ubifs overhead

[Daniel Drake]

Hi,

Building on some data produced earlier I've run a "more scientific"
test of determining the exact overhead of ubifs on XO-1, over jffs2.

I build 2 OS images from the same package set, one using jffs2, and
one using the jffs2 boot / ubifs root setup used in development
builds.
I installed the ubifs one on an XO-1, copied over a 50mb binary file
as many times as I could into /root, and counted the total amount of
data written before hitting disk full.

Then I did the same test (same 50mb file) on the same laptop but using
the jffs2 image.

Results are:

ubifs: I could write 309mb
jffs2: I could write 382mb

382-309 = 73

However, the ubifs layout includes a 24mb jffs2 boot partition which
must be excluded for the purpose of calculating the overhead of the
filesystem switch. 73-24=49mb
(this calculation loses a bit of accuracy as with compressed
filesystems you can often store more than 24mb of data in 24mb of disk
space, but as we're only talking 24mb this calculation can only be a
few mb "wrong" at the most)

Conclusion: the overhead of using ubifs instead of jffs2 on XO-1 is around 50mb
(That ignores the additional overhead through our choice of
partitioned layout and duplication boot partition contents on root
too, and focuses exclusively on the overhead caused by the fs switch
alone)

Daniel