I wrote:
> Paul Moody's mini-HOWTO on Embedded Linux at:
> http://users.bigpond.com/paulmoody/Mhow11c.html
> gives one approach to using a Flash memory drive with Linux, by loading the
> contents into a ramdisk at boot time.
>
> RAM is cheaper than Flash, so for the most part this makes sense. However, if
> you're stuck with less RAM than Flash, or if you're using a real hard drive,
> it may make more sense to mount the root filesystem from the drive read-only.
paulmoody replied:
> Hmmmmmmmmmm yes there are a myriad of configurations that can be implemented
> to produce a more robust system design and though the idea of a read only
> harddrive approach has merit in preventing corruption of filesystems during
> power failures or accidental erasures it was not the sole benefit to be
> obtained from a ramdisk / flash system.
> Briefly my design goals as follows ...
> No Mechanical parts - 1000 G rating ( vibration ).
> Small - It all fits on one card
> Low power - No fans at all in our micro servers. ( 5 - 8 watts total )
> Filesystem cannot be ( permanently ) corrupted ( its all in RAM )
> If critical processes / file CRC failure watchdog reboots.
> Largish capacitor on power supply keeps system up for 500 ms ... time
> enough to save critical data to flash.
> Flash can be reformatted / reloaded totally ( hoping power does not fail
> :-) )
> Flash can be removed whilst system up ( again hoping :-) )
> Flash a darn sight more convenient ( we use flash cards )
> File read / writes ( being ram based ) are ultra fast.
Certainly I agree that the best solution depends on the individual system's
design goals. Mounting the root file system read-only and mounting a RAM-disk
on /var is another way to meet the above goals, with the extra benefit of
saving RAM because the root file system doesn't need to be copied to the
RAM-disk. Of course, this means that the stuff on the flash drive either must
be stored uncompressed, or a compressed filesystem used. I've opted for the
former, since it's easier and my application all (just!) fits in the flash
drive uncompressed.
Regards,
Graham
--
Graham Stoney, Senior Hardware/Software Engineer
Canon Information Systems Research Australia
Ph: + 61 2 9805 2909 Fax: + 61 2 9805 2929
