Document that old backups are kept if rsync fails

README.md

@@ -49,6 +49,11 @@ For Arch Linux and Arch-based distros, two packages are available in the AUR (au
 - **simple_backup** for the release version
 - **simple_backup-git** for the git version
 
+## Old backups
+The number of old backups to keep can be set with the --keep (or -k) argument (or in the configuration file).
+Old backups are only removed if rsync completes successfully, so that they are not deleted when the new backup may be incomplete.
+The only rsync error that is ignored is return code 24 (source files vanished during the transfer), which is common when backing up a live system.
+
 ## Remote backup
 > **Warning**
 > This feature is experimental

man/simple_backup.1

@@ -61,6 +61,10 @@ will not be copied. Multiple elements can be specified, in the same way as for t
 .B \-k, \-\-keep N
 Specify how many old backups (so excluding the current one) will be kept. The default behavior
 is to keep them all (same as N=\-1).
+Old backups are only removed if rsync completes successfully, to avoid deleting them when the
+new backup may be incomplete. rsync return code 24 (source files vanished during the transfer)
+is treated as success, while any other error (including return code 23, partial transfer due to error)
+will prevent old backups from being removed. This does not apply when using \-\-remove\-before\-backup.
 .TP
 .B \-u, \-\-user USERNAME
 Explicitly specify the user running the backup (in case it is needed for home directory expansion).