Size of Volume Group

[Ralph Burnette]

Has anyone seen any documentation or have
done any testing on the size of volume
groups? Is there any advantage to having
a small number of volume groups on a server
versus a large number, etc.?

[A. Clay Stephenson]

The number of volume groups is essentially unimportant. Of much greater import is the number of physical devices and the number of paths to these devices and how the data is stripped across these devices. You really have to provide more data about what you are trying to do.

1) Are you talking about arrays or physical disks?
2) Large sequential or random I/O? Database?
3) Raw or cooked I/O? Type of filesystem?
4) Number of disks or arrays involved?

Almost certainly you want at least 2 or 3 volume groups. vg00 is used for the OS. vg01 might be used for applications and vg02 might be used for data.

[Duncan Edmonstone]

Depends what you want to do...

-Every system needs at least one VG - vg00 which should contain the OS + NOTHING ELSE (some people like to put their backup client s/w in here to to aid recovery)

-The usual rule of thumb after that is 1 VG per application, but thats not hard and fast e.g. if you want to split an applications IOs over seperate disks you can guarantee the split by putting the filesystems in different volume groups (e.g. Oracle datafiles vs. redo logs)

-From a performance perspective it usually makes sense to keep disks with wildly different geometry and performance stats in different VGs (e.g. a VG made up of an XP512 LUN and an old 7200rpm 2GB seagate will give inconsistent performance).

HTH

Duncan

[S.K. Chan]

Just want to add a few points (Clay and Duncan pretty much had answered your question).

I doubt anyone would setup 255 VGs (the max) to test the performance. Same goes to having 255 (also the max) PVs in a single VG. Thus I don't think you would be able to find any documentaion about this test. Having 3 VGs vs say 10 VGs, I would think it won't make any "significant" difference.

[James R. Ferguson]

Hi:

Aside from "reserving" vg00 for the operating system's standard filesystems the number of volume groups is a matter of taste.

Keep in mind, however, that some memory is necessary for each volume group as represented in kernel structures. This is controlled by the 'maxvgs' kernel paramter. Each volume group consumes about 4-8 KB of lockable memory. By default, ten (10) volume groups are allowed.

Regards!

...JRF...

[Jeff Schussele]

Very good advice Clay!
I bumped into maxvgs the hard way - not being able to create a VG, knowing the command(s) were correct & pulling my hair out......'til I searched the ITRC KnowledgeBase & finding that wonderful gem of advice - INCREASE MAXVGS!

Rgds,
Jeff

[Byron Myers]

I once bumped into the Max PE per VG limit of 65535 on a very large VG - my only explanation is that this value must be stored as a short int in the kernel (short int limit is 65535). The killer is that the kernel adds PE's to this value for pv-links, so a disk with one pv-link actually doubles the number of PE's used in the VG. This potential problem can be alleviated by setting the PE Size to something like 16 instead of the default 4 MB.

[Dave Wherry]

Others have pretty much covered performance concerns so I won't go back there.
Another thing to think about is how you manage you volume groups. Some one mentioned a volume group per application. I agree with that line of thinking. I have vg00 for the OS. I have another vg for my Oracle and SAP executables. I have 4 other vg's for my 600+GB database. I keep the executables and data seperate. Bi-monthly we refresh our QA database from our production database. We do a Business Copy on our XP256. All we want is the data so we keep them in different vg's. In the long run I want the data in one vg to simplify the QA refresh proceudre. I originally set it up with 5 because there was a bug with lvcreate. I'm gradually moving toward one to simplify management of them, aka my life.