feat: create sparse images

[ygerlach]

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Description

Instead of allocating the whole image on the disk, only allocate the parts that actually contain data. This reduces the required disk space to store or create images. The image does appear to have the normal size but if inspected with du or ls -lsh the difference becomes apparent.

Example with the image i build:

$ ls -lsh
total 244G
234G -rw-r--r-- 1 root root 235G Nov 20 17:30 image-x86_64-mender_old.img
 11G -rw-r--r-- 2 root root 235G Nov 20 17:54 image-x86_64-mender_sparse.img

This is done by the conv=sparse flag of dd. Alternatively a file can made sparse with fallocate -d <file>. But this may only collapse a file, not reduce its footprint while it is created. This may also improve read and write performance of those files in the empty areas, but i did not observe a big speed-up during the image creation.

[mender-test-bot]

Merging these commits will result in the following changelog entries:

Changelogs

mender-convert (sparsesImage)

New changes in mender-convert since master:

Features

• create sparse images