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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>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.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>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.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Does anyone think they would find this useful?<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Regards<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Rob<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:
"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'> simh-bounces@trailing-edge.com
[mailto:simh-bounces@trailing-edge.com] <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<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'>
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<p class=MsoNormal>In case anyone is interested I am adding a differencing disk
feature that<br>
allows you to store only the changes to a virtual disk file, so you can<br>
rollback to a known state, or keep the differences from a base image in a<br>
smaller file, like undo disks in Virtual PC.<o:p></o:p></p>
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</blockquote>
<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>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? <o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>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?<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>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.<o:p></o:p></p>
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