Re: Mounting a in-ROM filesystem efficiently

H. Peter Anvin (hpa@zytor.com)
13 Dec 2001 12:38:28 -0800

Followup to: <20011213160007.D998D23CCB@persephone.dmz.logatique.fr>
By author: Thomas Capricelli <orzel@kde.org>
In newsgroup: linux.dev.kernel
>
> Hello,
>
> I'm looking for a way to put a filesystem into ROM.
> Seems pretty trivial, isn't it ?
>
> My understanding is (the way initrd does, and the way I do as of today)
> * create a RAMDISK
> * loads the data into ramdisk
> * mount the ramdisk
>
> problem is that I don't want to waste the RAM as the data in the ROM is
> already in the address space. (it's an embedded system, btw)
>
> Speed is not an issue here. ROM access might be slower than RAM, it will
> always be so much quicker than a disk access. (wrong?)
>

Frequently wrong. Depending on the interface, ROM can be just
unbelievably slow.

> Ideally, i would give address/length of the fs in ROM to a function, and I
> would get a ramdisk configured to read its data exactly there, and not in
> ram.

The right thing for you to do is to write a block device driver, and
then mount that block device like any order device. Your in-use data
will be copied to RAM (i.e. cached), but it can be dropped and
re-fetched as necessary. This should be the desired behaviour.

-hpa

-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt	<amsp@zytor.com>
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