Subject: Re: LKM support
To: None <>
From: Perry E. Metzger <>
List: current-users
Date: 11/10/1996 09:44:07
"John S. Dyson" writes:
> One minor clarification -- MFS allocates VIRTUAL MEMORY, or normal
> process memory space.  So, that space is backed by swap, and can
> (with dubious effects) be paged out.  You really want to avoid
> paging your MFS, but it can/will be under tight memory conditions.
> Actually, I think of the paged MFS as having an advantage over a pure
> ramdisk (that takes up wired memory all of the time.)  When memory
> does get tight, that allocated space for metadata, and unused files
> is pushed out to swap space...  The key is to NOT take advantange
> of its pageability to excess.

I agree that it is good to have an MFS paged out.

I also agree with Chris, however, that the design is suboptimal.

Sun's equivalent of MFS has lots of nice features -- automatically
grows and shrinks (obviously, as everyone notes, you want to be able
to configure a cap), etc. Just the fact that the file blocks have to
be copyin'ed/copyout'ed seems like a lose to me.

The characteristics I think one really wants are:

1) No fixed size, just an ability to set the maximum.
2) Data structures for "file" storage that take advantage of the fact
   that you are in RAM, not on disk.
3) pageable if necessary
4) Efficient storage; minimal number of memory copies needed to get to
   the bits.