PersistentRAMFileSystem < DULG

%SECTION0{name=FilesystemPRAMFS}% 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=.
                                                                                
%SECTION1{name=MountParameters}% 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.

%SECTION1{name=Example}% 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):

<verbatim>
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# 
</verbatim>

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:

<verbatim>
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
</verbatim>

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:

<verbatim>
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#
</verbatim>
9.7. Overlay File Systems		1. Abstract		10. Debugging
Prev		Home		Next