What should the PE size BE

[Tony Escujuri_1]

Hello all,

What should the PE size be for disks this size.
The installation created vg00 with 16Mbytes for the root extents...:

PE Size (Mbytes) 16
Total PE 4374

Now I need to create a new VG on another 72Gbyte drive... what should I make the extent size? This will be running RME(CiscoWorks 2000) if that helps.


Any assistance would be wonderful!
Tony Escujuri
tony@unixadm.net

[Bill Hassell]

Can't quite answer the question since the original limits of the VG are not shown. When you first create a volume group, you get 1016 extents unless you override that value. Since the 72 Gb disk will require 4500 extents that are 16 megs in size, you'll need Max PE per PV larger than: 4374+4500 or 8874. If MAX PE is not that large, then the disk cannot be fully utilized.

This is a common problem as massively large disks have become available. VG00 is created many months ago with no idea that a much larger disk will be needed, so MAX PE is often too small and there is no way to increase thos value--the volume group must be removed and rebuilt. Since this is VG00, that is not easy, but can be done quickly with make_tape_recovery in the interactive mode. If you don't have make_tape_recovery, download a copy as soon as possible from Ignite/UX.

However, I would question the need to increase VG00. VG00 is for the operating system and needs to be perhaps 3-5 Gbytes in size. Large data files and applications are better managed on another volume group, in which this MAX PE issue on VG00 will no longer apply.

[Steven Sim Kok Leong]

Hi,

The maximum PEs you can have for a VG is 65535, the default is 1016.

You have a 72 GB (73728 MB) harddisk. If you use this harddisk in a new VG, then

73728 MB / 1016 approx equals 73.

The PE size must be a power of 2. If you use the default max_pe of 1016, then your PE size has to be 2^7 or 128 MB. If you wish to use a PE size of 64 MB (2^6) or 32 MB (2^5), then you have to increase your max_pe accordingly.

Use the option -e max_pe in vgcreate to increase your max_pe. If you don't wish to increase your max_pe, you will need to stick with 2^7 or 128 MB PE size.

Hope this helps. Regards.

Steven Sim Kok Leong

[Tony Escujuri_1]

Here is that requested information....
# vgdisplay -v vg00 | more
--- Volume groups ---
VG Name /dev/vg00
VG Write Access read/write
VG Status available
Max LV 255
Cur LV 8
Open LV 8
Max PV 16
Cur PV 1
Act PV 1
Max PE per PV 4384
VGDA 2
PE Size (Mbytes) 16
Total PE 4374
Alloc PE 738
Free PE 3636
Total PVG 0
Total Spare PVs 0
Total Spare PVs in use 0

[Tony Escujuri_1]

What is the benefits with going higher or lower with PE sizes? This is the question I should of asked. Will performance be sacraficed with larger PE size or vis versa?

Thank for the input in advance!!
Tony Escujuri
tony@unixadm.net

[Steven Sim Kok Leong]

Hi,

>> Now I need to create a new VG on another 72Gbyte drive... what should I make the extent size?

Did I hear you correct that you want to create a _NEW_ VG with a _NEW_ disk or you just want to add a _NEW_ disk to an _EXISTING_ VG? :-)

In addition to the computation, note the following from the vgcreate man page:

However, if the size of any physical volume exceeds 1016 times the pe_size, the default value for max_pe is adjusted to match the physical volume size. The maximum number of physical extents can be a value in the range 1 to 65535.

Hope this helps. Regards.

Steven Sim Kok Leong

[Animesh Chakraborty]

Hi,
We had lot of discussions here on this topics

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0xed7246ff9277d511abcd0090277a778c,00.html

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0x6deb5f260cafd4118fef0090279cd0f9,00.html

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0x5e7b5220af9bd5118ff10090279cd0f9,00.html

[Steven Sim Kok Leong]

Hi,

One disadvantage of a large PE is that when it turns bad (turns stale), it affects more of your data.

The other disadvantage I can think of is that when you perform PE striping, you have to end up with a larger stripe size.

Since LE sits on PE, another disadvantage is OS LV striping at the LV level since alike PE striping, you have to resort to a larger stripe size.

Thus, if you are intending software implementation of RAID 0, it may not be an efficient size depending on how large a chunk your application reads at each go.

If you are not into striping, I don't think there is too much of a handicap using large PE (except every stale or faulty extent will affect a larger set of data).

Since the filesystem block size and fragment size is not dependent on the PE size, it should logically not affect filesystem performance.

Hope this helps. Regards.

Steven Sim Kok Leong