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Appendix
The Journalling Flash File System
There are two different types of memory available on the dimmPCI CPU module, SDRAM and
flash memory. The SDRAM is virtually the same as the RAM in a personal computer; anything
stored in this memory will be lost when the power is turned off. This is also referred to as
volatile memory. The flash memory is analogous to the hard drive in your computer since it
retains stored data even when the computer is turned off. This is called non-volatile memory.
The journaling flash file system (JFFS) is an interface that allows data to be written/read to/from
the flash memory randomly. It arranges files in the memory, keeps track of how large they are and
where they are located etc.
But JFFS is not the only thing that uses flash memory; along with the JFFS there exists the kernel
and the read-only file system. Basically the kernel is the operating system, it handles memory
allocation, multi-tasking, etc. It is located in the lowest portion of the flash memory. Stored after
the kernel in flash is the read-only file system. It contains configuration scripts and user tools,
such as ‘cp’, ‘ls’, ‘pwd’ and so on. The remaining flash memory has been allocated for the JFFS,
depending on the size of the kernel and read-only file system this can range up to 1.0 Mbytes in
size.
In order to have access to this memory, the kernel must have been compiled with JFFS enabled.
Next the JFFS needs to be mounted on the JFFS on your root file system. The default location is
to mount it under the ‘/usr’ directory. So the command would be:
/bin/mount –t jffs /dev/flash0 /usr
After executing this command on the dimmPCI console, the ‘/usr’ directory will be a nonvolatile
filesystem with read/write capability. Any data stored here will survive turning the power off.
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