NetCube-Systems-Austria/kumquat-buildroot
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

manual: add section about dealing efficiently with big image files

Browse Source 
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 <rjbarnet@rockwellcollins.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ryan Barnett <rjbarnet@rockwellcollins.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
This commit is contained in:
Yann E. MORIN 2014-01-07 23:46:08 +01:00 committed by Peter Korsgaard
parent f96e4d7f34
commit abd4124e5f
1 changed files with 39 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

39
docs/manual/common-usage.txt
View File

@ -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
pages.
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.