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Date:      Sat, 9 Jun 2001 11:17:24 -0700 (PDT)
From:      Matt Dillon <dillon@earth.backplane.com>
To:        Terry Lambert <tlambert2@mindspring.com>
Cc:        patl@Phoenix.Volant.ORG, Peter Wemm <peter@wemm.org>, FreeBSD-FS@FreeBSD.ORG, FreeBSD-Arch@FreeBSD.ORG
Subject:   Re: nullfs fixes [Was: Support for pivot_root-like system call?]
Message-ID:  <200106091817.f59IHO108472@earth.backplane.com>
References:  <ML-3.4.992018636.5562.patl@asimov.phoenix.volant.org> <3B225E32.484672C@mindspring.com>
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:It will _NOT_ work for cryptographic FS's, where the number
:of pages or the page boundaries change witing a file,
:compressing FS's, FS's with different block sizes (such as
:...
:-- Terry

    Woa... sure it will!  The VM Object is representing the
    clear text file, not the actual underlying crypted file.  So
    it should work just fine. 

    Also, in the filespace the physical sector size is irrelevant.
    Take the VN (now MD) device for example.  If you use a file for
    backing store the sector size from the point of view of the MD
    device can be 512 bytes no matter what the actual sector size of
    the filesystem backing the file is.  But if you use, say, swap
    for backing store the sector size from the point of view of
    the MD device is one page (4K on IA32), period end of story.

    Also, even in UFS the physical topology of a file varies... a
    file might be partially represented by a fragment (e.g. 1K) as
    well as a full file block (8K) in the physical topology.  But
    the VM Object representing the file doesn't know and doesn't care.

    So VM Objects for files are 100% independant of the topology
    of the filesystem backing those files.  This means that the fixed
    NULLFS should work just dandy for any filesystem.  It may not be
    able to overload devices transparently, but it should definitely
    be able to overload a filesystem.

					    -Matt


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