> Date:         Fri, 7 Jul 1995 14:08:47 PST
> From: Mark Klein <[log in to unmask]>
> Subject:      Re: Tape drive alternatives
>
> With the cost of larger disk drives getting less and less
> (and in most instances, far less than corresponding tape devices),
> perhaps considering disk to disk backups is a viable alternative
> to tape based backups.
 
Mark makes an important point.  We use a rather antiquated suite
of financial applications that knows only about the current general
ledger and one previous year's general ledger.  For at least two
months after end of year roll-over, Finance Department need frequent
access to some prior prior-year general ledger.  Mindful of the
fact that changes may need to be made to the prior year ledger
during this period, it creates potential for disaster when rolling
in a temporary over-lay of the immediately preceeding year's ledger.
 
What we have done (thanks to a generous amount of free disc space)
is to set up a job to do the following
 
        -check for access to current prior ledger and abort if true
        -if idle, do a disk-to-disk copy of current prior ledger
                to a backup group
        -purge current prior ledger in production group
        -restore whatever prior prior-year ledger is required
                from tape (2 million sectors, takes about 25
                minutes)
 
When Finance Department have said they're finished with the old ledger:
 
        -check for access to prior ledger and abort if true
        -if idle, purge prior ledger and RENAME current prior
                ledger from backup group to production group
 
This step takes exactly 59 seconds.
 
If they weren't in the throes of tendering for a new suite of
financial applications, I would make a case for adding even more
spare disc capacity to keep multiple prior-year ledgers on disc
and do simple renames/disk-to-disk copies every time they wanted
access to an old one.
 
FWIW,
Ron
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