mfs and vnconfig questions out of curiosity

[Oliver Fromme]

Andy Smith <andy at strugglers.net> wrote:
 > So you can make a swap-backed filesystem in -STABLE with vnconfig
 > -S, but why would you ever want to do this?  Why is this preferred
 > over just using a regular file?

Upon unmount of the FS, the regular file will stay there,
so you have to remove it yourself.  A swap-backed FS will
disappear completely upon unmount.

But more importantly, a swap-backed FS is more efficient,
because you don't have all the overhead of the filesystem
which contains the regular file.

 > Then there is malloc-backed filesystem as created with mfs in
 > STABLE.  Presumably this works with virtual memory and lazy
 > allocation the same way as malloc() from a program would do, i.e.
 > it is possible to create an FS that is bigger than the amount of
 > physical memory in the system, and whenever files are stored in the
 > FS it is similar to any other request to the VM system, may be
 > satisfied with real memory or go into swap?

Yes, that's correct.  Basically, the file system data is
contained in the process image of the mount_mfs process.
You can see it in "ps" and "top":

top:
32 root      10   0   101M 76416K mfsidl   0:29  0.00%  0.00% mount_mfs

df -k:
mfs:32        100750  254 100496     0%    /tmp

Regards
   Oliver

-- 
Oliver Fromme, secnetix GmbH & Co KG, Oettingenstr. 2, 80538 München
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