[pbs-devel] [PATCH v3 proxmox-backup 56/58] docs: add section describing change detection mode

[Fabian Grünbichler]

Quoting Christian Ebner (2024-03-28 13:37:05)
> Describe the motivation and basic principle of the clients change
> detection mode and show an example invocation.
> 
> Signed-off-by: Christian Ebner <c.ebner at proxmox.com>
> ---
> changes since version 2:
> - not present in previous version
> 
>  docs/backup-client.rst | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/docs/backup-client.rst b/docs/backup-client.rst
> index 00a1abbb3..e54a52bf4 100644
> --- a/docs/backup-client.rst
> +++ b/docs/backup-client.rst
> @@ -280,6 +280,39 @@ Multiple paths can be excluded like this:
>  
>      # proxmox-backup-client backup.pxar:./linux --exclude=/usr --exclude=/rust
>  
> +.. _client_change_detection_mode:
> +
> +Change detection mode
> +~~~~~~~~~~~~~~~~~~~~~
> +
> +Backing up filesystems with large contents can take a long time, as the default
> +behaviour for the Proxmox Backup Client is to read all data and re-encode it
> +into chunks. For some usecases, where files do not change frequently this is not
> +feasible and undesired.

Maybe

File-based backups containing a lot of data can

and maybe also add something along the lines of "before being able to decide
whether those chunks need uploading or are already available on the server."

> +
> +In order to instruct the client to not re-encode files with unchanged metadata,

re-read (as that is what gives us the performance bonus ;))

> +the `change-detection-mode` can be set from the default `data` to `metadata`.
> +By this, regular file payloads for files with unchanged metadata are looked up

s/By this/In this mode

do we want to explain "payload" here? or just use "contents" or "data"?

> +and re-used from the previous backup runs snapshot when possible. For this to

s/runs//

> +be feasible, the pxar archives for backup runs using this mode are split into

s/ runs/s/

> +two separate files, the `mpxar` containing the archives metadata and the `ppxar`
> +containing a concatenation of the file payloads.
> +
> +During backup, the current file metadata is compared to the one looked up in the

During the backup 

or maybe

When creating the backup archives, 

> +previous `mpxar` archive, and if unchanged, the payload of the file is included
> +in the current backup by referencing the indices of the previous snaphshot. The

by reusing the chunks containing that payload in the previous snapshot.

> +increase in backup speed comes at the cost of a possible increase of used space,
> +as chunks might only be partially reused, containing unneeded padding. This is
> +however minimized by selectively re-encoding files where the padding overhead
> +does not justify a re-use.

nit: reused is not hyphenated consistenly

> +
> +The following shows an example for the client invocation with the `metadata`
> +mode:
> +
> +.. code-block:: console
> +
> +    # proxmox-backup-client backup.pxar:./linux --change-detection-mode=metadata
> +
>  .. _client_encryption:
>  
>  Encryption
> -- 
> 2.39.2
> 
> 
> 
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