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PE size

himacs
Super Advisor

PE size

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
8 REPLIES
Steven E. Protter
Exalted Contributor

Re: PE size

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.

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Steven E Protter
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James R. Ferguson
Acclaimed Contributor

Re: PE size

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...
Bill Hassell
Honored Contributor

Re: PE size

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.


Bill Hassell, sysadmin
Taifur
Respected Contributor

Re: PE size

Hi Himacs

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


Rgds//
Taifur
James R. Ferguson
Acclaimed Contributor

Re: PE size

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...
Sajjad Sahir
Honored Contributor

Re: PE size

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

Viktor Balogh
Honored Contributor

Re: PE size

2.4 kernel?? hey, it's an HP-UX forum!
****
Unix operates with beer.
Bill Hassell
Honored Contributor

Re: PE size

> 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.


Bill Hassell, sysadmin