Scuzzy and beyond Part II

CHIPS, April-June, 2005 by Patrick G. Koehler, Stan Bush

In the CHIPS Jan-Mar 2005 edition, we discussed the recent advances and evolution of the small computer system interface (SCSI) standard. You can view this article at http://www.chips.navy.mil/archives/05_jan/web_pages/ scuzzy.htm. In Part II, we will look at some of the overarching issues surrounding when and why SCSI may or may not be the preferred solution for you, and we will compare SCSI to some of the other competing standards.

Raw specifications are fine, but to evaluate an entire system you must look at all of its components. So what are some of the criteria that might be a factor in this process? Typically, we would be concerned with the following: (1) Reliability/Maintainability for example, mean time between failures; (2) Fault Tolerance what is the impact of a failure on the system as a whole; (3) Speed/Data Throughput how fast can data pass; (4) Storage capability disk or array size; (5) Cost usually expressed as cost per gigabyte; and (6) Scalability/Flexibility how hard is it to change the configuration or increase storage.

Comparing Drives

SCSI's performance can be compared with Serial Advanced Technology Attachment (SATA) and Integrated Development Environment (IDE). When evaluating drives alone, the information in Figure 1 summarizes the current state of technology. However, manufacturers are working on larger and faster drives for both SCSI and SATA.

As you can see, SCSI clearly outperforms the other two types of drives, but it should be noted that when implemented in a RAID (Redundant Array of Independent Disks) environment, differences in individual drive parameters become much less pronounced as even lower performance drives can saturate the data bus and other system components. This is because in a multi-drive array each individual drive has to do less work since the workload is shared across all the drives in the array.

Comparing Bus Types

In addition to evaluating the drives, there are factors regarding the data bus to consider. The SCSI bus is supported via cabling that can handle up to 15 devices per channel in series. It can support both internal (in the case with the central processing unit) and external devices. While cabling that supports SCSI U160 and U320 standards are more expensive than other storage architectures they can be easier to work with because multiple devices can be connected to a single cable run and each cable run can be several meters in length.

Each channel on a SCSI bus supports up to the maximum data transfer rate supported by the specification (i.e., U160 or U320), but since they are connected in series, bandwidth is shared by all devices on the channel. Currently, the IDE and SATA data buses require a separate cable between each device and the controller. So while the cables are relatively inexpensive you will need significantly more of them, and if they are not properly installed they tend to clutter the inside of the enclosure and can restrict air flow required for cooling.

IDE cables are usually .at ribbon cables, although rounded versions are available and are limited to 36 inches in length. SATA cables are smaller and easier to manage and can be up to 40 inches in length. SATA also implements multi-lane cabling that combines four device connectors into a single cable. In addition to the newly available 300 MBps devices, SATA has a 600 MBps specification in the works. New features will include native command queuing, an external interface (to allow the use of external enclosures), a port multiplier to further decrease cable clutter and hot-swap capability--features that up until now were only available in a SCSI environment.

Other Factors Affecting Overall Performance

At this point we have examined individual drive issues as well as those associated with each type of data bus. Now we will turn to some of the other factors that can influence overall system performance. This discussion will only address issues involving when the storage subsystem is installed as a direct component of a server versus as a Serial Attached SCSI (SAS) or Internet SCSI (iSCSI) implementation.

The disk controller acts as the interface point between the drives, data bus and system bus. If the controller is functioning only as a basic disk controller then it should perform at the same rate as the data bus. But if it is also performing as a RAID controller then the performance of the RAID function can have a significant impact on the overall storage system performance especially if it is an older or less capable device. But a RAID implementation with low performing hardware would probably still outperform one implemented via software only.

Another system component that can have a significant impact on performance is the system bus. Currently, the most popular bus is the Peripheral Component Interconnect (PCI) 2.0 bus, which replaced the Industry Standard Architecture (ISA) bus several years ago. However, a new bus called PCI-Express is available, which provides significant increases in bandwidth to pass data between the system components and installed interface cards.