LVM Mirroring

[P Arumugavel]

Hi Guru's, Let me have a clear cut idea on LVM mirroring as follows.
1. Whether Mirrored PE's can be placed on disks conncted to different controllers?
2. If the 1st point is possible then what is the concept of PVG introduced?
Give me good explanation on PVG.

[James R. Ferguson]

Hi:

LVM mirroring is implemented at the *logical volume* level.

By default, mirrored extents are allocated on separate physical volumes. This is controlled by the 'strict' allocation setting for the logical volume (the default). To defeat this is to loose all high-availability since a physical disk failure could then mean the destruction of all mirrored copies of a particular extent.

With regard to question-1, yes, mirrored extents can reside on physical disks that are connected to different controllers.

The idea of Physical Volume Groups is that this is a way to limit allocation to specific physical volumes. The '/etc/lvmpvg' files is used for this purpose. This can be useful with mirrored logical volumes as it allows you to specify the physical volumes that should be used to extend a mirrored logical volume. Regardless, LVM always ensures that physical
extent allocation can satisfy the current allocation policy or policies. To use PVG, the allocation policy of the logical volume must be set as 'PVG-strict'.

With 'strict' allocation, mirrored extents of a logical volume cannot share
the same physical volume. With 'PVG-strict' allocation, mirrors of a logical extent cannot share the same physical volume group.

See the manpages for 'lvcreate(1M)', 'lvextend(1M)', 'lvchange(1M)' and 'lvmpvg(4)' for more information.

Regards!

...JRF...

[Patrick Wallek]

With regard to question 1 -- The answer is "yes" as JRF pointed out. Additionally this is ABSOLUTELY the recommended way to mirror. You WANT your primary and mirror disks to be on different controllers. That way if a controller breaks you still have data accessible (which is the point of mirroring).

[P Arumugavel]

hi JRF / PW,
If my 1st point is possibele, so i can map the logical extents to any of the disks, connected to different controller as i desired, in terms of HA of data. Then I think i dont need of PVG-strict, which does the same. Am i right?

[Steven E. Protter]

Shalom,

You are right, but you must make sure you don't have both mirror copies of a logical volume on the same disk.

It is always best to lay out your mirrors yourself, not use sam or let the command decide which disk to use to make the mirror copy.

There are instances when strict PV rules get in the way and need to be turned off.

SEP

[James R. Ferguson]

Hi (again):

> If my 1st point is possibele, so i can map the logical extents to any of the disks, connected to different controller as i desired, in terms of HA of data. Then I think i dont need of PVG-strict, which does the same. Am i right?

'PVG-strict' is a form of 'strict' allocation as we already noted. It is designed to help you manage mirroring and the expansion of mirrored logical volumes. The '/etc/lvmpvg' file can be created before or during volume group creation.

You could certainly manage placing mirrored extents on different physical volumes without PVG-strict allocation in force. When you perform a 'lvextend' you have the option of specifying either the physical volumes or the physical volume *group* in question.

Regards!

...JRF...

[P Arumugavel]

Hi JRF,
It seems like totally mixed up. Give me some points on below, whold help me to have keen note on this concept.
What are the main advantages of PVG? (Greatful if it would be compared with some examples) and how to differ this concept?

[Torsten.]

>> What are the main advantages of PVG?

I try to keep this simple.
Let's say, you have 4 disks, 2 on each channel, HBA or whatever

A-B

C-D

You want to mirror A to C; B to D.

LVM itself makes sure you cannot mirror A to A (strict); but mirror A to B would be OK.

If you now create a group1 A and B; group2 C and D; you can create an LVOL in group1 and say "please mirror" - so the mirror is for sure on group2.



Now imagine more disks (e.g. 10) in each group. So you mirror group1 to group2, not within a group.

This concepts helps to keep this simple.




I hope this is clear, not more confusing ...

;-)

[James R. Ferguson]

Hi (again):

This guide might help you understand. In particular, see page-51:

http://bizsupport1.austin.hp.com/bc/docs/support/SupportManual/c02023734/c02023734.pdf

Regards!

...JRF...

[P Arumugavel]

Hi Torsten...,
I am coming to the scenario you shown, Disk A&B connected to Adapter1 and Disk C&D connected to Adapter2.
>>LVM itself makes sure you cannot mirror A to A (strict); but mirror A to B would be OK.
Query since my first post is LVM itself, can I mirror A to C & A to D?

[Torsten.]

>> Query since my first post is LVM itself, can I mirror A to C & A to D?

Yes, you can either mirror

A to C and A to D (double mirror)

or

A to C and B to D

with or without PVG.

[P Arumugavel]

If yes, so i can either mirror

A to C and A to D (double mirror)
or
A to C and B to D
(with or without PVG).
Then what is the need of PVG-Strict? I can have one good copy of my data accessible in the event of hardware(Adapter) failure as above...

[Torsten.]

Exactly.

If you have 2 sets of disks; each has an HBA, a disk enclosure and several disks, you can loose the HBA, some disks or even the complete enclosure and you have still access to your data.

[P Arumugavel]

So is there any special benefit by using PVG-Script? (Except DAP)

[Torsten.]

In the past in many configuration there were many external disks used. I have seen up to ~150 single disks connected to a single server.

Nowadays you have SAN based arrays.

But with so many disks you learn to love PVGs.


In addition, you can easily create extend based striped mirrors that way.

[sudheerch]

By default logical volume is created with strict allocation policy, with this policy we can mirror logical volume with in the physical volume, so with this policy there is no redundancy on disk level

So, on PVG Strict allocation policy, a group of physical volume within the volume group is a PVG we can create many no of PVG's on a volume group. here we can mirror the logical volume in two PVG's

[Torsten.]

>> By default logical volume is created with strict allocation policy...

correct.


>> ... with this policy we can mirror logical volume with in the physical volume, so with this policy there is no redundancy on disk level ...

not correct.


Strict means

"...Mirrors of a logical extent cannot share the same physical volume. This is the default."

source: man lvcreate

[sudheerch]

strict allocation policy is not enforced, both mirrors will be able to reside on the same physical volume
And PVG Strict allocation policy is also doing the same but here we are diving the PV's on the volume group

what is the main difference of PVG and Strict

[Torsten.]

>> strict allocation policy is not enforced, both mirrors will be able to reside on the same physical volume


But only if you set strict to false.