max size of a volume group

[irfan shahmiri_1]

Hi again,

I have 2 17G disks in a volume group. Want to add another one and its giving the following message.

"At least one fo the disks you have chosen is larger than the maximum disk size allowed by BG vg04 (17364 MB). These disks can be still used, but any capacity beyond the vg limit will be wasted. Do you want this anyways?"

What is the max size of a volume group?

thanks

--irfan

[RAC_1]

You are stuck with it. When creating a vg you did not check -e option (max pes) the default is 1016 pes. check vgdisplay vgxx. Check max pe.

You will have to recreate the vg

Anil

[Jeff Schussele]

Hi,

It's set when the VG is first created. It's the MaxPE/PV value in a vgdisplay output & it will default to the largest disk in the group being used to create the VG. Unfortunately there's no way to increase the value w/o recreating the VG.
I always override the default in the vgcreate command with -e XXXXX where XXXXX=equals a value 2 to 4 times the largest disk being used in the initial creation to avoid this problem down the road.

Rgds,
Jeff

[RAC_1]

Jeff,

If -e set to max (65535), will add some overhead somewhere/somehow even though largest disk size is less than that?

Just curious.

Anil

[Jeff Schussele]

Hi RAC,

Not really - only overhead would be the size of the space required for the VG info on the disk.
Remember that -e works *with* -s (pe size) to define the largest disk usable. BUT the greater the maxPE the greater the PE size must be because the VG info *must* fit in one PE.

Rgds,
Jeff

[Marvin Strong]

The downside of creating the vg structures as big as you might physically ever require, is that they use space on the disk and physically memory which will never get paged out. Therefore assuming maxvgs set to 10 (the default), you could quite easily use 320mb of physical memory on lvm structures.

i would think long and hard about doing this.

[Jeff Schussele]

Hi Marvin,

I agree to some extent but what I see as a bigger kernel memory hog related to LVM is the kernel parm vx_ninode.
One should *never* need more than 40,000, but the default setting is 0 which causes an auto-calc at boot & on an 8GB RAM system you'll get 256,000, 32GB yields 512,00 & on a 128GB system you'll get over a *million*. Now there's a huge waste of kernel space. This was made tunable in PHKL_24783 and it's use can free up valuable system memory if used properly.

My 2 cents,
Jeff

[irfan shahmiri_1]

Thanks for your input folks. Here is my vgdisplay -v vg04 output.

***************************************

--- Volume groups ---
VG Name /dev/vg04
VG Write Access read/write
VG Status available
Max LV 255
Cur LV 1
Open LV 1
Max PV 16
Cur PV 2
Act PV 2
Max PE per PV 4341
VGDA 4
PE Size (Mbytes) 4
Total PE 8680
Alloc PE 8680
Free PE 0
Total PVG 0
Total Spare PVs 0
Total Spare PVs in use 0

--- Logical volumes ---
LV Name /dev/vg04/lvol1
LV Status available/syncd
LV Size (Mbytes) 34720
Current LE 8680
Allocated PE 8680
Used PV 2


--- Physical volumes ---
PV Name /dev/dsk/c7t12d0
PV Status available
Total PE 4340
Free PE 0
Autoswitch On

PV Name /dev/dsk/c8t11d0
PV Status available
Total PE 4340
Free PE 0
Autoswitch On
*******************************************

So does it mean taht I will have to delete this vg and recreate it again.
If I add the new 17G disk to it, how much space will it add. What does Max PE per PV 4341 mean?

thanks
--irfan

p.s. I have inherited these systems from the guy who used to work here before me.

[Marvin Strong]

well not to hijack the thread :)

But yes vx_ninode is another problem,

personally I, do the same thing you do for MaxPE/PV of 2 or 4 * largest disk.

however, I would not recommend to anyone setting it to the max 65535, so they dont run into problems. It would be better to tweak, PESize and go with a lower MaxPE/PV number.

Of course thats just my opinion.

[Marvin Strong]

irfan,
if you take the maxPE/PV * PEsize you get the max size a single disk can be in that volume group.

for you thats 4341*4 = 17368 so 17GB.

If you wanted to add a 36GB disk you would only be able to use 17GB of it, in this volume group.