[Info-vax] Best way to get what I can off a failed VMS Disk?
Phillip Helbig undress to reply
helbig at asclothestro.multivax.de
Sat Oct 24 17:54:28 EDT 2020
In article <rn1qo9$oms$2 at dont-email.me>, Simon Clubley
<clubley at remove_me.eisner.decus.org-Earth.UFP> writes:
> > Would forming a shadow set be slower than an offline disk duplicator?
> >
>
> Will forming a shadow set cause random I/O all over the failing
> disk or will the disk be read exactly once from start to finish
> in strict sequential order without any additional I/Os ?
There are shadowing experts here who might reply. I believe it moves
sequentially through the source, writing to the target, and keeping
track of what portions were updated since the begin of the copy, then
after the initial pass updates those sections, and so on, until it
converges. I forget how large these sections are, but there is a bitmap
(used for MINICOPY and MINIMERGE as well) which keeps track of whether
that section needs to be updated or not, and if so the whole section is
copied. (This is a balance between writing too much unnecessary stuff
and having a bitmap as large as the disk itself.)
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