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Optimum settings for VG parameters

 
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Scott Pelletier
Occasional Contributor

Optimum settings for VG parameters

I've been bit too many times by MAX PE/PV not being high enough to add new larger disks into a VG. The HP-UX mirroring and pvmove functions work great for migrating to new disk technology, but new storage almost always means larger LUNs and I want to think ahead by creating my VGs with parameters large enough for larger LUNs. I know I can change the default parms when creating a VG, but what numbers are safe? If I increase parms like MAX PE/PV, PE Size, MAX PV/VG am I going to loose anything or hurt myself in some other way? For example, why wouldn't you set the MAX PE/PV to the maximum number allowed?

Thanks,
Scott
3 REPLIES 3
Patrick Wallek
Honored Contributor
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Re: Optimum settings for VG parameters

It is probably wise to increase the Max PE/PV value, but remember that with that value you need to take into account the PE Size parameter as a part of it. By default PE Size is 4 MB, but this can be increased.

Here is a document that outlines one pitfall you can hit when useing some of the new very large disks and the VG parameters to change to avoid it.

http://us-support2.external.hp.com/cki/bin/doc.pl/sid=1f7629f4070c8e3ec2/screen=ckiDisplayDocument?docId=200000054599371
James R. Ferguson
Acclaimed Contributor

Re: Optimum settings for VG parameters

Hi Scott:

The LVM table size (VGDA) is a factor of the 'max_pe', 'max_pv', 'max_lv' and 'pe_size' parameters. The essential constraint is that the LVM structure defined must fit into one physical disk extent.

Thus, the problem of creating large volume groups can be attacked by by adjusting one or more of the parameters 'max_pv', 'max_pe', 'max_lv' and 'pe_size'.

I'd give consideration to the size of your volume group as opposed to capriciously choosing maximum values.

For instance, with EMC2 disk, I generally have 4MB disk presented to the kernel. Thus, I don't need to inflate 'max_pe' but certainly want to choose a large 'max_pv' to allow my volume group to grow.

Regards!

...JRF...
Wodisch
Honored Contributor

Re: Optimum settings for VG parameters

Hello Scott,

whta you need is a current "magic crystal globe", of
course (out of stock, I am afraid, though).

But with a little bit of "common sense" (TM by someone,
I am shure ;-) you can be save for at least a few years
ahead:

How many filesystems will you need at max. in this VG?
=> rounded up that becomes your MAX_LV parameter.

What is the biggest disk you have right now in that VG?
What do you expect do be a "deliverable" disksize for
the next, say, 2 or 3 years be?
=> That becomes the size we have to "tune" for.

What is the amount of diskspace you are permitted to
"waste"? Say, up to 1/1000 of the disk? e.g. up to 31MB
of a 32GB disk?
=> That determines the maximum PE_SIZE.
I do not think you will have a problem in using more
space for a small LV than needed, like a 32MB LV
instead of 10MB one...

How many disks can be connect to your computer?
And if you add some more controllers?
=> That will be the maximum number of PVs, so choose
=> something suiting your possible growth.

What have we got now?
Well, perhaps something like 40 to 50 LVs, possibly
about 100 to 200 PVs and a PE_SIZE of 32 or 64MB
(maximum right now is 256MB, IIRC). The disks you
get (from a HP XP512, perhaps, or an EMC) are most
often sized for about 4 to 8 GB each (which I do not
like, not even the "OPEN-E" of the XPs) or even up to,
say, 100GB per disk (or "LUN").

That would be then
vgcreate -p 200 -l 50 -s 64 -e 2048 /dev/vgBIG ...

And since your PE_SIZE is bigger than 4MB you will NOT
run into problems with many PEs (as some of us did).

If one of your drives crashes next year, say, a 30GB one,
and the smallest disk available as a substitute would be
about 48GB, well, no problem, get it (and you can even
make use of the additional space!).

If you manager decides to buy another RAID and connect
it your your controllers, well, still some PV slots empty in
your VG, aren't they?

I never ever create VGs with PE_SIZEs of less tha 16MB
these days!

Just my ?0.02,
Wodisch