changing path

[Kim Kendall]

I'm new to this site and am documenting (mapping) the EMC san to the servers. I have one server that has a volume group with one of the devices going down what should be the alt path!

vgdisplay -v - shows me that 3 of the 4 physical volumes (LUN's) are down one path, while the 4th was added using the alt path!

/dev/dsk/c16t0d1 0/2/0/0.2.23.0.0.0.1
/dev/dsk/c16t0d2 0/2/0/0.2.23.0.0.0.2
/dev/dsk/c16t0d3 0/2/0/0.2.23.0.0.0.3
/dev/dsk/c17t0d4 1/0/0/0.1.23.0.0.0.4

I've never had to do this, so my question is .... is it safe to :
vgreduce /dev/dsk/c17t0d4
and then :
vgextend /dev/dsk/c16t0d4

[Kim Kendall]

PS - there currently are no alternate paths configured on any of the devices!

[James R. Ferguson]

Hi Kim:

Using 'vgreduce' and 'vgextend' to swap alternate (pv) links is quick and without hazard.

Regards!

...JRF...

[Jeff Schussele · Accepted solution]

Hi Kim,

NO...
You WANT to ensure that ALL PVs (both paths) ARE in the VG.
Then what really matters is what path the LVs are using.
Sounds to me like whoever created this VG did not add both paths to the VG - which MUST be done.
So if you have 4 LUNS & 2 paths - there SHOULD be 8 devices in the VG.

Then what order you specify in the lvcreate command will determine which is pri or alt.
And in that case it is safe to lvreduce out the pri you want to be alt & then add it back. It will then be the alt.
It should be noted, however, that it's OK to use both paths & under loads may be preferable as your ARE using both adapters. It's a crude way to load balance.

SO definitely do an lvdisplay to see just what devs are there & what paths they are using.
You're probably going to have to do some vgextends AND some lvextends to get them ALL in there & then you can adjust which you want to be pri & alt.

HTH,
Jeff

[S.K. Chan]

Sometimes adding alt path to an existing VG just have to be done manually if the VG was not created with the alt defined at the first place. So the answer is yes you can safely "vgreduce" and "vgextend" PV-paths that you're going to use as alt path.

[Kim Kendall]

Any heartburn by doing this to a live system or should it be unmounted first?

(and BTW - Nothing has an alt path defined - on any of the servers!... I'll be doing vgextend's on everything today)

[Jeff Schussele]

Guys...

I respectfully disagree.
IF she vgreduces out the 4th PV her VG will be w/o the PV for a period of time...albeit short.
She should add the "primary" path for the 4th PV BEFORE she reduces the "alternate"....at the least, just to be safe.

But I think she should get ALL the PV paths in the VGs & LVs BEFORE she reduces or adjusts anything. Then sort her paths as she sees fit.

My 2 cents,
Jeff

[Sanjay_6]

Hi Kim,

Using vgreduce and then vgextend to change the alt and primary path for a vg is not a problem. Make sure the primary and alternate are defined before you do the vgreduce.

You can do it on a live system. If this is a production server, you may like do it afterhours if possible.

Hope this helps.

Regds

[Jeff Schussele]

Hi Kim,

No it's perfectly safe to do extends on a live system. In fact the VG has to be activated to do so. The FSs can be mounted or not...doesn't matter. But they're not required to be either.
As I pointed out I'd get ALL your paths in the VGs & LVs first....THEN it'll be safe to do reduces on the LVs to get your paths where you want them.

HTH,
Jeff