10.6. Nested LVM configuration with DRBD

10.6. Nested LVM configuration with DRBD
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It is possible, if slightly advanced, to both use Logical Volumes as backing devices for DRBD and at the same time use a DRBD device itself as a Physical Volume. To provide an example, consider the following configuration:

We have two partitions, named /dev/sda1, and /dev/sdb1, which we intend to use as Physical Volumes.
Both of these PVs are to become part of a Volume Group named local.
We want to create a 10-GiB Logical Volume in this VG, to be named r0.
This LV will become the local backing device for our DRBD resource, also named r0, which corresponds to the device /dev/drbd0.
This device will be the sole PV for another Volume Group, named replicated.
This VG is to contain two more logical volumes named foo(4 GiB) and bar(6 GiB).

In order to enable this configuration, follow these steps:

Set an appropriate filter option in your /etc/lvm/lvm.conf:
```
indexterm:[LVM]indexterm:[filter expression (LVM)]
```

filter = ["a|sd.*|", "a|drbd.*|", "r|.*|"]

This filter expression accepts PV signatures found on any SCSI and DRBD devices, while rejecting (ignoring) all others.

After modifying the lvm.conf file, you must run the vgscan command so LVM discards its configuration cache and re-scans devices for PV signatures.

Disable the LVM cache by setting:

write_cache_state = 0

After disabling the LVM cache, make sure you remove any stale cache entries by deleting /etc/lvm/cache/.cache.

Now, you may initialize your two SCSI partitions as PVs:

pvcreate /dev/sda1
Physical volume "/dev/sda1" successfully created
pvcreate /dev/sdb1
Physical volume "/dev/sdb1" successfully created

The next step is creating your low-level VG named local, consisting of the two PVs you just initialized:

vgcreate local /dev/sda1 /dev/sda2
Volume group "local" successfully created

Now you may create your Logical Volume to be used as DRBD’s backing device:

lvcreate --name r0 --size 10G local
Logical volume "r0" created

Repeat all steps, up to this point, on the peer node.
Then, edit your /etc/drbd.conf to create a new resource named r0:

resource r0 {
  device /dev/drbd0;
  disk /dev/local/r0;
  meta-disk internal;
  on <host> { address <address>:<port>; }
  on <host> { address <address>:<port>; }
}

After you have created your new resource configuration, be sure to copy your drbd.conf contents to the peer node.

After this, initialize your resource as described in Section 5.4, “Enabling your resource for the first time”(on both nodes).
Then, promote your resource (on one node):

drbdadm primary r0

Now, on the node where you just promoted your resource, initialize your DRBD device as a new Physical Volume:

pvcreate /dev/drbd0
Physical volume "/dev/drbd0" successfully created

Create your VG named replicated, using the PV you just initialized, on the same node:

vgcreate replicated /dev/drbd0
Volume group "replicated" successfully created

Finally, create your new Logical Volumes within this newly-created VG:

lvcreate --name foo --size 4G replicated
Logical volume "foo" created
lvcreate --name bar --size 6G replicated
Logical volume "bar" created

The Logical Volumes foo and bar will now be available as /dev/replicated/foo and /dev/replicated/bar on the local node.

To make them available on the peer node, first issue the following sequence of commands on the local node:

vgchange -a n replicated
0 logical volume(s) in volume group "replicated" now active
drbdadm secondary r0

Then, issue these commands on the peer node:

drbdadm primary r0
vgchange -a y replicated
2 logical volume(s) in volume group "replicated" now active

After this, the block devices /dev/replicated/foo and /dev/replicated/bar will be available on the peer node.

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