Software Disk Image
Martin Roclawski
software installed on a Compaq ProLiant.
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.
The machine does not have a raid controller.
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.
Thanks for any help
Martin Roclawski
Stephen M. Dunn
$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 <ste...@stevedunn.ca>
>>>----------------> http://www.stevedunn.ca/ <----------------<<<
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Say hi to my cat -- http://www.stevedunn.ca/photos/toby/
Martin Roclawski
It seemed alot easier when discussing the concept with my people here!!!
Stephen M. Dunn <ste...@stevedunn.ca> wrote in message
news:HHrzv...@stevedunn.ca...
> In article <3f0b...@news.userve.net> "Martin Roclawski"
<m.roc...@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.
If software raid works like hardware they have to be in an array prior to
attempting to rebuild one from another.
However software might be different.
The dd approach seems a possibility, especially if I can stop services
through a script and then start them after the dump has taken place.
The only thing that may cause a problem is try to figure out what is running
in order to stop it!!
May try experimenting with this first.
to...@aplawrence.com
>Cheers for the info Steve.
>It seemed alot easier when discussing the concept with my people here!!!
Yeah, it always does. I must hear this a couple of times a month,
and while it's not necessarily a BAD idea if it's combined with
other real backup, it's usually proposed a a way to avoid tapes or
other storage and as that, it's dumb. See
http://www.pcunix.com/Unixart/diffback.html
Bill Vermillion
>Martin Roclawski <m.roc...@xremovexdenisonmayesgroup.com> wrote:
>>Cheers for the info Steve.
>>It seemed alot easier when discussing the concept with my people here!!!
>Yeah, it always does. I must hear this a couple of times a month,
>and while it's not necessarily a BAD idea if it's combined with
>other real backup, it's usually proposed a a way to avoid tapes or
>other storage and as that, it's dumb.
Speaking of that - [I may have mentioned this before] - I had been
prodding a client to take his backup tapes off site for years.
Then one day while reading his insurance policy to see what was
covered - he found that his business interuption service would not
cover any losses because of lost data if he did not have off-site
backups.
I've had weekly backups go off-site every week for the past 4 years
now - with never a miss. I also get email on the backups just in
case they don't read their email - which happened to another
customer of mine. He wound up with other people advising him and
when a fire totally destroyed everthing the only data he had was
made by the other people copying the data onto an MS machine and
making a CD.
When I told him the cost it would take to figure out all the real
file names and processes in the database - that were converted from
14 characters to a 8.3 MS format - he about fell over. Then the
people who did this told him he could take the HDs in the machine
and send them to a data-recovery company. That was also very
expensive.
The cost of a tape drive and decent software - the supertars often
recommend here are great [I have sites running both BackupEdge
and LoneTar] - are very cheap when you consider
1) the cost of rebuilding data that is not recoverable
2) the cost of recovering data that is not in an easily
recoverable format
3) the lost income while waiting for data to be restored.
With the emergency disks and off-site backups you can often start
restoring the system in under 10 minutes if your hd contollers
match.
Here's an old anecdote about the person above who lost his whole
building to fire and thus the data.
Years ago - when he was on SCO and filePro - he lost his tape
drive - an old Archive 525MB. I found out about this after
a call about something else let me know they had not had a good
backup for months - the new person did NOT know she was supposed to
get email on backups each day - or to check for them.
He was complaining about the cost of the tape drive being over
$500.
I said something like this.
Q: "Do you want to save some money - have a tape drive - and
even have money left over?"
A: "Yes"
"Then cancel your fire insurance, buy a tape drive, and have a
party with the rest of the money, because your computer will crash
long before you building burns down".
This was in another building and about 10 years before the current
building burned down - and he did have a couple of computers replaces
in the interim.
Bill
--
Bill Vermillion - bv @ wjv . com