
 
@@ -222,17 +224,31 @@ datastore or interfering with other backups.
 The garbage collection (GC) process is performed per datastore and is split
 into two phases:
 
-- Phase one: Mark
-  All index files are read, and the access time of the referred chunk files is
-  updated.
-
-- Phase two: Sweep
-  The task iterates over all chunks, checks their file access time, and if it
-  is older than the cutoff time (i.e., the time when GC started, plus some
-  headroom for safety and Linux file system behavior), the task knows that the
-  chunk was neither referred to in any backup index nor part of any currently
-  running backup that has no index to scan for. As such, the chunk can be
-  safely deleted.
+- Phase one (Mark):
+
+  All index files are read, and the access time (``atime``) of the referenced
+  chunk files is updated.
+
+- Phase two (Sweep):
+
+  The task iterates over all chunks and checks their file access time against a
+  cutoff time. The cutoff time is given by either the oldest backup writer
+  instance, if present, or 24 hours and 5 minutes after the start of garbage
+  collection.
+
+  Garbage collection can consider chunk files with access time older than the
+  cutoff time to be neither referenced by any backup snapshot's index, nor part
+  of any currently running backup job. Therefore, these chunks can safeley be
+  deleted.
+
+  Chunks within the grace period will not be deleted and logged at the end of
+  the garbage collection task as *Pending removals*.
+
+.. note:: The grace period for backup chunk removal is not arbitrary, but stems
+   from the fact that filesystems are typically mounted with the ``relatime``
+   option by default. This results in better performance by only updating the
+   ``atime`` property if a file has been modified since the last access or the
+   last access has been at least 24 hours ago.
 
 Manually Starting GC
 ^^^^^^^^^^^^^^^^^^^^
-- 
2.39.5