FlashDisk vs RamDisk+FlashDisk partitions

[Adi Linden]

Hi Andrei,

PeeWeeLinux supports different ways of loading a target device. This
includes having a single large partition on the flash drive (ext2 or FAT)
that contains the kernel and ramdisk image (compressed filesystem). In
this arrangement there is no easy way to save anything, though.

Another alternative (already in PeeWeeLinux) is to have an uncompressed
filesystem on flash which is loaded into memory and run from a ramdisk.
This allows for the flash to be mounted (read-write) and changes to be
made that will take effect once the system is rebooted.

Lastly there is a mixed approach. Quite close to your #1 suggestion. Have
a read-only root partition on flash. Load /etc, /dev/ and /var into 
ramdisks. The reason for /etc is that the system needs write access to
some files in /etc, the reason for /dev is that in order to write to
devices the partition the node is created on needs to be writable. And
/var you've explained already. Depending on how the /etc branch is created
it could be saved to flash by mounting the appropriate partition
read/write.

Future kernel releases will include devfs and reiserfs or ext3. devfs
allows for a virtual device filesystem, similar to how /proc works. This
would eliminate the need for a /dev ramdisk and preserve a moderate amount
of memory and flash since each device currently consumes one inode on the
filesystem it resides on.

You can corrupt flash if you remove power during a write operation.
Apparently ext3 and reiserfs would automatically recover and fall back to
a last known good state. Now that's my understanding based on rumours
rather than research on my part.

The read-only root filesystem with ramdisks for /etc, /var and /dev
that's implementedin PeeWeeLinux works but it could use some more fine
tuning, I think.

TTYL,
Adi

  

On Fri, 5 Jan 2001, Andrei Bulucea wrote:

> Happy New Year everybody! 
> 
> While reading your postings I noticed  some
> messges related to fsck/journaling filesystem
> implementations for PeeWeeLinux. 
> 
> How about having the folowing ramdisk/flashdisk arrangement
> thus avoiding running fsck based on the assumption that
> fsck gets invoked due to a *modified* filesystem, not corruption,
> (CF doesn't get corrupted on power off)
> 
> 1) I think it is quite possible to have your / partition
> ,read only, mounted on the FlashDisk and something like
> /var mounted on a ramdisk. So your endless logging and 
> 'scratchpad area' will be volatile, therefore each time the 
> machine crashes or is powered off, /var will be lost, but no 
> changes will be recorded on "/" filesystem, therefore fsck 
> will not be invoked.
> 
> 2) Compress the entire filesystem into the FlashDisk and
> then load it into RamDisk. This consumes more RAM, but it 
> seems to be the fastest, however costly due to the fact 
> that the compression radio is 1:3 and CF/RAM price ratio 
> is only 1:2...  
> 
> Am I missing something? I prefer #1, Which one would you choose?
> 
> Best Regards and thank you in advance,
> Andrei
> 
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