Host adapter
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In computer hardware, a host adapter or host bus adapter (HBA) connects a host system (the computer) to other network and storage devices. The terms are primarily used to refer to devices for connecting Fibre Channel, eSATA, and SCSI devices (see SCSI host adapter), but devices for connecting to ESCON, Ethernet, and other systems may also be called host adapters. Recently, the advent of iSCSI has brought about Ethernet HBAs, some including TCP Offload Engines.
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[edit] SCSI
A SCSI host adapter connects a SCSI bus to a computer. The host adapter bridges the physical and logical chasm that separates the SCSI bus from the host computer's internal bus. Modern host adapters contain all the electronics and firmware required to execute SCSI transactions, and often include a BIOS that not only allows the host system to boot from a SCSI device, but also facilitates configuration of the host adapter. Typically a device driver, linked to the operating system, controls the host adapter itself.
In a typical SCSI subsystem, each device has assigned to it a unique numerical ID. As a rule, the host adapter appears as SCSI ID 7, which gives it the highest priority on the SCSI bus (priority descends as the SCSI ID descends; on a 16-bit or "wide" bus, ID 8 has the lowest priority, a feature that maintains compatibility with the priority scheme of the 8-bit or "narrow" bus).
The host adapter usually assumes the role of "initiator," in that it issues commands to other SCSI devices.
A computer can contain more than one host adapter, which can greatly increase the number of SCSI devices available.
Current server and motherboard manufacturers are excluding on-board SCSI for SAS/SATA connectors. The only way those who purchase these machines who need SCSI connectivity will be able to access SCSI is through the purchase of a SCSI HBA.
Major HBA manufacturers are ATTO Technology, Adaptec, and LSI. ATTO Technology currently has the only PCIe SCSI adapter which fits in new Apple PCs including Mac on Intel.
[edit] Fibre Channel
Today, the term host bus adapter is most often used to refer to a Fibre Channel interface card. Fibre Channel HBAs are available for all major open systems, computer architectures, and buses, including PCI and SBus (obsolete today). Each HBA has a unique World Wide Name (WWN), which is similar to an Ethernet MAC address in that it uses an OUI assigned by the IEEE. However, WWNs are longer (8 bytes). There are two types of WWNs on a HBA; a node WWN, which is shared by all ports on a host bus adapter, and a port WWN, which is unique to each port. Major HBA manufacturers are Emulex, Qlogic, JNI, and LSI, ATTO Technology.
[edit] eSATA
External Serial ATA enclosures and drives are increasingly common in the consumer computing market, but many SATA compatible motherboards and disk controllers do not include external SATA ports. As such, adapters to connect external SATA devices to ports on an internal SATA bus are commonly available.
[edit] SAS/SATA
SAS or Serial Attached SCSI is the current connectivity to replace the previous generation of PAS or Parallel Attached SCSI devices. Ultra320 was the highest level of PAS available, SAS has extended and increased the performance of this technology.
SATA is a similar technology from the aspect of connection options. HBAs can be created using a single connector to connect both SAS and SATA devices.
SATA drives are comparable to ATA drives, being low MTBF and reliability.
SAS drives are comparable to SCSI drives, with high MTBF and high reliability.
Major SAS/SATA HBA manufacturers are Qlogic, LSI and ATTO Technology. ATTO announced a combination low profile SAS/SATA Host Adapter to the market at SNW in Fall of 2006.
