This guide describes DRBD version 8.4 and above. For 8.3 please look here.


10.4. Configuring a DRBD resource as a Physical Volume

In order to prepare a DRBD resource for use as a Physical Volume, it is necessary to create a PV signature on the DRBD device. In order to do so, issue one of the following commands on the node where the resource is currently in the primary role:

# pvcreate /dev/drbdX

or

# pvcreate /dev/drbd/by-res/<resource>/0
[Note]Note

This example assumes a single-volume resource.

Now, it is necessary to include this device in the list of devices LVM scans for PV signatures. In order to do this, you must edit the LVM configuration file, normally named /etc/lvm/lvm.conf. Find the line in the devices section that contains the filter keyword and edit it accordingly. If all your PVs are to be stored on DRBD devices, the following is an appropriate filter option:

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

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

[Note]Note

By default, LVM scans all block devices found in /dev for PV signatures. This is equivalent to filter = [ "a|.*|" ].

If you want to use stacked resources as LVM PVs, then you will need a more explicit filter configuration. You need to make sure that LVM detects PV signatures on stacked resources, while ignoring them on the corresponding lower-level resources and backing devices. This example assumes that your lower-level DRBD resources use device minors 0 through 9, whereas your stacked resources are using device minors from 10 upwards:

filter = [ "a|drbd1[0-9]|", "r|.*|" ]

This filter expression accepts PV signatures found only on the DRBD devices /dev/drbd10 through /dev/drbd19, 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.

You may of course use a different filter configuration to match your particular system configuration. What is important to remember, however, is that you need to

In addition, you should 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.

You must repeat the above steps on the peer node.

[Important]Important

If your system has its root filesystem on LVM, Volume Groups will be activated from your initial ramdisk (initrd) during boot. In doing so, the LVM tools will evaluate an lvm.conf file included in the initrd image. Thus, after you make any changes to your lvm.conf, you should be certain to update your initrd with the utility appropriate for your distribution (mkinitrd, update-initramfs etc.).

When you have configured your new PV, you may proceed to add it to a Volume Group, or create a new Volume Group from it. The DRBD resource must, of course, be in the primary role while doing so.

# vgcreate <name> /dev/drbdX
[Note]Note

While it is possible to mix DRBD and non-DRBD Physical Volumes within the same Volume Group, doing so is not recommended and unlikely to be of any practical value.

When you have created your VG, you may start carving Logical Volumes out of it, using the lvcreate command (as with a non-DRBD-backed Volume Group).


This guide describes DRBD version 8.4 and above. For 8.3 please look here.