What size should the PE size be?

[Tony Escujuri_1]

We are using extent-based stripping on our system and we want to know the optimum extend size we should be using.

We have a A500 with a DS2405(DiskArray) with 6 72Gbyte drives on each side of the split-bus setup.

We will be running oracle on this LVM configuration.

Any guidance in this area would be GREAT!

TIA.

Regards,
Tony Escujuri
escujuri@wellsfargo.com

[S.K. Chan]

What I noticed is in a fresh OS install (11.x) LVM knows how to adjust my PE size according to the size of my root disk. For instance if I install my OS on a 36GB disk, my PE size is set to 8MB. There is a formula to determine upfront what's the ideal extent size be for a given amount of disks that you're going to configure in a VG. It's described in ..
DOCID=KBRC00009035
http://www1.itrc.hp.com/service/cki/docDisplay.do?docLocale=en_US&docId=200000062909770
From the doc, I've calculated that in your case (total of 6x72GB=432GB) you would want to set your max PE to 212MB. I may be wrong, please double check.

[S.K. Chan]

Sorry, typo ..
"..you would want to set your max PE to 212MB"
should be
"..you would want to set your PE size to 212MB"

[Anil C. Sedha]

Hi Tony,

If i would have been in your place, i would go for a PE size of 4 Mb.

Regards,
Anil

[Ted Ellis_2 · Accepted solution]

based on the man pages for vgcreate, the smallest pe-size is 2 - MB.... it states that the pe-size must be a factor of 2 (1,2, etc). The default size is 4 MB. Since striping tends to be optomized with smaller stripes, go with the smallest of 2 MB. Don't hold your breath on performance. Most software striping uses stripes around 64 K... so you will get a lot more writes on a stripe before it moves to a different spindle. Still.. better than a kick in the head. We run a similar envoronment... 2405 disk system with 8 - 72 GB disks. Built one large volume group, with 2 physical volume groups in the volume group (PVG's)... then created each logical volume with a single mirror and strict allocation tuned to the "g" setting, which keeps the mirrors balanced across the PVG... more info than you asked for, but we looked into your questions and took this approach

Ted

[Tony Escujuri_1]

I agree with the smallest for stripping I have found via document.

Powers of 2 are (1, 2, 4, 8, etc.) Smallest extent size it 1Mbyte.

Thank you all for your replies.

Regards,
Tony Escujuri

[Bill Hassell]

Unlike striping across multiple disks where smaller values are often more effective, the PE size for LVM is an allocation unit and for really big (terabyte) volume groups, the PE size must be significantly larger than 4mb simply to keep the number of extents under control. There is an upper limit to the maximum extents allowed so for large VG's, 8mb or 16mb or even 32mb would be useful. There is no significant difference in performance for sequentially allocated PE's that are larger versus smaller.

[Wodisch_1]

Hi Tony,

since there are limits on how many PEs you can have, and how big the PVRA and the VGRA can be, you simply have to use PE sizes of more than 4MB with today's disk drives (and LUNs)!
So I usually go for a "vgcreate" like this:

vgcreate -l 32 -p 24 -s 64 -e 4096 /dev/vg...

Which would enable you to use:
-l 32 # up to 32 LVOLs in that VG
-p 24 # up to 24 disk drives in that VG
-s 64 # PEsize of 64MB each
-e 4096 # disks of up to 256GB can be added later

Remember that you will have to substitute a broken disk sooner or later - will you be able to buy another 72GB drive within the lifetime of that disk array? Or could it be the smallest drive you could get then would be 100GB at least?

FWIW,
Wodisch