Re: ramdisk/tmpfs/ramfs/memfs ?

[Padraig Brady]

David Woodhouse wrote:
> 
> [e-mail] said:
> > btw I get my initial root filesystem from a compact flash that can be
> > accessed just like a hardisk. It's writeable also like a harddisk, but
> > we boot with it readonly, and only mount it rw if we want to save
> > config or whatever. We definitely wouldn't swap to it as it has
> > limited erase/write cycles. The filesystem is compressed ext2.
> 
> Why copy it into RAM? Why not use cramfs and either turn the writable
> directories into symlinks into a ramfs which you create at boot time, or
> union-mount a ramfs over the top of it?

? I never said I copied it into RAM. The ramfs is just used for /tmp &
/var
which are symlinks on the flash. I didn't know you could read/write
cramfs
like ext2/JFFS2? I still want to be able to update/create/delete files
like a normal ext2 partition. Oh I see the confusion, sorry my fault,
when
I said the root filesystem is compressed ext2, I meant it's ext2 with
chattr +c done on all files (the e2compr patch implmenents it). By the
way why isn't e2compr a standard part of the kernel. I've have no
problems
at all with it.

I didn't know about union-mounting, seems useful.

As for whether to use tmpfs/ramfs I think they do basically the same
thing 
only the implementation is different. tmpfs uses shared mem and so is 
probably more portable than ramfs which it tightly coupled with VFS. I
think I'll use ramfs (with limits patch) so.

> [e-mail] said:
> > As for using JFFS2 + MTD ramdisk intead of ext2+e2compr+ramdisk is not
> > an option as the only advantage would be journalling, you still can't
> > resize. IMHO JFFS is only required where you have flash without an IDE
> > interface.
> 
> True. We are currently lacking a compact, compressing, journalling
> filesystem for use on block devices. It's been suggested that we could make
> JFFS2 work on them, by making a fake MTD device which uses a block device
> as backing store. Nobody's yet shown me the code though :)

How much more efficent is JFFS than say ext3+e3compr, wrt:
logic size/logic speed/RAM requirements/filesystem structure size.

> --
> dwmw2

Padraig.