-Valid values for <backup-type> are `cpio' (uses cpio to produce
-tar-format backups), `dump' (uses dump to dump entire filesystems;
-<directory name> should be a mount-point for this), and `zafio' (uses
-afio to compress each file as it is backed up). Only `dump' type
-backups perform incremental backups.
+Valid values for <backup-type> are
+ cpio
+ uses cpio to produce tar-format backups
+ dump
+ uses dump to dump entire filesystems
+ <directory name> should be a mount-point
+ gtar
+ uses GNU tar to produce GNU tar format backups and -N-based
+ semi-incrementals (not --incremental or --listed-incremental)
+ zafio
+ uses afio to compress each file as it is backed up
+ ntfsimage
+ for NTFS volumes, requires device name
+Only `dump' and `gtar' type backups perform any kind of incremental
+backups.
+
+<options> is a comma-separated list of <option> or <option>=<value>.
+Options supported:
+
+ gz[i][=<compressionlevel>]
+ Indicates that the whole stream should be compressed with gzip.
+ The compression level defaults to 1 if gz is specified by the
+ level isn't. gzi appliies only to the incrementals; gz applies to
+ both unless gzi is also specified. compression level 0 means not
+ to run gzip at all and is the default if gz[i] is not mentioned.
+
+ snap=<snapkind>
+ Indicates that the filesystem should be frozen before the backup
+ by using /etc/chiark-backup/snap/<snapkind>. See the head comment
+ in /etc/chiark-backup/snap/lvm for details of how this works.
+ When snap= is used, the block device must be specified.
+
+ noinc
+ Suppress incrementals.