How to turn off mirroring

[John Downs_1]

Greetings!

My system disk has one mirror. If a problem occurs with either disk they both go down, that's what I'm told. How do I turn off mirroring in order to bring the system back up with a single non-mirrored disk?

Thanks
-john

[James R. Ferguson]

Hi John:

There are instances with a mirrored boot disk where a failure of the primary copy will necessitate a reboot to recover. However, mirroring has protected your data.

To turn off mirroring for a logical volume do the following:

# lvreduce -m 0 /dev/vgXX/lvolN /dev/csk/cXtYdZ

Remember that mirroring in HP-UX LVM is done at the logical volume level, not at the physical disk or volume group levels.

Regards!

...JRF...

[Dietmar Konermann]

Hi,

Please do also an "lvlnboot -R vgXY" after reducing the mirror copies from the lvols.

Regards...
Dietmar.

[John Downs_1]

Can I do this after the system has failed? Can I bring is up in single user mode and use these commands to turn off mirroring?

-john

[Bill Hassell]

Both go down? Hummmm. I can think of a very rare case where an electronic failure might lockup the entire SCSI bus, but normally, if a disk goes bad (bad sectors, unable to access data, etc) then the mirror will continue without intervention. In fact, several forum questions ask how to detect that a disk goes bad--in one case, the disk went bad months before and no one noticed. In this case, lvdisplay -v will show 'current' or 'stale' extents.

For the boot disks, make sure the boot string has been setup on the secondary boot disk to use hpux -lq (if you have only one mirror--some sysadmins have 2 mirrors) so LVM will reboot automatically on the secondary without a quorum requirement.

[Trond Haugen]

If your question is how you boot with only one disk should the other fail, the answer is that you will have to boot without quorum. That is boot to the ISL prompt (from the good disk) and issue 'hpux -lq'.

Regards,
Trond

[Carlos Fernandez Riera · Accepted solution]

Not only the secondary disk, both disks...


First use setboot -a "primary disk HW/path" -a "secondary disk HW/PATH"


now mkboot
mkboot -a "hpux -lq " /dev/rdsk/cxxxx # primary
mkboot -a "hpux -lq " /dev/rdsk/cyyyy # secondary disk.


Now you are sure that if your system goes down you will be able to startup even one of the boot disks is out/off.

Bill: The first disk could break too...

[Vincent Farrugia]

Hello,

Could you tell us how come both your harddisks will both go down in the event of either one of them failing?

Vince

[John Downs_1]

Hi Vincent, I was told that in a two disk mirrored setup that when one of the disk becomes unmirrorable (has problems) that the mirror system will take itself out because the mirroring (integraty) has failed; it has to maintain the mirror. In a three disk mirrored system a failure of one disk can be tolerated because mirroring is still occuring between the remaining two. This is what I was told by some HP experts (which I am not).

-john

[Sajid_1]

John:

That description about mirroring not actually true. If that is the case, then nobody will keep their disk mirrored! Mirror/UX and LVM are more capable of finding out all these issues and defects and is intelligent to make sure that atleast one disk is good. Keep your disk as mirrored, find out the SCSI issues, apply the patches and keep tracking your log files.

For reducing mirror:
# lvreduce -m 0 ...

HTH

[Scott_14]

Hi John:

Well, let me tell you from experience, all our root disks are mirrored, and I have had a failure or 2 but with it being mirrored, the system didnt even hic up for the users. I usually find out by a messge in syslog, or an event monitor message, but both the "good" disk was never effective. I just had to bring the system down to single user only to replace it, and replace the boot parms on it, then re-sync them up. I would keep the mirror, and not reduce it if you can, its a good fail safe.
and like mentioned, in a rare instance they both can go. But if you reduce your mirror, what is the point if you only root drive goes bad?

I would also have an Ignite tape made, just incase of that rare time when both would corrupt.

Scott

[Vincent Farrugia]

Hello,

Well, it's true that when you have mirroring and one disk fails, the other disk "expects" that mirroring is still there. This will result in unsynced mirroring or a stale harddisk.

But... this does not mean that the healthy disk will fail too. Otherwise, what is the use of mirroring? Mirroring is there so that, if a disk fails, you can continue working. We recommend every customer to have at least software mirroring of their root disks. With mirroring, the other disk will automatically take control and continue normal operation. Sometimes you won't even notice you have a harddisk failure unless you have EMS notification.

With only one disk, take regular Ignite backups and make sure that they're good.

HTH,
Vince

[John Downs_1]

Thanks, You guys have been great! I had some bad information.

-john

[Sean OB_1]

John,

I think what you need is not to reduce the mirror but to tell the system to boot even though it hasn't met the quorum.

To do this boot the system, and when it asks if you want to interact with the ISL say yes.

Then issue "hpux -lq".

This will boot the system even though only 1 of the 2 disks in the mirror is available.