[Linux-cluster] CLVM clarification

[Angelo Compagnucci]

Ok, this could be clear, but in the Cluster_Logical_Volume_Manager.pdf I've
read (bottom of page 3):
"The clmvd daemon is the key clustering extension to LVM. The clvmd daemon
runs in each cluster computer and distributes LVM metadata updates in a
cluster, presenting each cluster computer with the same view of the logical
volumes"

This is a picture of wath I have in mind:

-----------------------------------------
|      GFS filesystem            |
-----------------------------------------
|            LV                         |
-----------------------------------------
|            VG                        |
-----------------------------------------
|  PV1     |  PV2     |   PV3   |
-----------------------------------------
| GNBD1 | GNBD2 | GNBD3 |
-----------------------------------------
| hda1     |  hda1    |   hda1   |
| Node1   | Node2  |   Node3 |
-----------------------------------------

In this case the clvm features are not useful because there is only one
machine (that could not be a node of a cluster) that have the lvm over GNBD
exported devices. So the nodes doesn't know nothing about the other nodes.

Let's pose this situation:

-----------------------------------------------
|            GFS                           |
-----------------------------------------------
|                LV                          |
-----------------------------------------------
|         VG1         |        VG2      |
-----------------------------------------------
|         PV1         |        PV2       |
|      Node1         |      Node2      |
-----------------------------------------------
|        CLVM coordinates           |
-----------------------------------------------

In this situatuation makes sense to have a clustered lvm because if I have
to make some maintenance over VGs, CLVM can lock and unlock the interested
device.

Is this the correct behaviour??

In the contrary, which is the CLVM role in a cluster?


2008/10/2 Xavier Montagutelli <xavier.montagutelli at unilim.fr>

> On Wednesday 01 October 2008 17:39, Angelo Compagnucci wrote:
> > Hi to all,This is my first post on this list. Thanks in advance for every
> > answer.
> >
> > I've already read every guide in this matter, this is the list:
> >
> > Cluster_Administration.pdf
> > Cluster_Logical_Volume_Manager.pdf
> > Global_Network_Block_Device.pdf
> > Cluster_Suite_Overview.pdf
> > Global_File_System.pdf
> > CLVM.pdf
> > RedHatClusterAdminOverview.pdf
> >
> > The truth is that I've not clear a point about CLVM.
> >
> > Let's me make an example:
> >
> > In this example CLVM and the Cluster suite are fully running without
> > problems. Let's pose the same configuration of cluster.conf and lvm.conf
> > and the nodes of the cluster are joined and operatives.
>
> Does your example include a shared storage (GNBD, iSCSI, SAN, ...) ?
>
> >
> > NODE1:
> >
> > pvcreate /dev/hda3
> >
> > NODE2:
> >
> > pvcreate /dev/hda2
> >
> > Let's pose that CLVM spans LVM metadata across the cluster, if I stroke
> the
> > command:
> >
> > pvscan
> >
> > I should see /dev/sda2 and /dev/sda3
> >
> > and then I can create a vg with
> >
> > vgcreate /dev/sda2 /dev/sda3 ...
> >
> > The question is: How LVM metadata sharing works? I have to use GNBD on
> the
> > row partion to share a device between nodes? I can create a GFS over a
> > spanned volume group? Are shareable only logical volumes?
>
> I have the feeling that something is not clear here. I am not an expert,
> but :
>
> GNBD is just a mean to export a block device on the IP network. A GNBD
> device
> is accessible to multiple nodes at the same time, and thus you can include
> that block device in a CLVM Volume Group. Instead of GNBD, you can also use
> any other shared storage (iSCSI, FC, ...). Be careful, from what I have
> understood, some SAN storage are not sharable between many hosts (NBD, AoE
> for example) !
>
> After that, you have the choice :
>
>  - to make one LV with a shared filesystem (GFS). You can then mount the
> same
> filesystem on many nodes at the same time.
>
>  - to make many LV with an ext3 / xfs / ... filesystem. But you then have
> to
> make sure that one LV is mounted on only one node at a given time.
>
> But the type of filesystem is independant, this is a higher component.
>
> In this picture, CLVM is only a low-level component, avoiding the
> concurrent
> access of many nodes on the LVM metadata written on the shared storage.
>
> The data are not "spanned" across the local storage of many nodes (well, I
> suppose you *could* do that, but you would need other tools / layers ?)
>
> Other point : if I remember correctly, the Red Hat doc says it's not
> recommended to use GFS on a node that exports a GNBD device. So if you use
> GNBD as a shared storage, I suppose it's better to specialize one or more
> nodes as GNBD "servers".
>
>
> HTH
>
> >
> > Thanks for your answers!!
>
> --
> Xavier Montagutelli                      Tel : +33 (0)5 55 45 77 20
> Service Commun Informatique              Fax : +33 (0)5 55 45 75 95
> Universite de Limoges
> 123, avenue Albert Thomas
> 87060 Limoges cedex
>
> --
> Linux-cluster mailing list
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>
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