About clusters

What a cluster is

A cluster is a pair of NetApp filer heads (or nodes) that are connected through an interconnect adapter and cables and are configured so that both filer heads share access to a set of Fibre Channel disks, subnets, and tape drives to provide fault tolerance. In most cases the clustered nodes are two separate filers sharing access to the same disk shelves

Note
NetApp also offers combined internal filer head and disk shelf appliances (such as the FAS270c) in which the cluster consists of two internal filer heads that reside in and share the same disk shelf.

What a cluster does

When one filer in a cluster undergoes a system failure and cannot reboot, the partner filer in the cluster can take over the failed filer functions and serve the data on the failed filer disks. This is known as a takeover.

In addition to enabling automatic takeover, you can also force a takeover at any time. You can cause a clustered filer to immediately halt and be taken over by its partner, allowing scheduled maintenance to be performed on the filer. Alternatively, you can cause a filer to halt while preventing its partner from taking over, allowing scheduled maintenance on both the filer and disks.

Cluster diagram

The following figure diagrams a common configuration of the cluster hardware components.

Cluster characteristics

The filers in a cluster have the following characteristics:

They are connected to each other through a cluster interconnect consisting of adapters and cable through which they
- Continually check whether the other filer is functioning
- Mirror log data for each other's NVRAM
- Synchronize each other's time
They use two Fibre Channel Arbitrated Loops (FC-AL loops), in which
- Each filer uses the A loop to manage its own disks
- Each filer uses the B loop in takeover mode to manage its partner's disks
They own their spare disks and do not share them with the other filer.
They may reside on the same NT domain or on different domains.

Cluster considerations

The following considerations are necessary to have a successful cluster environment:

Architecture compatibility
Both filers should have roughly the same performance and architecture so that no undue performance degradation occurs during takeover.

The number of Fibre Channel disks shared by the two filers must not exceed the capacity of either filer.
Disks and disk shelf compatibility
You can use only FC-AL disks and disk shelves in clustered configurations.

Preparing to configure a cluster

In addition to the cluster considerations described earlier, you must also ensure the following conditions before configuring a cluster:

System hardware is compatible between filers.
Disks are connected through Fibre Channel to both filers.
Interconnect host adapters and cables are installed.
Filers are attached to the same networks.
Network card interfaces are configured correctly.
Cluster software is licensed on both filers.
Cluster software is enabled on both filers.
The same system software, such as CIFS, NFS, or SyncMirror, is licensed and enabled on both filers, except in the case of vFiler, which does not have to be licensed on the takeover filer for takeover of the partner filer to succeed.
Note
Filers in a cluster can reside on the same NT domain or on different domains.