Re: Idea about a disc backed ram filesystem

[Horst von Brand <vonbrand@inf.utfsm.cl>]

Sascha Nitsch <Sash_lkl@linuxhowtos.org> wrote:
> this is (as of this writing) just an idea.
> 
> === current state ===
> Currently we have ram filesystems (like tmpfs) and disc based file systems
> (ext2/3, xfs, <insert your fav. fs>).

Right.

> tmpfs is extremely fast but suffers from data losses from restarts, crashes
> and power outages.

Part of the design tradeoffs.

>                    Disc access is slow against a ram based fs.

On-disk filesystems (and block device handling) are designed around that
fact.

> === the idea ===
> My idea is to mix them to the following hybrid:
> - mount the new fs over an existing dir as an overlay
> - all files overlayed are still accessible
> - after the first read, the file stays in memory (like a file cache)
> - all writes are flushed out to the underlying fs (maybe done async)
> - all reads are always done from the memory cache unless they are not cached
>   yet
> - the cache stays until the partition is unmounted
> - the maximum size of the overlayed filesystem could be physical ram/2 (like tmpfs)

But the current on.disk filesystems use caching of data in RAM extensively,
/without/ having to keep the whole file in memory, just the pieces
currently in active use. Your proposal negates the RAM for caches, so it
would be much /slower/ than the current on-disk filesystems.

BTW, many of the live-CD distributions do exactly this (RAM overlay over a
CD-based filesystem).
-- 
Dr. Horst H. von Brand                   User #22616 counter.li.org
Departamento de Informatica                     Fono: +56 32 654431
Universidad Tecnica Federico Santa Maria              +56 32 654239
Casilla 110-V, Valparaiso, Chile                Fax:  +56 32 797513
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