Allow to override part of the backup filename

doc/raketasks/backup_restore.md

@@ -195,6 +195,17 @@ To use the `copy` strategy instead of the default streaming strategy, specify
 sudo gitlab-rake gitlab:backup:create STRATEGY=copy
 ```
 
+### Backup filename
+
+By default a backup file is created according to the specification in [the Backup timestamp](#backup-timestamp) section above. You can however override the `[TIMESTAMP]` part of the filename by setting the `BACKUP` environment variable. For example:
+
+```sh
+sudo gitlab-rake gitlab:backup:create BACKUP=dump
+```
+
+The resulting file will then be `dump_gitlab_backup.tar`. This is useful for systems that make use of rsync and incremental backups, and will result in considerably faster transfer speeds.
+
+
 ### Excluding specific directories from the backup
 
 You can choose what should be exempt from the backup up by adding the environment variable `SKIP`.

lib/backup/manager.rb

@@ -234,7 +234,11 @@ module Backup
     end
 
     def tar_file
-      @tar_file ||= "#{backup_information[:backup_created_at].strftime('%s_%Y_%m_%d_')}#{backup_information[:gitlab_version]}#{FILE_NAME_SUFFIX}"
+      if ENV['BACKUP']
+        @tar_file ||= ENV['BACKUP'] + "#{FILE_NAME_SUFFIX}"
+      else
+        @tar_file ||= "#{backup_information[:backup_created_at].strftime('%s_%Y_%m_%d_')}#{backup_information[:gitlab_version]}#{FILE_NAME_SUFFIX}"
+      end
     end
 
     def backup_information