In many cases, existing DRBD resources that were created manually some time in the past can be migrated, so that LINSTOR can be used to manage those resources afterwards.
While resources managed by LINSTOR can coexist with resources that were created manually, it can often make sense to migrate such existing resources. When resources are migrated to LINSTOR manually, there are some settings that must be carefully adjusted to match the resource’s existing configuration in LINSTOR.
Here is an overview of the migration process. It is, however, important to note that there are countless special cases, such as existing resources backed by different storage pools on different nodes or resources that use external metadata, where additional steps may be required for a successful migration.
This article describes the migration of an existing DRBD resource with a rather typical configuration.
LINSTOR must be installed, configured and running already.
Sample configuration assumed by this article for the sample commands:
1 existing DRBD resource:
TCP/IP port number 15500
Volume number 10
Minor number 9100 (/dev/drbd9100)
LVM volume group datastore
Logical volume name legacy (/dev/datastore/legacy)
Logical volume size 1,340 MiB
Node eagle Node ID 4
Node rabbit Node ID 8
Meta data type internal
Peer slots 4
Create a resource definition for the existing resource
This may require renaming the resource in some cases, because LINSTOR’s naming rules are stricter than DRBD’s naming rules. The TCP/IP port number can be assigned automatically by LINSTOR if you don’t mind the change, and if a short disconnect/reconnect cycle during the migration is not going to cause problems. Otherwise, you can specify the port number manually to set it to the value that the resource is currently using.
resource-definition create legacy -p 15500
Adjust the DRBD peer slots count for the resource definition
If LINSTOR needs to create additional resources, or has to recreate DRBD meta data, the current peer count used by the resource’s volumes can be important, because it has a direct influence on the net size of the DRBD device that is created.
Note: If the peer count of a DRBD volume is unknown, it can be extracted from a meta data dump, which can be displayed using the drbdadm dump-md command (this requires the volume to be detached, or the resource to be stopped, and will normally require execution of the drbdadm apply-al command first).
resource-definition set-property legacy PeerSlotsNewResource 4
Create a volume definition
While manual resource creation typically involves specifying the size of the backend storage device, in this case a logical volume, which will result in DRBD using whatever space is left after meta data creation, LINSTOR works with the net size of a DRBD volume.
To figure out the correct size to use for the LINSTOR volume definition, you can query the net size of the DRBD device using the blockdev utility:
blockdev –getsize64 /dev/drbd9100
LINSTOR stores volume sizes in kiB internally, and the value yielded by the blockdev utility is in bytes, so you will have to divide by 1024. There should be no rest (modulo-division by 1024 should yield zero), unless you are dealing with a special case that might require additional steps to migrate successfully.
In the case of the sample configuration assumed by this article, a 1,340 MiB logical volume with DRBD meta data with a peer count of 4 will result in a net size of 1,371,956 kiB. This is the value to use for the volume definition in LINSTOR.
The volume number (10) and the minor number (9100) should also be specified manually, unless you want LINSTOR to allocate new numbers automatically, which will typically require a reconfiguration of the applications or filesystem entries that are using the existing DRBD resource.
volume-definition create legacy -n 10 -m 9100 1371956KiB
If, for some reason, the backing storage volume is significantly larger than what would be required to fit the net size reported by the DRBD volume, then there is an additional property that can be set on the volume definition:
volume-definition set-property legacy 10 AllowLargerVolumeSize true
Setting this property is not normally required. It should be set if the LINSTOR satellite presents an error regarding the backing storage volume’s size being larger than expected.
Set the name of the manually created backing storage volume
LINSTOR normally generates the name for the backing storage volume, but the name of that volume can be overridden by setting a property on the volume definition as long as the backing storage volume name is the same on each node.
volume-definition set-property legacy 10 OverrideVlmId legacy
Create resources from the resource definition
Finally, resources must be created from the resource definition in LINSTOR. When creating resources, the resource’s DRBD node id on each node must match the node id used by the manually created resource. You can lookup the node-id in the existing, manually created DRBD resource configuration file.
Before the final step of creating LINSTOR resources from the resource definition can be executed, the existing manually created DRBD resource configuration file must be moved away, otherwise drbdadm will complain about duplicate definitions.
mv /etc/drbd.d/legacy.res /etc/drbd.d/legacy.res.disabled
It is a good idea to stop the LINSTOR satellites before creating the resources, otherwise the DRBD resource will disconnect from other nodes as the first resource is created, and will then reconnect as the other resources (on other nodes) are added.
By creating the resources first and starting the satellites after all resources have been created, the satellites will immediately configure all connections, thereby normally avoiding a disconnect/reconnect cycle.
resource create --node-id 4 -s fatpool eagle legacy resource create --node-id 8 -s fatpool rabbit legacy
(If the satellites are currently stopped, add the –async parameter to the command line to avoid having the client wait for the creation of each resource, which would not take place if the satellite is offline)
After finishing the migration, LINSTOR can manage the DRBD resource just like resources that were originally created by LINSTOR.