In article <3f0bfe7a@news.userve.net> "Martin Roclawski" <m.roclawski@XremoveXdenisonmayesgroup.com> writes:
$We have just aquired a second hard disk the same size as the first and
$wondered if it was possible to do a software mirror of the current hard
$disk.

You could add Virtual Disk Manager to your server. This is
software RAID and is an optional (i.e. extra-cost) SCO product.
I've never used it at all, let alone tried to migrate to it
from a single-disk configuration, so I can't tell you if there
are any gotchas to be aware of.

$We were thinking maybe there is something we can put into the crontab to run
$overnight which would do a full mirror of the hard disk.

Sure, you could do something like that, but there are gotchas
with most of these approaches, too.

If the new disk is no smaller than the first one (not all
"18.4 GB" drives are exactly the same size, for example), you
could look at using dd to make an image of the whole first
disk onto the whole second disk. Since the whole disk includes
things like the masterboot block and partition table, you should
(at least theoretically) be able to make a duplicate disk that
will be bootable, though I haven't tried this. One possible
gotcha with this approach is that if filesystem data is changing
while you're doing it (even something as simple as a nightly
job that cleans up old files, or an email message being received),
you could end up with corruption on the second drive, because
you could (for example) copy the inode table before the data
changes, but the directory entries and the actual file data
after the change takes place, and then you will have inconsistencies
that could make some files vanish or be corrupt.

You could also set up filesystems on the second drive that
match what's on the first drive, then copy the data at the
filesystem level (e.g. using cpio -p). You won't end up with
metadata problems on the new filesystem, but if you're in
the middle of copying a file while it's being updated, the file
data could still be messed up, and since different files will
be copied at different times, you could have problems if the data
in two files (e.g. your customer list and the customer order
history) is supposed to match but one got updated before the copy
and one after. You'll also have a bit more work to do to get
the second drive set up and bootable.

You could set up a job that restores your nightly tape backup to
the second drive. This has similar problems to the file-by-file
copy approach.
--
Stephen M. Dunn <stephen@stevedunn.ca>
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