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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:

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 up while running
ramdisk 16.3 sec 6.58 MB whole image yes
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
JFFS2 21.4 sec 10.3 MB per file only non-active files

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.

9.5.6. Root File System in a Read-Only File in a FAT File System 1. Abstract 9.7. Overlay File Systems
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