diff options
authorYann E. MORIN <>2014-01-07 22:46:08 (GMT)
committerPeter Korsgaard <>2014-01-09 20:26:36 (GMT)
commitabd4124e5fcae93337d755ea5e913368d1c3efa1 (patch)
parentf96e4d7f348ce1b7b53763f102bbedb1ef6a1991 (diff)
manual: add section about dealing efficiently with big image files
As reported by Ryan, it is not well-known that most tools can deal efficiently with big sparse files. Add a section in the manual about this, with tar and cp used as examples, and a hinting to the man pages for the others. Reported-by: Ryan Barnett <> Signed-off-by: "Yann E. MORIN" <> Cc: Ryan Barnett <> Cc: Peter Korsgaard <> Cc: Thomas De Schampheleire <> Signed-off-by: Peter Korsgaard <>
1 files changed, 39 insertions, 0 deletions
diff --git a/docs/manual/common-usage.txt b/docs/manual/common-usage.txt
index c7e4568..1d15c05 100644
--- a/docs/manual/common-usage.txt
+++ b/docs/manual/common-usage.txt
@@ -104,3 +104,42 @@ or +g+++ for building helper-binaries on your host, then do
$ make HOSTCXX=g++-4.3-HEAD HOSTCC=gcc-4.3-HEAD
+Dealing efficiently with filesystem images
+Filesystem images can get pretty big, depending on the filesystem you choose,
+the number of packages, whether you provisioned free space... Yet, some
+locations in the filesystems images may just be _empty_ (eg. a long run of
+'zeroes'); such a file is called a _sparse_ file.
+Most tools can handle sparse files efficiently, and will only store or write
+those parts of a sparse file that are not empty.
+For example:
+* +tar+ accepts the +-S+ option to tell it to only store non-zero blocks
+ of sparse files:
+** +tar cf archive.tar -S [files...]+ will efficiently store sparse files
+ in a tarball
+** +tar xf archive.tar -S+ will efficiently store sparse files extracted
+ from a tarball
+* +cp+ accepts the +--sparse=WHEN+ option (+WHEN+ is one of +auto+,
+ +never+ or +always+):
+** +cp --sparse=always source.file dest.file+ will make +dest.file+ a
+ sparse file if +source.file+ has long runs of zeroes
+Other tools may have similar options. Please consult their respective man
+You can use sparse files if you need to store the filesystem images (eg.
+to transfer from one machine to another), of if you need to send them (eg.
+to the Q&A team).
+Note however that flashing a filesystem image to a device while using the
+sparse mode of +dd+ may result in a broken filesystem (eg. the block bitmap
+of an ext2 filesystem may be corrupted; or, if you have sparse files in
+your filesystem, those parts may not be all-zeroes when read back). You
+should only use sparse files when handling files on the build machine, not
+when transferring them to an actual device that will be used on the target.