topic Re: Multiple VG on SIngle Disk in Operating System - HP-UX

Multiple VG on SIngle Disk

Amit Dixit_2 — Wed, 25 Aug 2004 01:45:53 GMT

Hi,
Can we have multiple VG on single disk
IF not why ?

Thanks,
Amit

Re: Multiple VG on SIngle Disk

melvyn burnard — Wed, 25 Aug 2004 01:47:43 GMT

No you cannot.
A single disk, or PV, can only belong to one VG .
The VG information gets written on the first few blocks of the diszc.

Re: Multiple VG on SIngle Disk

Michael Tully — Wed, 25 Aug 2004 01:53:22 GMT

You cannot have more than one volume group on a single LUN (disk)
Once a volume group has been designated to a disk, it writes a volume header on it. It cannot then be used again for another group.

Re: Multiple VG on SIngle Disk

Bharat Katkar — Wed, 25 Aug 2004 01:59:52 GMT

Hi Amit,
Multipe VG's cannot exists on Single Disk. VG's is made of PV's and in multiples of PV's. It can span accross no of disks. Disk is your PV.
Now a PV can have mulitple volumes like lvol1 lov2 etc. Again one Logical Volume can hold only a single FileSystem..so as many lv's on a PV that many filesystems you can create on the PV and thereby on the VG.

VG == made up of =>> No. of PV's
PV's == contain ==>> No. of LV's
LV's == contain ==>> One FileSystems each

This how our LVM works.
Hope that helps.

Regards,

Re: Multiple VG on SIngle Disk

Bharat Katkar — Wed, 25 Aug 2004 02:09:29 GMT

Amit,
Sorry for multiple replies.. forgot mention why:

Whenever you add a disk to VG using vgcreate or vgextend then it write some information on the disk which is called as VGRA i.e. Volume group reserved Area. This make that particular disk reserved to that volume group. Becuase these information is specific to that group only.
Now VGRA contains two things i.e.
VGSA -- Volume Group Status Area
VGDA -- VOlume Group Descriptor Area which contains device driver information used by the Volume Group.

Hope that clarifies.
Regards,

Re: Multiple VG on SIngle Disk

Geoff Wild — Wed, 25 Aug 2004 09:42:38 GMT

This kind of depends....if the disk is attached directly to the HP server - then no...If however, you are in a SAN environment, then you could "slice" up a disk any way you like and present it to HPUX as multiple disks (even though it is physically 1).

But as others have said, the answer is really no as vgreate writes a vgid on each disk in a volume group - so you can't have multiple vg's on a "single" disk...

Rgds...Geoff

Re: Multiple VG on SIngle Disk

Rick Garland — Wed, 25 Aug 2004 10:23:10 GMT

As Geoff stated, in a SAN environment you could slice and dice the disk array however you see fit. Look at EMC, NetApp, EVA, etc., disk arrays.

Re: Multiple VG on SIngle Disk

Tanmay_1 — Wed, 25 Aug 2004 14:40:03 GMT

If the system recognizes the disk as a single disk you can not create multiple vg in it. I think you are not considering that at the back end the devices are slices among many physical devices, typically RAID 5 configuration. So in that case, at the storage box, one device may have multiple vg. But as per as system is concern you can not use multiple VG in a single disk.