Longhorn supports to use more than one disk on the nodes to store the volume data. To add a new disk for a node, heading to
Node tab, select one of the node, and click the edit disk icon. By default,
/var/lib/rancher/longhorn on the host will be used for storing the volume data.
To add any additional disks, user needs to:
- Mount the disk on the host to a certain directory.
- Add the path of the mounted disk into the disk list of the node.
Longhorn will detect the storage information (e.g. maximum space, available space) about the disk automatically, and start scheduling to it if it's possible to accomodate the volume in there. A path mounted by the existing disk won't be allowed.
User can reserve a certain amount of space of the disk to stop Longhorn from using it. It can be set in the
Space Reserved field for the disk. It's useful for the non-dedicated storage disk on the node.
The kubelet needs to preserve node stability when available compute resources are low. This is especially important when dealing with incompressible compute resources, such as memory or disk space. If such resources are exhausted, nodes become unstable. To avoid kubelet
Disk pressure issue after scheduling several volumes, by default, longhorn reserved 30% of root disk space (
/var/lib/rancher/longhorn) to ensure node stability.
Nodes and disks can be excluded from future scheduling. Notice any scheduled storage space won't be released automatically if the scheduling was disabled for the node.
There are two global settings affect the scheduling of the volume as well.
StorageOverProvisioningPercentage defines the upper bound of
ScheduledStorage / (MaximumStorage - ReservedStorage) . The default value is
500 (%). That means we can schedule a total of 750 GiB Longhorn volumes on a 200 GiB disk with 50G reserved for the root file system. Because normally people won't use that large amount of data in the volume, and we store the volumes as sparse files.
StorageMinimalAvailablePercentage defines when a disk cannot be scheduled with more volumes. The default value is
10 (%). The bigger value between
MaximumStorage * StorageMinimalAvailablePercentage / 100 and
MaximumStorage - ReservedStorage will be used to determine if a disk is running low and cannot be scheduled with more volumes.
Notice currently there is no guarantee that the space volumes used won't exceed the
- Longhorn volume can be bigger than specified size, due to the snapshot contains the old state of the volume
- And Longhorn is doing over-provisioning by default.