Host-Attached Storage

• Host-attached storage is storage accessed through local I/O ports.
• These ports use several technologies.
  • The typical desktop PC uses an I/O bus architecture called IDE or ATA. This architecture supports a maximum of two drives per I/O bus.
  • A newer, similar protocol that has simplified cabling is SATA.
  • High-end workstations and servers generally use more sophisticated I/O architectures, such as SCSI and fiber channel (FC).
• SCSI is a bus architecture;
  • The SCSI protocol supports a maximum of 16 devices on the bus. Generally, the devices include one controller card in the host (the SCSI initiator) and up to 15 storage devices (the SCSI targets).
  • A SCSI disk is a common SCSI target, but the protocol provides the ability to address up to 8 logical units in each SCSI target.
  • A typical use of logical unit addressing is to direct commands to components of a RAID array.
• FC is a high-speed serial architecture that can operate over optical fiber or over a four-conductor copper cable.
• A wide variety of storage devices are suitable for use as host-attached storage. Among these are hard disk drives, RAID arrays, and CD, DVD, and tape drives.
• The I/O commands that initiate data transfers to a host-attached storage device are reads and writes of logical data blocks directed to specifically identified storage units (such as bus ID, SCSI ID, and target logical unit).