Volume Group Creation: PE Size

[Conal O'Kelly]

Hi
Can anybody advise on an optimal PE size(-s option)when creating Vol Grps (vgcreate). If the number of PV's(-p) and maximum physical extents per PV (-e) are sized appropriately is there any need to worry about extent size.
I've seen from previous discussions that extent size doesn't impact performance. Assuming I set PV's (-p) to 50 and max extents (-e) to 4205 and extent size (-s) to 8, then I can use up to a 33GB disk (LUN)in this Vol Grp and have a potential maximum VG size of 1650 GB (33 * 50).
Is there any advantage in confuring a larger extent size (such as 16 or 32) apart from being able to configure larger disks into the VG?
Also is there much overhead in setting the no. PV's to the maximum (ie 255)?

Thanks for any help.

Cheers
Con

[harry d brown jr]

Unless you have to, you normally don't change the PE size. the larger the extent size, the larger the contiguous space created on the disk.


live free or die
harry

[Michael Tully]

Hi Con,

Generally there is no need to change the PE
size, unless there is great reason to do so
because of the way your database may be laid out.

Be careful about trying to attempt to add
larger disks to your volume group. If for
example you have created you VG with a 4GB
disk, you can add a 9GB disk but you will not
be allowed to use the full number of extents.
It will size the disk as a 4GB disk only.

-Michael

[harry d brown jr]

Michael,

You make a GREAT point, that all disks in a VG should be identical, in every way possible. Mixing disk types or models will negatively impact performance. They must all spin at the same speed, have the same IO rate, seek time, scsi or whatever type of access. And of course that means that Mirrors of LV's have the same identical disks.

live free or die
harry

[A. Clay Stephenson]

Hi:

In reality, PE size has no performance impact. Bear in mind that the PE's are specified in MB's while any reasonable I/O occurs in KB's. The PE size is much more a resource variable (i.e. total number of PE's) than it is a performance variable. Stripping within LVOL's also in measured in KB's so again - no big difference. If you are mixing heterogeneous drives within a volume group, create the volume group with the largest drive; you will then always have enough PE's per physical device.

Regards, Clay

[Conal O'Kelly]

Thanks for the info, I'll assign points shortly.
Just to clarify. I am creating new VG's - is there any reasons not to use an 8MB extent (assuming I set the other parameters as previously mentioned). I did read somewhere on the forums that LVM data structures can only reside in one physical extent but I'm not sure what impact this has.

Cheers
Con

[Sanjay_6 · Accepted solution]

Hi Con,

Also take a look at this thread,

http://us-support3.external.hp.com/cki/bin/doc.pl/sid=a23ea0940254d1d48d/screen=ckiDisplayDocument?docId=200000052474625

Hope this helps.

Regds

[Michael Tully]

Hi Con,

There is really nothing wrong with using 8MB
extents, as it depends on what application your
using, what database, raw/cooked.... One thing
that is clear is once you create your volume
group you cannot change the extent size. As
mentioned 4Mb is the default. If you create
yours with 8MB, that's what you got.

Just remember what I said about using different
sized disks in a volume group.

-Michael

[Wodisch]

HEllo Con,

why stopping at half the way?
We had that discussion more than once, and to me the basic point is to be able to add more disk space later one, especially larger disks.
Hence I ususally use "vgcreate" with a lot of options, e.g.:
vgcreate -p 100 -l 20 -s 32 -e 2048 /dev/vg...
That permits me to have up to 100 PVs, each of up to 64GB, and I never use hundreds of LVs, so the maximum of 20 of them is fine for me.
And since ALL the header information MUST fit into a single PE, you will NOT run into those problems (which you would with 4MB PEs).
In my opinion the default parameters are just plain useless (they were not that bad back at HP-UX 9).
Just my $0.02,
Wodisch

[Duncan Edmonstone]

PE size is important if you are using extent-based mirrored stripes as this will give you your stripe size. See the -D option of lvcreate.