[Info-vax] Best way to get what I can off a failed VMS Disk?

[Michael Moroney]

Simon Clubley <clubley at remove_me.eisner.decus.org-Earth.UFP> writes:

>On 2020-10-24, Phillip Helbig (undress to reply) <helbig at asclothestro.multivax.de> wrote:
>> In article <rn1ids$78o$1 at panix2.panix.com>, kludge at panix.com (Scott
>> Dorsey) writes: 
>>
>>> Get a drive image as soon as possible.  You can do it on the Alpha, you
>>> can pull it off and put it on a PC with Clonezilla or with dd, although
>>> the best thing to do is to drop it into one of the offline disk duplicators.
>>
>> 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 ?

>Lots of extra random I/O is very bad on a failing disk.

Yes, shadowing will cause excessive I/Os.

Shadowing uses an algorithm for copy/merge that doesn't require the file that the blocks 
being copied represent to be locked against modification, but it requires read/compare as 
well as writes to verify the file system hasn't done anything.  MOUNTing the disk into the 
shadowset and adding the new drive(s) also causes a few writes to the original disk which 
you want to avoid.

Mount the bad disk read-only and use BACKUP/PHYSICAL to another drive or saveset.
BACKUP/PHYSICAL is also quicker than a shadow copy.

(aside: in my previous suggestion for using two disks, you can use a saveset instead of
the first disk if that's easier for your configuration)