vgreduce when PVG exists

[Tim Medford]

I have a volume group "vg05" that is configured using extent-based striping across 2 fibre channel cards. I need to remove the alternate link so that it has primary path only.

I have used vgreduce to do this before, but not when the volume group was part of a PVG. Does this complicate things or is the syntax the same? Do I need to manually edit /etc/lvmpvg once I've issued the vgreduce command?

Here is an excerpt from vgdisplay and /etc/lvmpvg:

vgdisplay -v /dev/vg05
--- Physical volumes ---
PV Name /dev/dsk/c9t0d0
PV Name /dev/dsk/c10t0d0 Alternate Link
PV Status available
Total PE 25077
Free PE 8437
Autoswitch On


--- Physical volume groups ---
PVG Name PVG5
PV Name /dev/dsk/c9t0d0
PV Name /dev/dsk/c10t0d0

/etc/lvmpvg:

VG /dev/vg05
PVG PVG5
/dev/dsk/c9t0d0
/dev/dsk/c10t0d0


Thanks,
Tim

[Devender Khatana · Accepted solution]

Hi,

PVG will not make any changes to earlier steps . You will have to check your /etc/lvmpvg file after reducing the VG and will have to update it manually because I think it is updated from vgcreate and vgextend and not perhaps vgreduce.

HTH,
Devender

[Vincente Fernandes]

I have used vgreduce to do this before, but not when the volume group was part of a PVG. Does this complicate things or is the syntax the same? Do I need to manually edit /etc/lvmpvg once I've issued the vgreduce command?

Syntax is the same.
vgreduce /dev/vg05 /dev/dsk/cxtxdx
edit the /etc/lvmpvg file and remove the Alternate link entry i.e. /dev/dsk/c10t0d0 in your case.

By the way i assume you have more then 1 disk on the primary for extend based stripping correct? Since you have listed on 1 drive. 2 drives are needed to perform extend based stripping.

[Devender Khatana]

Hi,

What he has done is correct only because he is stripping it with the disk accessed through alternate path ( Other controller). So it is providing a sort of load balancing.

It is a very good practice indeed if your controller is heavily loaded (When no. of disks is very high)

HTH,
Devender

[Klaas D. Eenkhoorn]

Hi Tim,

No it does not complicate things.
If a disk is included in a PVG and the VG is reduced of the disk it will remove that disk form the PVG config.

If the disk is the alternatepath nothing will happen, if the primarypath is reduced the I/O will switch to the 'alternatepath', the last one left in your case, and that will become the primarypath.

Beware: If you reduce the disk no failover will take place in case of a problem with the primarypath. But i assume you are familliar with that fact.

Klaas

[Tim Medford]

Thanks every for the replies.

Vincent - Extent based striping is a little different than normal LVM stripes. You use a single disk/LUN and stripe across controllers. It will only work if you use physical volume groups and have a valid /etc/lvmpvg entry. If setup correctly it will round-robin through each physical volume in the PVG as extents are taken.

Keep in mind, an extent is usually fairly large (1mb, 4mb, 8mb etc...). It is load balancing at the extent level, not filesystem/block level.

Tim

[Devender Khatana]

Hi,

Yes it is a controller based stripping rather than a disk based stripping. As a result of this it supports load balancing in LVM which is not otherwise supported.

Vgreduce here also will remove the second path like normal alternate link being reduced.

HTH,
Devender