Re: [RFC] Advanced XIP File System

["Josh Boyer" <jwboyer@gmail.com>]

On 5/2/06, Jared Hulbert <jaredeh@gmail.com> wrote:
> > Nice, this is the first time I heard of anyone using filemap_xip on MTD.
>
> Actually we don't use the MTD.  Well, we may use it to provide the
> physical address of the volume, thats not really _using_ it.

Can you explain a bit more?  If you aren't going to use the MTD, why
rely on it at all?

> >> - Design allows for tighter packing of data and higher performance
> >> than XIP cramfs
> >
> > why? by how much?
>
> Data packing:
> 1) When mkcramfs is writing files to the image it mixes compress and
> XIP files.  XIP files are page aligned.  Compressed files are not.  I
> think it was about 3.3% wasted I measured on an actual production
> linux phone.
> 2) Choosing page by page to XIP or compress means you can save space,
> but it depends on what is more precious, RAM or Flash, given real
> designs (ie - you don't buy 36MiB of RAM you get 32MiB or 64MiB)
>
> Performance:
> 1) The way we are storing the metadata should make for quicker access.
> 2) Being able to store specific pages of RO data from an otherwise XIP
> file such that they end up in RAM has speed thing up for us a great
> deal in the lab.

Have you done comparisons vs. squashfs at all?  It does better at both
performance and compression that cramfs, so I'm curious.

josh