regarding physical group

[Tvs]

hai all

pls help me to find out what is a physical groyp

what is the differnce between a physical disk and physical group. what is the advantage of having physical group

thanks in advance

[Mahesh Kumar Malik]

Hi

When physical group is used to group say multiple physical disks while creating the volume group, physical links are maintained. This feature is useful while mirroring the drives. Drive extents will follow physical links as per PVGroup configuration.

Following link may also be of help

http://docs.hp.com/en/B2355-90692/vgcreate.1M.html

Regards
Mahesh

[Tvs]

pls helpme
1. how to create a physcial group

2. what is the advantage using physical group instaed of physcial disk

[Alessandro Pilati · Accepted solution]

1) You can create PVGs directly editing the /etc/lvmpvg ascii file.
Here it is an example of the format:

VG VG01
PVG PVG1
pv_path1
pv_path2
...

VG VG01
PVG PVG2
pv_path3
pv_path4
...

VG VG02
PVG PVG1
pv_path5
pv_path6
...

You can use the same name for PVG across different VGs, but not the same inside a VG
Be sure that the PVs owned by a PVG are owned by a VG.

2) The advantages are for example a simpler management of displacing mirror copies for a LV only to PVs you want ( example: to the lvextend command you can specify a specific PVG )

Rgds,
Alex

[Sudeesh]

If there are multiple host bus adapters (SCSI or fibre channel) available on the system it is useful in terms of high availablility to have mirror copies located on different adapters. The strict allocation policy for mirrored LVs guarantees that the mirror copy will not be placed on the same disk but it could be placed on a disk that is on the same adapter. The latter case can be avoided by using physical volume groups. A PVG is a subset of PVs within a VG that can be defined using -p option of vgcreate/vgextend or simply by creating an ascii file called /etc/lvmpvg.

Sudeesh

[Sudeesh]

See page 21 and 22 in the attached doc for more details.


Sudeesh

[Tim Nelson]

I believe the biggest purpose of the PVGs was for extent based striping. Once upon a time in order to to extent based striping you had to write a script that would allocate one extent from one disk, then mirror to another, then add another extent to the next disk, and mirror that, over and over and over ( ie. script the creation, and it took forever to build). In late 10.20 HP added the -D option to lvcreate. This required some type of grouping i.e. and the use of the -s g option. -D (distributed stripe) -s g ( use strict groupings so overlap of mirrors and primaries did not occur).

The extent based mirrored stripe ( psuedo raid 0+1, although the stripe size is now 4MB or the size of pesize) works great and is loads faster than concantenated volumes. Pound for pound as fast as some array stripes in 0+1 without the cache assist.

If you have local disks, use it. Our typical config was two trays of DS2100 disk, 4 36GB drives in each, extent based stripe on the 4 mirrored to the other 4 in the other tray.