tlug: Re: root mirror (was: LILO Vs. 1024??)

["Stephen J. Turnbull" <turnbull@example.com>]

>>>>> "Rex" == Rex Walters <rex@example.com> writes:

    Rex> On Wed, Oct 07, 1998 at 12:39:34PM +0900, Stephen J. Turnbull
    Rex> wrote:
    >> Paranoid structure:
    >> 
    >> /dev/?da1  50 MB / 
    >> /dev/?da2 128 MB <swap>
    >> /dev/?da3  50 MB /.root_mirror

    Rex> I also recommend this, but I caution against the word
    Rex> "mirror".  I prefer the word "clone" or "backup".  That is I
    Rex> don't think you're really referring to raid1 mirroring, are

No, I'm not a physical FS kind of guy, I'm a logical FS (eg NFS, AFS,
URL ;-) kind of guy.  I'm talking about the kind of mirror that the
mirror script does.  I don't think of it as a "backup", because that
implies to me offline/hard to access.  "Clone" would be OK.

    Rex> Out of curiosity, how are you populating the /.root_mirror
    Rex> filesystem?

1.  Insert distribution CD, install to /dev/hda1.
2.  When I feel like it, mount /dev/hda3, and do

	      cp -options `find / -xdev` /.root_mirror/

or something like that.

I don't need to be _that_ paranoid.  "When I feel like it" = when
/etc/passwd changes (very inconvenient for other users if I miss
this), or when I install new packages.

    Rex> My approach would probably be to unmount it, dd from ?da1 to
    Rex> ?da3 when you "know" (hmmphf) nothing is being modified in /,
    Rex> then fsck ?da3 to be sure and remount.

Too much trouble; I just do a sync, diff, umount, and fsck.

I don't need a true clone, just something that's close enough to get
rolling again; the housekeeping data that dpkg and rpm et al keep is
in /var, and there's very little in /etc that would _kill_ me if I
corrupted a file.

I use backups (as defined above) to deal with /var and /home, and try
hard to avoid polluting /usr and /playpen with locally generated
data.  As long as I've still got /, I've still got the localhost
hardware services (eg, I only have one floppy tape drive, should I
ever need it) and can get to the net for other hosts.

    Rex> The only thing that's missing to be able to accomplish the
    Rex> same thing with a Linux system is the ability to "clone" an
    Rex> ext2 filesystem reliably.  On an Auspex, ax_clonefs will
    Rex> ensure that any modifications that occur while the copy is
    Rex> running are handled correctly (there is actually a brief
    Rex> lockout at the very end to ensure that all metadata is
    Rex> synched on both sides correctly).

That sounds like a very hard problem without designing the fs around
that requirement from the ground up.

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