Disk Attachment

Disk Attachment

Disk drives can be attached either directly to a particular host (a local disk) or to a network.

a. Host-Attached Storage

Local disks are accessed through I/O Ports. The most common interfaces are IDE (Integrated Drive Electronics) or ATA (Advanced Technology Attachment), each of which allow up to two drives per host controller. SATA (Serial ATA) is similar with simpler cabling.

High end workstations or other systems in need of larger number of disks typically use SCSI disks:

·         The SCSI standard supports up to 16 targets on each SCSI bus, one of which is generally the host adapter and the other 15 of which can be disk or tape drives.

·         A SCSI target is usually a single drive, but the standard also supports up to 8 units within each target. These would generally be used for accessing individual disks within a RAID array.

FC is a high-speed serial architecture that can operate over optical fiber or four-conductor copper wires, and has two variants:

·         A large switched fabric having a 24-bit address space. This variant allows for multiple devices and multiple hosts to interconnect, forming the basis for the storage-area networks, SANs, to be discussed in a future section.

·         The arbitrated loop, FC-AL,  (that can address up to 126 devices drives and controllers.)

b. Network-Attached Storage

Network attached storage connects storage devices to computers using a remote procedure call, RPC, interface, typically with something like NFS file system mounts. This is convenient for allowing several computers in a group common access and naming conventions for shared storage.

NAS can be implemented using SCSI cabling, or iSCSI uses Internet protocols and standard network connections, allowing long-distance remote access to shared files.

NAS allows computers to easily share data storage, but tends to be less efficient than standard host-attached storage.

c. Storage-Area Network

A Storage-Area Network, SAN, connects computers and storage devices in a network, using storage protocols instead of network protocols. One advantage of this is that storage access does not tie up regular networking bandwidth.

SAN is very flexible and dynamic, allowing hosts and devices to attach and detach on the fly. SAN is also controllable, allowing restricted access to certain hosts and devices.