System Disk mirroring

[Brando Sumayao]

If the system disk is corrupted (somehow), what is the value of mirroring a system disk if you have good backups anyway?

[Roger Baptiste]

hi,
Mirroring is a protection against a disk going faulty or hardware errors with a disk. In such a case, it seamelessly takes over the function of a primary disk, without any downtime. WHereas, if you did not have a mirror disk, it would have meant a delay
in waiting for a new disk to be fixed and
on top of that restoring the data from the
backup.

Ofcourse, even the best of hardware cannot
protect against human errors like rm -r or
logical corruption. ;-)

Your question is very valid in a BCV
setup. (bcv being third mirror copy). I do not
sync the BCV all the time. Rather, i establish it only when i intend to take a database backup. That way, i have an offline BCV copy sitting on the disk.

-raj

[David Allen]

Hi Brando,

Further to RajMan's comment. Whilst mirroring protects your system from hardware failures, you should also keep regular Ignite-UX backups (man make_recovery - provide you have installed Ignite-UX). This is important from a system recovery perspective. If the data on your system disk becomes corrupted or has been accidentally deleted, then you can boot off of the Ignite-UX tape and recover your system.

At the very least you should insist on having your system disk mirrored, as it can be very time consuming trying to recover a system disk.

Regards,
Dave

[Michael Tully]

Hi,

The basic difference between having mirrors is
for hardware failure contingency. The use of
having backups is to restore files that have
been removed or corrupted. That is where
having an ignite tape is an essential part
of your system administration. If your OS gets
corrupted and becomes unusable you can recreate
very easily with an ignite tape. This is also
useful when you don't have a mirrored root disk.
You don't say what OS version you are:

To create an ignite tape do this:

For 10.20
# /opt/ignite/bin/make_recovery _ACvd /dev/rmt/0mn

For 11.x
# /opt/ignite/bin/make_tape_recovery -x inc_entire=vg00 -I -a -v /dev/rmt/0mn

Make note of the no-rewind device.

If you don't have it installed, load it from your
applications CD or download it from here:

http://www.software.hp.com/products/IUX/download.html

-Michael

[James R. Ferguson]

Hi:

A clarification. Regardless of whether you are running 10.20 or 11.x, you should use Ignite's 'make_tape_recovery' in lieu of the older 'make_recovery'. 'make_tape_recovery' provides many newer features and is designed to replace 'make_recovery'.

When you download and install Ignite note that the "A" version can install multiple versions of HP-UX 10.x from a server running 10.x.

The "B" version requires a server *running* 11.x (32 or 64 bit), but can can install 10.20 or 11.x targets.

*Both* the "A" and the "B" versions fully support 'make_tape_recovery'.

See here for Ignite documentation and software. Installation takes just a few minutes and does not require a reboot:

http://www.software.hp.com/products/IUX/index.html

Regards!

...JRF...

[A. Clay Stephenson]

The main advantage is that your system stays up with a failed disk. In fact, if you have planned well, you can replace a mirrored boot disk and your users will never even know it and the machine doesn't have to be shutdown.
That's the good news. The bad news is that a mirror is just that - if you do something really stupid (or install a really bad patch), you will do exactly the same thing on the mirror. I take this to one more level. At least once a week or before a patch is to be installed, I do a dd of one of the raw boot disks to an identical disk to create a 'LIFEBOAT'. Then no matter what happens or how stupid I am, I simply remove the original boot disk and mirror and replace the boot with the LIFEBOAT. I am back up in minutes. The system has to fsck the filesystems but after that I'm up and running. It's much faster than
bringing the system back up with Ignite.

The real answer to your question is how much downtime can you afford and how does the system get restored if you are not there.

Food for thought, Clay