Modern computer networks can include various types of storage servers. Storage servers can be used for many different purposes, such as to provide multiple users with access to shared data or to back up mission critical data. A file server is one type of storage server, which operates on behalf of one or more clients to store and manage shared files in a set of mass storage devices, such as magnetic or optical storage based disks or tapes. The mass storage devices are typically organized into one or more volumes of Redundant Array of Independent (or Inexpensive) Disks (RAID).
One configuration in which a file server can be used is a network attached storage (NAS) configuration. In a NAS configuration, a file server can be implemented in the form of an appliance, called a filer, that attaches to a network, such as a local area network (LAN) or a corporate intranet. An example of such an appliance is any of the Filer products made by Network Appliance, Inc. in Sunnyvale, Calif.
A storage server can also be employed in a storage area network (SAN). A SAN is a highly efficient network of interconnected, shared storage devices. In a SAN, the storage server (which may be an appliance) provides a remote host with block-level access to stored data, whereas in a NAS configuration, the storage server provides clients with file-level access to stored data. Some storage servers, such as certain Filers from Network Appliance, Inc. are capable of operating in either a NAS mode or a SAN mode, or even both modes at the same time. Such dual-use devices are sometimes referred to as “unified storage” devices. A storage server such as this may use any of various protocols to store and provide data, such as Network File System (NFS), Common Internet File system (CIFS), Internet SCSI (ISCSI), and/or Fibre Channel Protocol (FCP).
Historically, file server systems used in NAS environments have generally been packaged in either of two forms: 1) an all-in-one custom-designed system that is essentially just a standard computer with built-in disk drives, all in a single chassis; or 2) a modular system in which one or more sets of disk drives (each set being mounted in a separate chassis) are connected to a separate external file server “head”. Examples of all-in-one file server systems are the F8x, C1xxx and C2xxx series Filers made by Network Appliance, Inc. Examples of modular filer heads are the F8xx and FAS9xx heads made by Network Appliance, Inc.
In this context, the term “head” means all of the electronics, firmware and/or software that is used to control access to storage devices in a storage system; it does not include the disk drives themselves. In a file server, the head normally is where all of the “intelligence” of the file server resides. Note that a “head” in this context is not the same as, and is not to be confused with, the magnetic or optical head used to physically read or write data to a disk.
In a modular file server system, the system can be built up by adding multiple disk enclosures in some form of rack and then cabling the disk enclosures together. The disk drive enclosures are often called “shelves”, and more specifically, “just a bunch of disks” (JBOD) shelves. The term JBOD indicates that the enclosure essentially contains only physical storage devices and no substantial electronic “intelligence”. Some disk drive enclosures include one or more RAID controllers, but such enclosures are not normally referred to as “JBOD” due to their greater functional capabilities.
Modular storage systems and all-in-one storage systems each have various shortcomings, as noted in Reger (referenced above). Reger describes a standalone network storage server that overcomes some of the shortcomings of modular storage systems and all-in-one storage systems. The standalone storage server includes multiple internal single-board heads and multiple internal disk drives, all contained within a single chassis and connected to each other by an internal passive backplane. Each head contains the electronics, firmware and software along with built-in I/O connections to allow the disks in the enclosure to be used as a NAS file server and/or a SAN storage device.
Reger also describes that the standalone storage server can be easily converted into a JBOD shelf (essentially, by removing the internal heads and replacing them with I/O modules) and then integrated into a modular storage system such as described above. This allows a storage system to be grown in capacity and/or performance by combining the converted JBOD shelf with one or more separate (modular), more-powerful file server heads, such as Network Appliance F8xx or FAS9xx series heads, and additional JBOD shelves.
Although this convertability makes the standalone storage server very versatile, reconfiguring a storage system in this manner is not a trivial task. This type of system reconfiguration can require fairly extensive rerouting and addition of cables to allow the modular heads to control the disks in the newly-converted JBOD shelf (converted from the standalone storage server). In many storage systems with redundant heads, each disk is “owned” by (primarily accessed by) only one head, and disk ownership is determined by the cable connections. For example, in some systems, each disk has two external ports, port A and port B, which are connected (at least indirectly) to two separate heads. Only the head connected to port A owns the disk, while the head connected to port B assumes a backup role for purposes of accessing that disk.
To integrate a converted JBOD shelf into a modular system in the manner described above requires reassigning ownership of all of the disks in the converted JBOD shelf (which were owned by the removed internal heads) to an external modular head. As indicated above, such reassignment of ownership can require moving disks from one enclosure to another as well as extensive rerouting of cables and/or addition of new external cabling, all of which is inconvenient and complicated.