Subject: Re: swapfs filesystem design (and mount/umount question)
To: Chuck Silvers <>
From: Simon Burge <>
List: tech-kern
Date: 03/20/2000 15:29:37
Chuck Silvers wrote:

> On Mon, Mar 20, 2000 at 12:18:03AM +1100, Simon Burge wrote:
> > Folks,
> > 
> > Here's some rough notes on how I think a swapfs filesystem should be
> > implemented from a layout POV.
> > 
> >  B- The filesystem is contained in one aobj.  This is split up into four
> >     parts:
> > 
> >     1) a bitmap for each page that is used
> >     2) a bitmap for each inode that is used
> >     3) an page map for the pages that contain inodes (see C below).
> >     4) the filesystem inodes and data
> you might consider making each of these types of data a separate aobj.
> if you put the inodes in an aobj (ie. one aobj contains all the inodes),
> then you don't need the "page map" (if I understand what that's supposed
> to be),

I've chopped and changed on multiple aobjs a couple of times (well, one
for data and one for the maps - but one for each map makes more sense if
I'm gonna split things up).  The maps themselves are relatively small
(roughly 0.2% of the total filesystem size).  With my tinkering I've
been using a 8192 page aobj, and the maps total just 17 pages.  Perhaps
it would be simpler to just malloc the maps instead of setting up aobjs
for them.  With say a gigabyte swapfs you'd then need just over two
megabytes of wired kernel memory - is this not an unreasonable size to
malloc for a filesystem?

> and you don't need the "si_number" in the swapfs_inode.
> I don't think you need the "si_dev" in each inode either, since that
> should be the same for all inodes in the filesystem.

I think we'll still need the si_number and si_dev so that when given a
pointer to a vnode we know where it comes from - remember that we need
to keep info that's usually spread across both in-memory and on-disk
structure on a "normal" filesystem in the one structure.  As I work out
more about filesystems this may change - for example I might be able to
fairly simply work out the inode number by the address of the inode.
For a couple of bytes per inode it may end up easier to leave things as
is.  Opinions from any filesystem gurus appreciated ;).