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Evolution of Storage Protocols: SCSI to SAS

Synopsys recently announced the availability of industry’s first VIP to support the Serial-attached SCSI (SAS) 24G standard. Let’s look back how far we have come along, let’s time travel and re-live the interesting journey of storage and SCSI evolution.

There is no limit to generate, process, store, and restore data in today’s smart and connected world. One thing is for sure: we want our information, all of our information, to be stored securely and accessible whenever we want to and from wherever we are. Storage drives are becoming smaller in size with evolving storage technologies, yet data centers are becoming bigger by every passing day to support the ever increasing rate of data generation. What does the future of data storage hold? Will our data continue to leap from our myriad of devices to the cloud?

Storage media has evolved from magnetic tape to floppy disks and CD/DVD, to hard disk drives, and flash based scsi 1storage. It is projected that in one single day we exchange as much information as our grandparents did in early 1950s in a year. In 1950s, magnetic tapes were first used by IBM to store data, that was quite popular until the mid-1980s. In 1969, the first floppy disk (read only, stored 80kB of data) was introduced. Four years later in 1973, same size floppy disk could store 256kB of data and could be re-written. Now the kids of connected-to-cloud generation, who haven’t seen floppy, will recognize the image just as an icon to save your work in some popular software.

In 1981, Shugart Associates teamed up with NCR Corporation, and convinced ANSI to set up a committee to standardize a computer system interface for storage (SASI). In 1982, a technical committee was formed to work on standardizing SASI (very early predecessor of SCSI). A number of changes were made to the interface to widen the command set and improve performance. The name was also changed to SCSI; to make it as an industry standard. The first “true” SCSI interface standard was published in 1986, and evolutionary changes to the interface have been occurring since that time. Since then the trend has set to store more and more data into smaller size disks.

SCSI, the Small Computer System Interface, is a set of American National Standards Institute (ANSI) standard electronic interfaces that allow personal computers to communicate with peripheral hardware such as printers, scanners, disk drives, and CD-ROM drives etc. SCSI is often thought of as a hard disk interface, however, SCSI is not an interface tied specifically to hard disks. Many other type of devices can be present on the bus, what my point is that SCSI was designed from the ground up to be a high-performance, high-level, expandable interface. SCSI has made a name for itself as a storage technology that as a highly reliable solution and high performance and includes many commands and special features, to our forever increasing demands for speed and flexibility in storage devices. Now, SCSI is the storage technology of choice for storage applications in high-end computer users, server systems, network attached storage (NAS) and storage area network (SAN) applications.

scsi parallel to serial comparision table

SCSI began as a parallel interface, allowing the connection of devices to a personal computer or other systems with data being transmitted across multiple data lines. SCSI itself, however, has broadened greatly in terms of its scope, and today includes a wide variety of related technologies and standards, as defined in the SCSI-3 standard. SCSI-3 is extended into its own full portfolio, reflecting its new status as an “umbrella” standard containing several others.

All SCSI communication takes place between an initiator (host adapter) that sends commands, and a target drive that responds. Immediately after receiving a command sequence, the target returns a response code, so the initiator knows whether or not it will get the response it wants.

Common SCSI components

Initiator: An initiator issues service requests and receives responses. Initiators come in a variety of forms and may be integrated into a server’s system board or exist within a host bus adapter

Target: A SCSI target is typically a physical storage device. The target can be a hard disk or an entire storage device. It is also possible for non-storage hardware to function as a SCSI target

Service delivery subsystem: The mechanism that allows communication to occur between the initiator and the target; it usually takes the form of cabling

Expander: Only used with serial-attached SCSI (SAS); allows multiple SAS devices to share a single initiator port

Stay tuned for upcoming blogs on SAS – Serial Attached SCSI details and other interesting aspects.

To know more about Synopsys storage VIP, please visit http://synopsys.com/vip

Authored by Mansi Chadha