How to mirror a logical volume ?

[amreek bansal]

Hi everyone,

I have a question about mirroring logical volumes, If anyone can help.

Background:

We have secondary swap set up on hp-ux server, I have a new disk which I'm going add to the server. I then want to use that new disk to mirror the secondary swap ?

Can any one provide the steps for carrying this out please ?

Thanks

[avizen9]

hello,
what you want to do?
you want to create mirror for swap? or all other lv? clarifiy, thanks,

[James R. Ferguson]

Hi:

LVM mirroring occurs at the logical volume level. Given your description, add the new physical disk to the volume group on which your secondary swap lives. That is, 'pvcreate' the physical disk; and 'vgextend' it into the appropriate volume group.

Next, simply 'lvextend' with the '-m 1' option the logical volume that represents your secondary swap device.

See the manpages for more information.

Regards!

...JRF...

[Michal Kapalka (mikap)]

Hi,

at first you need a product Mirror/UX,

and if you have in your OE, you could mirror.

mikap

[amreek bansal]

To clarify, I just want to create a mirror for swap in case of a disk failure on the disk which the secondary swap is on. I dont want to mirror any other logical volumes.

So currently my existing secondary swap sits on c1t2d0 in a logical volume, I have a new disk which I'm about to add to server and want to use this new disk to mirror the existing secondary swap.

Thanks

[Michal Kapalka (mikap)]

Hi,

so add the new disk to the VG pvcreate -f /.../rnewdisk ( vgextend ) and them use lvextend -m 1 .... .


mikap

[Sajjad Sahir]

Dear Amreek

first initialize a disk
1.pvcreate /dev/rdsk/c#t3d#
then extend the logical volume with mirroring
2.lvextend -m1 /dev/vg00/lvol1 /dev/dsk/C#t#d#

thanks and regards

Sajjad sahir

[Sunny123_1]

Hi

Why you are mirroring your swap?????

If you want then add new disk and use this disk as new secondary swap disk .Create swap disk by using swapon.

Regards
Sunny

[Michal Kapalka (mikap)]

Hi sunny123,

to prevent HW failure.

mikap

[amreek bansal]

Guys, So from some of the replies, I'm doubting whether mirroring the secondary swap is actually needed? Can someone confirm this pls ... is it not normal practice to mirror secondary swap ? does it serve any purpose then ?

Thanks.

[Michal Kapalka (mikap)]

hi,

to mirror ( duplicate ) is the best practice,

for production system.

If something happend to your disk where is your swap located, and you don't have a mirror, your system will go down.

mirroring and duplicating is to prevent SPOF

Single point of failure

mikap

[Sunny123_1]

hi

I think you can go ahead to add new swap disk instead of mirroring old swap disk.

Regards
Sunny

[amreek bansal]

Okay, I see Sunny is suggesting to set up new swap on the new disk, howver I think I would prefer to duplicate (mirror existing one), so what would the commands be pls ?

Thanks

[James R. Ferguson]

Hi:

> So from some of the replies, I'm doubting whether mirroring the secondary swap is actually needed? Can someone confirm this pls ... is it not normal practice to mirror secondary swap ? does it serve any purpose then ?

Yes, you _do_ want to mirror your secondary swap. To not mirror it might mean that you have a server crash in the event that the disk on which the swap device exists fails.

If you don't care about high availablity then don't mirror in the first place.

Regards!

...JRF...

[Patrick Wallek]

YES YES YES YES YES!!!!!! YOU ABSOLUTELY WANT TO MIRROR YOUR SWAP!!!!!!!!

Consider this --- You have a swap LV that is not mirrored and resides on a single disk. Your server is cruising along without any apparent problems. Suddenly it crashes. The cause? The disk your swap is on went bad and you didn't know. When your sever tried to actually page out to that device, it couldn't, so your server crashed!

Now, if that swap LV were mirrored, the server would not have crashed.

Having multiple swap areas is NOT THE SAME as having your swap areas mirrored.

Mirroring your swap areas provides redundancy and prevents a single point of failure (which everyone usually wants).

Adding additional swap areas provides just that, more swap that is NOT redundant.

[amreek bansal]

Thanks alot for the last replies, I understand now that I should mirror the swap, can any one be kind enough to show the steps in completing the task of mirroring.

Thanks

[Patrick Wallek]

You were given lots of advice on how to do this above, but in a nutshell:

Assume new disk is c1t3d0 (substitute whatever your disk is)

Assume swap is vg01/lvol1 (substitute whatever yours is)

# pvcreate /dev/rdsk/c1t3d0
# vgextend vg01 /dev/dsk/c1t3d0
# lvextend -m 1 /dev/vg01/lvol1 /dev/dsk/c1t3d0

Wait......

Once the mirror finishes syncing you are done.

NOTE: Depending on the size of your swap LV, this may take a while.

[amreek bansal]

Thanks Patrick, I understand your steps. I will do this now. Thanks for your help.