create --sparse, file map support for the "fixed" chunker, see #14

a file map can be:

- created internally inside chunkify by calling sparsemap, which uses
  SEEK_DATA / SEEK_HOLE to determine data and hole ranges inside a
  seekable sparse file.
  Usage: borg create --sparse --chunker-params=fixed,BLOCKSIZE ...
  BLOCKSIZE is the chunker blocksize here, not the filesystem blocksize!

- made by some other means and given to the chunkify function.
  this is not used yet, but in future this could be used to only read
  the changed parts and seek over the (known) unchanged parts of a file.

sparsemap: the generate range sizes are multiples of the fs block size.
           the tests assume 4kiB fs block size.

1 files changed, 4 insertions, 1 deletions

diff --git a/docs/usage/create.rst b/docs/usage/create.rst
index d8e00df21..4b3966a47 100644
--- a/docs/usage/create.rst
+++ b/docs/usage/create.rst
@@ -43,7 +43,10 @@ Examples
     $ borg create --chunker-params buzhash,10,23,16,4095 /path/to/repo::small /smallstuff
 
     # Backup a raw device (must not be active/in use/mounted at that time)
-    $ dd if=/dev/sdx bs=4M | borg create --chunker-params fixed,4194304 /path/to/repo::my-sdx -
+    $ borg create --read-special --chunker-params fixed,4194304 /path/to/repo::my-sdx /dev/sdX
+
+    # Backup a sparse disk image (must not be active/in use/mounted at that time)
+    $ borg create --sparse --chunker-params fixed,4194304 /path/to/repo::my-disk my-disk.raw
 
     # No compression (none)
     $ borg create --compression none /path/to/repo::arch ~