[Simh] SIMH Development

[Rob Jarratt]

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.

 

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.

 

Does anyone think they would find this useful?

 

Regards

 

Rob

 

From: simh-bounces at trailing-edge.com [mailto:simh-bounces at trailing-edge.com]
On Behalf Of Stephen Hoffman
Sent: 01 November 2009 17:43
To: simh at trailing-edge.com
Subject: Re: [Simh] SIMH Development

 

In case anyone is interested I am adding a differencing disk feature that
allows you to store only the changes to a virtual disk file, so you can
rollback to a known state, or keep the differences from a base image in a
smaller file, like undo disks in Virtual PC.

 

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?   

 

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?

 

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.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.trailing-edge.com/pipermail/simh/attachments/20091101/b2495d22/attachment-0003.html>