what's the adavantage of using PVG on root VG

[zs_hzh]

hi,

i understand little of PVG , only knows it's for limiting mirroring disk on different controller or SCSI bus

what i don't understand's my production system is using PVG on ROOT vg, Which only have 2 disks actually

Server RP4440 , OS 11.11

i'm curious of the why they design it that way ? i'd like to hear pros and cons

my thought is 2 root disks are always something like c2t0d0 and c3t0d0 , is it necessary to use PVG ?



--- Physical volumes ---
PV Name /dev/dsk/c2t1d0
PV Status available
Total PE 2170
Free PE 41
Autoswitch On

PV Name /dev/dsk/c3t0d0
PV Status available
Total PE 2170
Free PE 41
Autoswitch On


--- Physical volume groups ---
PVG Name scsi3_0
PV Name /dev/dsk/c3t0d0

hznkdbs1:/ (104) root% more /etc/lvmpvg
VG /dev/vg00
PVG scsi3_0
/dev/dsk/c3t0d0

[zs_hzh]

it's not like normaly example i see from docs

you have VG01,and PVG1 PVG2 , and disks under both PVG ,

in my system , VG00 are built on PV, but still 1 PVG on 1 single disk , i couldn't figure out why

(102) root% strings /etc/lvmtab
/dev/vg00
/dev/dsk/c2t1d0
/dev/dsk/c3t0d0

[Torsten.]

For a basic understanding see

http://docs.hp.com/en/B3921-60631/lvmpvg.4.html

It makes not so much sense to do this for a VG with 2 disks, but it cannot harm. But if you use it, you should use it the correct way and add all the disks from the VG.

For example (/etc/lvmpvg):

VG /dev/vg00
PVG scsi3_0
/dev/dsk/c3t0d0
PVG scsi2_0
/dev/dsk/c2t1d0

[James R. Ferguson]

Hi:

In my opinion, using physical volume groups (PVG) for a mirrored vg00 is needless.

Vg00 should be reserved solely for the standard operating system logical volumes and nothing else. Further, every logical volume thereon should be mirrored for high-availabliity. The first three (3) logical volumes must be mirrored in that order and there is no reason not to do all of them in ordinal fashion. This isn't hard to track.

Regards!

...JRF...

[Ganesan R]

Hi,

If you have only two disks you are not going to get any advantage of PVG. You can explicitly mention the mirror copies on other disks.

But imagine if you have huge no of disks on a VG and managing mirror copies on different disks and different controller is really painful job. PVG comes here to help you out.
If you created PVG to logically group the disks then lvm mirroring will be automatically done on other group when you specifiy "-s g " option.

[zs_hzh]

our system has been on this planet for over 15 years , of course it's evloving with technologies ,

but in this well designed and largely distributed system , why this seems half baked and no obvious good rack's my brain .

i saw a case of using PVG on root disk , but in his case , he has disk of 2G , and OS is 3.6G , so it has to span 2 disks at least , that's where PVG might help straight thing out ...

is it possible it's just a left-over of from the days when disk is rarely small ,from the time when i have not even graduated ,

and they forgot to update the auto-installing scripts , only have this strange looking desgin left there!

[zs_hzh]

things are there for a reason , i guess i'm obsessed by this trash !

at least you guys confirmed nowadays this kind of desgin looks wired and worthless !

any exception ?

[Torsten.]

The exception is when you have let's say 4 or even 3 disks in vg00. This was usual years ago due to the disk size.

Some has for example 2x 2GB and 1x 4GB disks mirrored, or 4x 2GB disk or whatever.

But it is not needed for 2 disks at all.

Remember, this file is always created by a user.

[zs_hzh]

yeah ,i checked case years back

it's suppose to be left-over i guess

7 years ago , when we are still using K380,and root disk is quite small , PVG is a useful feature ...but at least back then , the script create them make it good decent ,

i check the script
vgextend -g ${scsi_nbr} /dev/vg00 /dev/dsk/${mirror_device}

only this command will create /etc/lvmpvg for mirroed disks to be in PVG ?

what if i decides to add the primary to another PVG ?
what others should be done to achive it ?

thanks

HP recommends 3 disks for VG00 because of quorums , what mirror strategy better be adopt if i have 3 root disks ?

[James R. Ferguson]

Hi (again):

> only this command will create /etc/lvmpvg for mirroed disks to be in PVG ?

No, you can manually create the '/etc/lvmpvg' file too. see the manpages for 'lvmpvg(4)'.

> HP recommends 3 disks for VG00 because of quorums , what mirror strategy better be adopt if i have 3 root disks ?

This is entirely up to you. If you find/feel that having an '/etc/lvmpvg' file helps you when/if you add and/or remove mirrors then by all means do it.

Regards!

...JRF...

[Bill Hassell · Accepted solution]

> HP recommends 3 disks for VG00 because of quorums , what mirror strategy better be adopt if i have 3 root disks ?

In this case, HP is recommending triple mirroring, an ideal situation for instant backup solutions. You would use lvextend -m1 for the first mirror, and lvextend -m2 for the second mirror. lvsync the lvol and now a single disk failure results in a useable, mirrored lvol. And if your vg00 is triple mirrored, the system can boot up automatically without the hpux option -lq.

The other advantage with triple disk mirroring is that you can shutdown applications, run the sync command a few times, then split the 3rd disk off and start your applications again. Now the 3rd disk is a clean copy of the lvols and can be mounted for backup while the application continues to run.

[zs_hzh]

that's nice

thanks~~~~