topic Re: PE size in Operating System - HP-UX

PE size

himacs — Sun, 24 May 2009 18:33:10 GMT

Hi Admins,

While vgcreate we use -s to set PE size.Default is 4 MB.Why we required to change the PE size from default 4MB.whats the advantages from doing so

regards
himacs

Re: PE size

Steven E. Protter — Sun, 24 May 2009 18:47:33 GMT

Shalom,

With the possible exception of a new feature in 11.31, you must re-create the volume group to change the -s setting for PE size.

The advantages are bigger volume groups. Some performance advantage can be gained with bigger PE's and Oracle, that is a matter of dispute and debate.

SEP

Re: PE size

James R. Ferguson — Sun, 24 May 2009 20:34:29 GMT

Hi:

You choose a non-default 'pe_size' during a 'vgcreate' to allow yourself to increase the size of the physical disks that comprise a volume group as it subsequently grows. As the value of 'pe_size' increases (in powers of 2) so does the volume size.

Once set 'pe_size' is forever fixed. Even 'vgmodify' (available in 11.31 and in 11.23 with patches) can NOT alter its value.

See for more insight:

http://www.docs.hp.com/en/B2355-60130/vgcreate.1M.html

http://www.docs.hp.com/en/B2355-60130/vgmodify.1M.html

Regards!

...JRF...

Re: PE size

Bill Hassell — Mon, 25 May 2009 01:23:54 GMT

The PE size is a bookeeping item. LVM is limited to 65 thousand PE's so regardless of how big your volume group might be, it cannot have more than 65k PEs. So a 4 MB PE size is extremely limiting for large (multi-GB) volumes. The PE size has virtually no effect on disk I/O performance. If you will be dealing with large volumes (terabytes), the PE size will have to be 16, 32 or 64 MB.

Re: PE size

Taifur — Mon, 25 May 2009 08:21:42 GMT

Hi Himacs

The PE size is only used for bookeeping, and has nothing to do with actual read or write performance.

Rgds//
Taifur

Re: PE size

James R. Ferguson — Mon, 25 May 2009 12:09:04 GMT

Hi:

@ Taifur: That's a really nice plagarism of Bill's post about 7-hours earlier. Exactly what value did you add to this thread with your "me-too"?

...JRF...

Re: PE size

Sajjad Sahir — Mon, 25 May 2009 12:17:48 GMT

Dear Himacs

u can change pe size during volume group creation by using vgcreate -s option
Sets the physical extent size on physical volumes of this volume group. A size suffix (k for kilobytes up to t for terabytes) is optional, megabytes is the default if no suffix is present. The default is 4 MB and it must be at least 1 KB and a power of 2.
Once this value has been set, it is difficult to change it without recreating the volume group which would involve backing up and restoring data on any logical volumes. However, if no extents need moving for the new value to apply, it can be altered using vgchange -s.

If the volume group metadata uses lvm1 format, extents can vary in size from 8KB to 16GB and there is a limit of 65534 extents in each logical volume. The default of 4 MB leads to a maximum logical volume size of around 256GB.

If the volume group metadata uses lvm2 format those restrictions do not apply, but having a large number of extents will slow down the tools but have no impact on I/O performance to the logical volume. The smallest PE is 1KB.

The 2.4 kernel has a limitation of 2TB per block device.

thanks and regards

Sajjad Sahir

Re: PE size

Viktor Balogh — Mon, 25 May 2009 17:59:50 GMT

2.4 kernel?? hey, it's an HP-UX forum!

Re: PE size

Bill Hassell — Mon, 25 May 2009 21:33:03 GMT

> Sajjad: Linux comments...

Many of the above statements about Linux are incorrect for HP-UX. Always consult the man pages for the system you are using.