Serial attached SCSI is leading the pack of storage interfaces. (Storage Networking).Of the storage interfaces available today, as well as those that will soon enter the market, Serial Attached SCSI See SAS. is clearly in the leadership position as the most flexible, powerful interface. Serial Attached SCSI will be the first storage interface to offer extensive configuration options for any mission-critical enterprise applications. It will be the first I/O (Input/Output) The transfer of data between the CPU and a peripheral device. Every transfer is an output from one device and an input to another. See PC input/output. I/O - Input/Output to offer the investment protection advantages of SCSI SCSI in full Small Computer System Interface Once common standard for connecting peripheral devices (disks, modems, printers, etc.) to small and medium-sized computers. SCSI has given way to faster standards, such as Firewire and USB. software and middleware, which have evolved over the years and are the most robust in the industry today. Serial Attached SCSI's advantage is due, in part, to its compatibility with Serial ATA See SATA. Serial ATA - Serial Advanced Technology Attachment (SATA (Serial ATA) A serial version of the ATA (IDE) interface, which has been the de facto standard hard disk interface for desktop PCs for more than two decades. The original Parallel ATA (PATA) interface was launched in 1986. ), which is targeted to the desktop market. This compatibility feature will make it possible for SATA drives to operate in a Serial Attached SCSI system, either alone or along with Serial Attached SCSI drives, providing wider configuration options than any other interface. SATA Configurations and Flexibility A desktop PC with a low- to mid-capacity drive and ATA (1) (AT Attachment) The specification for IDE drives. See IDE. (2) See analog telephone adapter. ATA - Advanced Technology Attachment controller integrated onto the motherboard is a starting configuration that everyone is familiar with. This would probably satisfy most household computing requirements in which the driving metric is undoubtedly cost. (See Figure 1.) Moving up a notch or two with a dual processor, a multiport mul·ti·port adj. Having, relating to, or being a system of multiple ports for injecting fuel separately into each cylinder of an engine. SATA controller, matching drives and possibly a software RAID package, all of which improve the capacity and performance without too much impact on the savings account Savings Account A deposit account intended for funds that are expected to stay in for the short term. A savings account offers lower returns than the market rates. Notes: . (See Figure 2.) But if performance is a serious consideration, is SATA still viable? SATA Compared to Fibre Channel and Serial Attached SCSI SATA was designed for the desktop and its incorporation on disk drives is consequently motivated by the need to meet the overriding market expectation of low-cost storage. That doesn't mean the drive is lower quality or that its quality is in any way jeopardized by its production cost targets. The table on page 23 compares the differences in the three disk drives. SATA in Enterprise Applications A SATA drive is not optimized for fast access to data. The speed of the data coming off (or going to) the media is quite impressive, but it takes the drive longer to find it. However, if the data is stored sequentially, it can be accessed immediately. In this third configuration, a pair of SATA RAID controllers and their drive sets are hooked up to a server. The controllers and drives may be housed in the server or they may be in separate cabinets, tied to a server HBA (Host Bus Adapter) See host adapter. . In the latter case, the HBA-to-RATD connection could be any convenient serial interface, but first the drive must be considered. (See Figure 3.) If the application requires sequential access In computer science, sequential access means that a group of elements (e.g. data in a memory array or a disk file or on a tape) is accessed in a predetermined, ordered sequence. Sequential access is sometimes the only way of accessing the data, for example if it is on a tape. to data, these SATA RAID sets would provide a raw data rate that is comparable with enterprise drives. Candidates for such a configuration could include video applications, document imaging and backup storage A storage device used to hold copies of data for backup and recovery. In the IT world, tape drives and tape libraries have been the traditional backup storage medium; however, magneto-optic (MO) and other optical discs as well as regular magnetic disks are also used. See LAN free backup. . But before deciding that this configuration is the answer to all applications involving sequentially ordered data, one should look more closely at the pros and cons pros and cons Noun, pl the advantages and disadvantages of a situation [Latin pro for + con(tra) against] . Pros: * Initial hardware costs are minimized. * Storage performance is excellent under error-free conditions. * Sequential access prevents RV from being a limiting factor A factor or condition that, either temporarily or permanently, impedes mission accomplishment. Illustrative examples are transportation network deficiencies, lack of in-place facilities, malpositioned forces or materiel, extreme climatic conditions, distance, transit or overflight rights, in performance. * The RAID redundancy will take care of drive failures and compensate for the lower reliability of ATA vs. SCSI drives. Hot spares will replace failed units. Cons: * With no guarantee of data integrity, use in mission critical applications would be inappropriate. * Error recovery management must be handled at the controller level. * Lower reliability increases down time due to rebuild operations after failures. * Each hot spare represents additional hardware cost and rebuild time. The more critical the application, the more significant the downtime and the greater the impact on total cost of ownership. * Single ported drives do not support fail-over recovery. To handle large storage requirements, either racks of high capacity disk drives, a forest of moderately sized units, or a combination of both could be used. There are significant advantages and disadvantages to each of these solutions, but the common denominator common denominator n. 1. Mathematics A quantity into which all the denominators of a set of fractions may be divided without a remainder. 2. A commonly shared theme or trait. at the system level is connectivity and addressability. A Switched Environment Since SATA and Serial Attached SCSI are point-to-point interfaces, some form of switch is required to allow their controllers to access multiple drives. Fabric switches are synonymous with synonymous with adjective equivalent to, the same as, identical to, similar to, identified with, equal to, tantamount to, interchangeable with, one and the same as FC; so one solution would be to integrate FC into the SATA or Serial Attached SCSI topologies. Interconnects at the network (switch) level would then be handled by FC and a converter in the controller (bridge) would handle the SATA/Serial Attached SCSI translation to the drive. Using a bridging topology would allow connectivity of any disk interface to any network interface such as iSCSI or Ethernet. But what if the system could have more limited and inexpensive connectivity at the local level? Serial Attached SCSI has the answer built into its protocol. Expanders are used in the Serial Attached SCSI environment to permit multiple initiators and targets to be addressed while the cables and connectors are SATA compatible. In addition, the Serial Attached SCSI interface supports a controller-to-expander dialogue known as Serial Tunneling Protocol A network protocol that encapsulates packets at a peer level or below. It is used to transport multiple protocols over a common network as well as provide the vehicle for encrypted virtual private networks (VPNs). (STP STP or standard temperature and pressure, standard conditions for measurement of the properties of matter. The standard temperature is the freezing point of pure water, 0°C; or 273.15°K;. ). This allows the controller to send SATA-formatted data to the expander, which in turn transmits it via SATA protocol to a standard SATA drive. (See Figure 4.) From the SATA perspective, the advantages of using a Serial Attached SCSI platform are clear: * Compatible connections allow common cabinetry cab·i·net·ry n. Cabinetwork: finely detailed cabinetry. Noun 1. cabinetry - the craft of making furniture (especially furniture of high quality) cabinetwork for desktop and enterprise applications. * Addressability up to 1024 ports. * Built in upgrade path from SATA drives to Serial Attached SCSI drives and enterprise applications. FC and Serial Attached SCSI Both FC and Serial Attached SCSI offer the maturity, richness, depth and scope of the SCSI command In SCSI computer storage, a command is the basic unit of communication. The SCSI command architecture was originally defined for parallel SCSI buses but has been carried forward with minimal change for use with Fibre Channel, iSCSI and Serial Attached SCSI. set. They are differentiated by their drive-to-drive connectivity, their inter-box connectivity and their addressability. FC's combination of shared media access, data rate, optical support and fabric compatibility has made it the interface of choice for SANs and high-speed switching environments. Serial Attached SCSI systems are not yet on the market, but they will be built on the strong foundation laid by the SCSI protocol and the serial technology of FC. For the first time since the market has been segmented into desktop and enterprise applications, Serial Attached SCSI provides a direct link between them and provides the continuity required to make all three interfaces complementary storage solutions. The SCSI Trade Association The SCSI Trade Association, or SCSITA, is an industry trade group which exists to promote the use SCSI technology. It was formed in 1996 [1]. As of 2006, major members include Adaptec, HP, Intel, LSI Logic, Seagate, and IBM [1]. is based in San Francisco San Francisco (săn frănsĭs`kō), city (1990 pop. 723,959), coextensive with San Francisco co., W Calif., on the tip of a peninsula between the Pacific Ocean and San Francisco Bay, which are connected by the strait known as the Golden , CA.
DRIVE COMPARISON TABLE
SATA
PERFORMANCE (Access to Data)
Latency + Seek Time 13msec @ 7200rpm
Command Queuing and Reordering LBA based
Rotational Vibration 5 rads/sec/sec
Typical I/Os per sec/drive (no RV) 77
Typ I/Os per sec/drive 35
(10 rad/sec (2))
Typ I/Os per sec/drive < 7
(20 rad/sec (2))
Duplex Operation Half
CUSTOMIZATION
Unique Code and Hardware Limited
Variable Sector Sizes No
Mode Page Parameter Control No
Inquiry Data No
Diagnostic Pages No
Capacity Controls No
INDICATORS
Activity LED No
Fault LED No
RELIABILITY
MTBF 600K Hrs
Duty Cycle 8 X 5
Interactive Error Management No
Internal Data Integrity Checks No
Dual Port No
Serial Attached SCSI
or Fibre Channel
PERFORMANCE (Access to Data)
Latency + Seek Time 5.7msec @ 15K rpm
Command Queuing and Reordering LBA (1) and RPS (2) based
Rotational Vibration 21 rads/sec/sec
Typical I/Os per sec/drive (no RV) 319 (3)
Typ I/Os per sec/drive 319 (3)
(10 rad/sec (2))
Typ I/Os per sec/drive 310 (3)
(20 rad/sec (2))
Duplex Operation Full
CUSTOMIZATION
Unique Code and Hardware Extensive
Variable Sector Sizes Yes
Mode Page Parameter Control Yes
Inquiry Data Yes
Diagnostic Pages Yes
Capacity Controls Yes
INDICATORS
Activity LED Yes
Fault LED Yes
RELIABILITY
MTBF 1.2M Hrs
Duty Cycle 24 X 7
Interactive Error Management Yes
Internal Data Integrity Checks IOEDC (4)
Dual Port Yes
(1)Logical Block Address
(2)Rotational Position Sensing
(3)Queue = 16
(4)Input Output Error Detection Code
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