Specifications

Reading from files, creating new files, modifying already modified files
These operations are passed directly through to the lower native layer, and only impose an overhead
of 1-2%.
2.
Further information
This section discusses how the mini_fo overlay file system can be used in embedded systems. More general
information is available at the mini_fo project page: http://www.denx.de/wiki/Know/MiniFOHome.
9.8. The Persistent RAM File system (PRAMFS)
The pramfs file system supports persistent memory devices such as SRAM. Instead of having a block
emulation layer over such a memory area and using a normal file system on top of that, pramfs seeks to
induce minimal overhead in this situation. Most important in this respect is that the normal block layer
caching of the Linux kernel is circumvented in pramfs.
9.8.1. Mount Parameters
The most important parameters for normal usage are
physaddr: The physical address of the static memory.•
init: When given, it will initialize the file system to that size.•
9.8.2. Example
We will show a sample usage of pramfs in this section using normal DRAM on a board with at least 256MB
of memory. For pramfs we reserve the upper 32MB by appending mem=224M to the kernel command line.
First off we generate some testdata on a persistent file system (/tmp) to demonstrate that pramfs survives a
reboot (of course with power always applied to keep the DRAM refreshed):
bash-3.00# dd if=/dev/urandom bs=1M count=8 of=/tmp/testdata
8+0 records in
8+0 records out
bash-3.00#
Next we mount the 32MB that we reserved and initialize it to be 32MB in size and copy the testfile. A final
compare shows that the copy was indeed successful so we can reboot:
bash-3.00# mount -t pramfs -o physaddr=0xe000000,init=0x2000000 none /mnt
bash-3.00# cp /tmp/testdata /mnt
bash-3.00# cmp /tmp/testdata /mnt/testdata
bash-3.00# reboot
Having rebooted (using mem=224M on the kernel command line again of course) we mount the file system
but this time without the init parameter because it is preinitialized. We then check the contents again:
bash-3.00# mount -t pramfs -o physaddr=0xe000000 none /mnt
bash-3.00# ls /mnt
testdata
bash-3.00# cmp /tmp/testdata /mnt/testdata
bash-3.00#
9.8. The Persistent RAM File system (PRAMFS) 129