Specifications

9.6. Root File System Selection
Now we know several options for file systems we can use, and know how to create the corresponding images.
But how can we decide which one to chose?
For practical purposes in embedded systems the following criteria are often essential:
boot time (i. e. time needed from power on until application code is running)•
flash memory footprint•
RAM memory footprint•
effects on software updates•
The following data was measured for the different configurations. All measurements were performed on the
same TQM860L board (MPC860 CPU at 50 MHz, 16 MB RAM, 8 MB flash, 256 MB CompactFlash card):
File System Type Boot Time Free Mem Updates while running
ramdisk 16.3 sec 6.58 MB whole image yes
JFFS2 21.4 sec 10.3 MB per file only non-active files
cramfs 10.8 sec 10.3 MB whole image no
ext2 (ro) 9.1 sec 10.8 MB whole image no
ext2 on CF (ro) 9.3 sec 10.9 MB whole image no
File on FAT fs 11.4 sec 7.8 MB whole image yes
As you can see, the ramdisk solution is the worst of all in terms of RAM memory footprint; also it takes a
pretty long time to boot. However, it is one of the few solutions that allow an in-situ update while the system
is running.
JFFS2 is easy to use as it's a writable file system but it takes a long time to boot.
A read-only ext2 file system shines when boot time and RAM memory footprint are important; you pay for
this with an increased flash memory footprint.
External flash memory devices like CompactFlash cards or USB memory sticks can be cheap and efficient
solutions especially when lots of data need to be stored or when easy update procedures are required. -
9.7. Overlay File Systems
Introduction
Overlay File Systems provide an interesting approach to several frequent problems in Embedded Systems. For
example, mini_fo is a virtual kernel file system that can make read-only file systems writable. This is done
by redirecting modifying operations to a writeable location called "storage directory", and leaving the original
data in the "base directory" untouched. When reading, the file system merges the modifed and original data so
that only the newest versions will appear. This occurs transparently to the user, who can access the data like
on any other read-write file system.
9.7. Overlay File Systems 126