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Question about physical volume groups....

Dave Cast
Frequent Advisor

Question about physical volume groups....

What is a physical volume group? Is it any different from a volume group?
Today is different and tomorrow the same.
9 REPLIES
James R. Ferguson
Acclaimed Contributor

Re: Question about physical volume groups....

Hi Dave:

I suspect you are asking about the '/etc/lvmpvg' file which defines Physical Volume Groups. Have a look at the 'lvmpvg(4)' man pages.

Regards!

...JRF...
Ron Irving
Trusted Contributor

Re: Question about physical volume groups....

Physical Volumes, (PVs,) refer to the physical device, (hard-drive.)


ron
Should have been an astronaut.
Kenan Erdey
Honored Contributor

Re: Question about physical volume groups....

Hi,

yes it's different. You can group physical volumes. So you control which pv will be used for stripping or mirroring. In normal lvm takes the next pv in volume group. So you can do stripping with mirroring i.e.
Computers have lots of memory but no imagination
Mark McDonald_2
Trusted Contributor

Re: Question about physical volume groups....

Yes they are different.

If your volume group has 10 physical volumes for example. You can group the physical volumes in to 2 physical volume groups with say 5 PV's in each.

As mentioned above this is helpful when mirroring etc.

Suraj K Sankari
Honored Contributor

Re: Question about physical volume groups....

Hi,

Are you talking about LVM PVG if yes please go through below link.

(HP-UX 11i v2)
http://docs.hp.com/en/B2355-60103/lvmpvg.4.html

(HP-UX 11i v1)
http://docs.hp.com/en/B2355-90696/lvmpvg.4.html

Suraj
Johnson Punniyalingam
Honored Contributor

Re: Question about physical volume groups....

Physical volume is the actual physical disk drive. To use a disk with LVM

A disk has to be initialized before LVM can use it. The pvcreate command writes the PVRA
to the disk and such a disk is called a PV:

one or more Physical volumes Grouped are called "Volume Group"

Example:-
/dev/vg01
/dev/dsk/c12t13d0
/dev/dsk/c13t13d0
Problems are common to all, but attitude makes the difference
Wim Rombauts
Honored Contributor

Re: Question about physical volume groups....

There is clearly some confusion here. A Physical volume (PV) is not a Physical Volume Group (PVG).
PVG's are only useful in mirroring configurations. When you start mirroring your LV's, you have three options to specifiy whee LVM has to allocate mirrored blocks.
1) Anywhere where you have an extent available.
2) Anywhere where you have an extent available, as long as it is not on the same physical volume as the primary extent.
3) On a physical volume that belongs to another PVG.

For instance, you have two SAN boxes with each a set of LUN's and you want to mirror your data over these two boxes : Place all LUN's of box 1 in one PVG, and all LUN's of box 2 in another PVG. The create logical volumes with the "-s g" (PVG-strict allocation policy) and your data is mirror over the boxes.
sujit kumar singh
Honored Contributor

Re: Question about physical volume groups....

PVGs are used in HP-UX LVM as they can give only one solution to mirroring the striped Vols that is the Implementation of the mirrored and striped LVs from LVM Solution until the new LVM in 11iv3 where u can easliy mirror an LV which is basically a Stripped LV.
For a VG, using the /etc/lvmpvg file we define the member disks of the individual Physical Volume Groups. this looks like the following:
#cat /etc/lvmpvg
VG-Name vg02
PVG PVG0
/dev/dsk/c11t1d0
/dev/dsk/c11t2d0
/dev/dsk/c11t3d0
/dev/dsk/c11t4d0
PVG PVG1
/dev/dsk/c12t1d0
/dev/dsk/c12t2d0
/dev/dsk/c12t3d0
/dev/dsk/c12t4d0

this means that in VG vg02 whicg has 8 disks, we have 2 PVGs PVG0 and PVG1 that have the PV memebers as shown.

Now if u create an LV with PVG Strict Policy that is lvcreate -D y -s g option , The LV will be created with the PEs being taken from the PVs in only one PVG and also allocation of the PEs takes place in a round-robin fashion selecting the PEs from the disks in that PVG.

So that with this PVG-strict allocation if u mirror the LV then the PEs for the mirror of this LV will always be placed on the PEs from the Other PVG that is PVG1 of orig LV is on PVG0 and PVG0 if the Orig LV is on PVG1.

When u mirror such an LV using the PVG Strict Policy using the lvextend command using #lvextend -m 1 ; in the mirror that is created the allocation is in the same fashion that is Distributed Extent based Striping and the mirror resides on the disks belonging to the other PVG.


This is called the DEBM that is Distributed Extent Based Mirroring that is the only poassible implementation of RAID 01 in LVM prior to 11.31.

if u include in PVG0 and PVG1 for example disks belonging to different HBAs, this gives the HBA failure Redundancy for ur LV in case of HBA Level failures and providing the performance benifits of Striping.


Sujit
sujit kumar singh
Honored Contributor

Re: Question about physical volume groups....

Hp-UX 11iv3 Supports the direct mirroing of the Striped Volumes. Which Was not there in LVM prior to 11.31.

in 11iv3 u can create a striped LV and directrly go on to mirror the same using lvextend -m 1.

LVM did not support the mirroring of the Striped LVs before.