mfs and vnconfig questions out of curiosity

From: Andy Smith (
Date: 04/17/04

  • Next message: Joan Picanyol i Puig: "Re: vinum configuration broken"
    Date: Sat, 17 Apr 2004 13:30:50 +0000

    Hi there,

    I haven't yet had any cause to use memory filesystems in FreeBSD,
    but I was just reading about them the other day and I came across
    references to malloc-backed and swap-backed.

    I have read the man page for vnconfig and mfs on -STABLE, and for
    mdconfig on -CURRENT, but I'm still a little confused and would
    appreciate some clarifications. So I have a few questions which I
    will try to keep relevant to -STABLE, although if the situation has
    significantly changed in -CURRENT then a commnent about that would
    be appreciated too..

    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?

    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?

    I understand that in -CURRENT, mdconfig replaces vnconfig for
    managing MFS that are either malloc-backed, swap-backed or
    file-backed, right?

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