mirroring boot disk and mirroring swap necessity..

[Bill McNAMARA_1]

Hi all,

I've got a question relating to swap mirroring.
I use a procedure to mirror a boot disk and there is a step in it that requires to boot into maintenance mode to
lvchange -M n -c n /dev/vg00/lvol2

I need to know if it is absolutely necessary to boot into maintenance mode to do this, and whether it can be done without booting into maintenance mode.
If it is not necessary, I'd like to remove it from the procedure, and if it is necessary, I'd like to know if it is possible to do so via some lvm startup file ie, lvmrc
I can't access the site/console remotely...
(I have a vg00 and a vg01 if needed to be done via lvmrc)

procedure attached.
Bill

[Torsten.]

Where did you get this procedure from?

I don't see the sense of the lvchange command. Why changing the mirror consistency recovery setting?

Compare to one of the official procedures (Appendix) in

http://docs.hp.com/en/5991-1236/When_Good_Disks_Go_Bad.pdf

[Ivan Krastev]

Here is procedure for root and swap mirroring - http://www.docs.hp.com/en/B2355-90672/ch07s07.html

It's not necessary to boot into LVM maintenance mode.

regards,
ivan

[BUPA IS]

Hello Bill ,
We do not bother to to this .
I suspect the idea came to improve performance on older systems when disks were slower and swapping / paging i/o had more of an impact .
The lvol has to be closed to change the mirror write cache so until somebody invents a swapoff command a maintenance mode reboot will be needed .

Mike

[Bill McNAMARA_1]

Thanks for the replies,
is is possible via lvmrc to do this before vg activation?

Can anyone tell me exactly what
lvchange -M n -c n /dev/vg00/lvol2
does too?

Bill

[BUPA IS · Accepted solution]

Hello Bill ,
-c n turns off the consistency recovery for the mirroring after crash because on this occasion there would be no point if weren't saving the consistency data.

-M n turns off the mirror write consistency cache writes which save the data about which extent is the most recent and valid to make this recovery possible in the first place .

This mechanism allows the lvm system to make the two (or three) copies consistent at volume group activation even after crash (volume group stopped with files open for write) .

The -M n would save some i/os to the disk but primary swap is usually not large and has a low i/o rate compared to the rest of the boot disk. The time saved for -c n during a reboot would be quite small anyway .
Since the volume group is not active at that point lvmrc might help you, but read the /etc/lvmrc and /sbin/lvmrc scripts before you customise /etc/lvmrc only . If I were you I would simply adjust your procedure to note that it was no longer entirely nesscessary. The last time I had to fix a broken boot mirror it took 5 weeks to get a slot to reboot so I could check it .

Mike