PE Size for large PVs

[Mark Poeschl]

HPUX 11.23 running on an rx2600:

Is there some limit on the number of PEs/PV other than the documented 65535?

With a 500 GB PV on an EVA storage array, I'm trying to create a VG. When attempting a vgcreate using a PE size of 8, I get the warning message "Increasing to 63999 physical extents" and then the vgcreate hangs indefinitely (we let one sit overnight). Using a PE size of 64 works just fine.

From other messages in the forum, it looks like there's little performance implication of using larger extents, but I'm concerned about capacity utilization implications. Is a 64 MB PE size reasonable in this case? We'll be putting something like 10 LVs of various sizes between 20 and 150 GB in this VG.

[Steven E. Protter]

You probably saw a thread I posted on the topic.

The standard size didn't work for me when we got past a certain number of GB. I thought about using the size 64 to improve oracle performance and was told it would have no effect.

So I went with 16 to accommodate future growth and left it at that.


You are dealing now with roughly four times the data I am so 64 seems quite reasonable to me.

Good Luck

SEP

[A. Clay Stephenson]

The only downside of using such a large PE size is that you could leave a rather large chunk usused, if the PE's X PE size does not exactly fill the PV. Of course, worrying about 64MB out of 500GB is no big deal especially if this is only 1 PV. A PE size of 8 should have worked but almost certainly 16 will suffice.

[Elmar P. Kolkman]

One other thing you might try: split the disk on the EVA in two and create the vg with those two 'disks'... Your volume group is the same size...

Downsides might be on buffer level (writing a single byte will be done by re-writing 64 Mb, for instance), but with this size of vg I guess you have lots of memory too... And of course the loss of space because a lvol has to be a multitude of 64Mb.

[Mark Poeschl]

Just want to make sure I've got this right...

The "wasted" space in this scenario would be up to 64 MB per LV - not 64 MB per file. As I understand it, file system space allocation is independent of the LE and PE allocation done at the volume level.

Am I understanding this correctly?

[Sridhar Bhaskarla · Accepted solution]

Hi Mark,

Actually it is not per "LV". It is per PV.

For a crude example, consider you have a disk of exactly 127 MB (hypothetical). If you create a PV with extent size 64, then you can only grab one PE out of it. 63 MB is wasted there.

It has absolutely no performance implications at all.

-Sri