mfs and vnconfig questions out of curiosity
olli at lurza.secnetix.de
Tue Apr 20 07:48:38 PDT 2004
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":
32 root 10 0 101M 76416K mfsidl 0:29 0.00% 0.00% mount_mfs
mfs:32 100750 254 100496 0% /tmp
Oliver Fromme, secnetix GmbH & Co KG, Oettingenstr. 2, 80538 München
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