topic Re: Number of disks in volume group in Operating System - HP-UX

Number of disks in volume group

Don Spare — Mon, 13 May 2002 12:54:47 GMT

This past weekend I was adding disks to my system and trying to add them to an existing volume group when I ran into a 16 disk limit in a volume group. Is this a hard and fast limit or can it be adjusted? What are the repercussions if it is adjusted? Will data be lost in the process? If it can't be adjusted or the process is very complex I can always just create another volume group for the extra disks but I was hoping to have only 1 very large filesystem when I was done. BTW these are 4.3GB EMC RAID1 disks.

Thanks for any assistance with this.

Re: Number of disks in volume group

Patrick Wallek — Mon, 13 May 2002 12:57:39 GMT

The value for the maximum number of physical volumes in a VG is set at VG creation time.

The default value is 16, but it can be set to a value between 1 and 255.

This can ONLY be set at VG creation time. There is no way to reset it after you create the VG.

See 'man vgcreate', specifically the '-p' option, for more information.

Re: Number of disks in volume group

John Poff — Mon, 13 May 2002 12:57:57 GMT

Hello,

You can specify the maximum number of PVs when you create the volume group with vgcreate. The default is 16 PVs. The only way to increase it is to nuke the volume group and recreate it.

JP

Re: Number of disks in volume group

A. Clay Stephenson — Mon, 13 May 2002 12:58:24 GMT

Yes you can go above 16 (to a maximum of 255) but the bad news is that you must set this limit when the VG is created. If no -p argument is supplied to vgcreate, the default is 16.

Re: Number of disks in volume group

Vijeesh CTK — Mon, 13 May 2002 13:00:00 GMT

hi

while creating a VG by default the max pv will be 16. U cannot increase the max pv in an existing VG. what u have to do is create a new VG with -p option
ie, vgcreate -p 'pv nos'
u can select a value between 1 to 255.

then u have to make filsystems and move ur data

CTK

Re: Number of disks in volume group

Jeff Schussele — Mon, 13 May 2002 13:00:17 GMT

Hi Don,

Max phys volumes is set when the VG is created - default is 16. The absolute max is 255.
Unfortunately it cannot be changed w/o recreating the VG from the start.
So you'd have to back it up & recreate the VG - setting an appropriate max for your needs.

Rgds,
Jeff

Re: Number of disks in volume group

Sanjay_6 — Mon, 13 May 2002 13:00:18 GMT

Hi,

Once the VG has been created it is a hard limit. The parameter for Maximum no of PV's in the vg has to be defined in the vgcreate command itself.

use the -p option to set the maximum no of PV's you may add to this VG at the time of vg creation.

Once set, this cannot be changed for that VG, the only choice is to create a new VG.

Hope this helps.

Regds

Re: Number of disks in volume group

James R. Ferguson — Mon, 13 May 2002 13:00:25 GMT

Hi Don:

It is a "hard-and-fast-limit" *only* at creation time. That is, at 'vgcreate'. If you want to increase the value from its default, you must specify it ('-p max_pv') when the volume group is created. Once set, the only way to change it is to destroy and recreate the volume group.

Regards!

...JRF...

Re: Number of disks in volume group

Deshpande Prashant — Mon, 13 May 2002 13:10:52 GMT

HI
You are stuck with the Max PV number that was set during VG creation.
This option only can be changed during vg creation.

Thanks.
Prashant.

Re: Number of disks in volume group

Dave Wherry — Tue, 14 May 2002 13:44:35 GMT

As the others already covered, vgcreate, 16 default, 1-255 range of disks per volume group.
There is another option available to you, although I would just recreate the volume group. On the EMC frame you can use meta volumes. Combine several of those smaller LUNs into a larger one and present that to the host. However, if you accepted the maximun number of disks default value when you created the volume group, you probably also accepted the maximum number of physical extents per disk value which defaults to the largest disk you added when creating the volume group.
So even if you combined 2 of those 4GB LUNs into an 8GB LUN, your old volume group would only be able to use 4GB. You pretty much still have to recreate your volume group.
I just bring this up as an option if you want to limit the number of LUNs you present to the host. I had almost hit the limit of LUNs that I can have on an interface on my K570. This past weekend I reworked much of it by setting up LUSE volumes on my XP256 (essentially the same as meta volumes). I have the same amount of disk space available to me, but about half the number of LUNs. A little easier to manage from the host side.
On the XP I can setup LUSE volumes myself. When I last ran an EMC, 2.5 years ago, to do a meta volume would require EMC to rework the bin file, load it and reboot the frame. Of course your hosts would also be down. They also charged me for this type of work. I'm not sure of the state of EMC now. Whether this can be done without a frame reboot, how much the customer can do and if there is a charge if EMC has to do it. You might want to investigate it just for the knowledge. Also, read the man page for vgcreate. It will clear up many things for you.