Chapter 2. Why and how we can use a storage area network
2.3 Using the storage area network components
The foundation that a storage area network (SAN) is built on is the interconnection of storage devices and servers. This section further describes storage, interconnection components, and servers, and how the different types of servers and storage are used in a typical SAN environment.
2.3.1 Storage
This section briefly describes the main types of storage devices that can be found in the market.
Storage systems
By being contained within a single
box
, a sotrage system (HDD, SSD, Flash) usually has a central control unit that manages all of the I/O. This configuration simplifies the integration of the system with other devices, such as other disk systems or servers.Chapter 2. Why and how we can use a storage area network 21
Tape systems
Tape systems, in much the same way as disk systems, are devices that consist of all the necessary apparatus to manage the use of tapes for storage purposes. In this case, however, the serial nature of a tape makes it not possible for them to be treated in parallel. This treatment is because Redundant Array of Independent Disks (RAID) devices are leading to a simpler architecture to manage and use.
There are basically three types of tape systems: Drives, autoloaders, and libraries. An overview of each type of system is provided.
Tape drives
As with disk drives, tape drives are the means by which tapes can be connected to other devices. They provide the physical and logical structure for reading from, and writing to tapes.
Tape autoloaders
Tape autoloaders are autonomous tape drives that can manage tapes and perform automatic backup operations. They are usually connected to high-throughput devices that require constant data backup.
Tape libraries
Tape libraries are devices that can manage multiple tapes simultaneously and, as such, can be viewed as a set of independent tape drives or autoloaders. They are usually deployed in systems that require massive storage capacity, or that need some type of data separation that would result in multiple single-tape systems. Because a tape is not a random-access media, tape libraries cannot provide parallel access to multiple tapes as a way to improve
performance. However, they can provide redundancy as a way to improve data availability and fault-tolerance.
The circumstances under which each of these systems, or even a disk system, might be used, strongly depend on the specific requirements of a particular SAN implementation. However, disk systems are usually used for online storage because of their superior
performance. Whereas, tape systems are ideal for offline, high-throughput storage, because of the lower cost of storage per byte.
The next section describes the prevalent connectivity interfaces, protocols, and services for building a SAN.
2.3.2 Storage area network connectivity
Storage area network (SAN) connectivity consists of hardware and software components that make possible the interconnection of storage devices and servers. You are now introduced to the Fibre Channel (FC) model for SANs.
Standards and models for storage connectivity
Networking is governed by adherence to standards and models. Data transfer is also
governed by standards. By far the most common is Small Computer System Interface (SCSI). SCSI is an American National Standards Institute (ANSI) standard that is one of the leading I/O buses in the computer industry.
An industry effort was started to create a stricter standard allowing devices from different vendors to work together. This effort is recognized in the ANSI SCSI-1 standard. The SCSI-1 standard (circa 1985) is rapidly becoming obsolete. The current standard is SCSI-2. The SCSI-3 standard is in the production stage.
The SCSI bus is a parallel bus, which comes in a number of variants, as shown in Figure 2-2.
Figure 2-2 SCSI standards comparison table
In addition to a physical interconnection standard, SCSI defines a logical (command set) standard to which disk devices must adhere. This standard is called the Common Command Set (CCS) and was developed more or less in parallel with ANSI SCSI-1.
The SCSI bus not only has data lines, but also a number of control signals. An elaborate protocol is part of the standard to allow multiple devices to share the bus efficiently.
In SCSI-3, even faster bus types are introduced, along with serial SCSI buses that reduce the cabling overhead and allow a higher maximum bus length. It is at this point where the Fibre Channel model is introduced.
As always, the demands and needs of the market push for new technologies. In particular, there is always a push for faster communications without limitations on distance or on the number of connected devices.
Fibre Channel is a serial interface (primarily implemented with fiber-optic cable) and is the primary architecture for most SANs. To support this interface, there are many vendors in the marketplace that produce Fibre Channel adapters and other Fibre Channel devices. Fibre Channel brought these advantages by introducing a new protocol stack and by keeping the SCSI-3 CCS on top of it.
Fibre Channel: For more information about parallel and serial data transfer, see