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I think it is a neat and useful option.<br><br>
Bruce C.<br><br>
At 12:36 PM 11/1/2009, Rob Jarratt wrote:<br>
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It is a runtime mechanism. Before booting the OS in SIMH you attach the
differencing disk. Then during operation the differencing disk implements
a copy-on-write mechanism, so that all changes to the underlying disk are
stored in a separate file. This is useful (to me at least) because I can
experiment with changes non-destructively (I could copy the whole disk
image of course, but prefer to manage a small differences file instead),
I can keep multiple different configurations of the same underlying image
just by using a different differences file, and when using online backup
I only have to backup the underlying image file once, then the
(hopefully) smaller difference files can be backed up more quickly.<br>
<br>
I am sure that VMS has stuff like this, although I don’t anything about
these things in VMS, but this does not apply to older versions of VMS and
other historic operating systems, and it works independently of the
actual OS. If you don’t use it the overhead is negligible.<br>
<br>
Does anyone think they would find this useful?<br>
<br>
Regards<br>
<br>
Rob<br>
<br>
<b>From:</b> simh-bounces@trailing-edge.com
[<a href="mailto:simh-bounces@trailing-edge.com" eudora="autourl">
mailto:simh-bounces@trailing-edge.com</a>] <b>On Behalf Of </b>Stephen
Hoffman<br>
<b>Sent:</b> 01 November 2009 17:43<br>
<b>To:</b> simh@trailing-edge.com<br>
<b>Subject:</b> Re: [Simh] SIMH Development<br>
<br>
<dl>
<dd>In case anyone is interested I am adding a differencing disk feature
that<br>
<dd>allows you to store only the changes to a virtual disk file, so you
can<br>
<dd>rollback to a known state, or keep the differences from a base image
in a<br>
<dd>smaller file, like undo disks in Virtual PC.<br><br>
</dl> <br>
So that I might understand this, what you're offering here is analogous
to an (off-line) diffutils (diff and patch, et al) for Windows and that's
tailored for big wads of disk data? <br>
<br>
Or is this differencing mechanism implemented at run-time from some known
checkpoint where the data is consistent, where you're running some sort
of tracing and checkpointing akin to what OpenVMS host-based shadowing
and its minicopy or minimerge provides?<br>
<br>
FWIW, with OpenVMS VAX, it's feasible to do this sort of
known-starting-state task using a known install image and a real or
host-based InfoServer disk service.<br>
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<br>
Mr. Bruce Claremont, Software Preservationist<br>
Migration Specialties International, Inc.<br>
217 West 2nd Street, Florence, CO 81226-1403<br>
Bruce.Claremont@MigrationSpecialties.com<br>
<a href="http://www.migrationspecialties.com/" eudora="autourl">
www.MigrationSpecialties.com<br>
</a>+1 719-371-1711, Fax: +1 888-854-3417 <br><br>
Founded in 1992, Migration Specialties offers modern solutions for legacy
hardware & software. Look to us for VAX, Alpha, HP1000, PDP-11,
and DG Nova & Eclipse replacement options. Visit
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www.MigrationSpecialties.com</a> for more information.</body>
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