Implementing SAS storage
By Rick Cook
When it comes to storage, serial-attached SCSI (SAS) is as flexible and powerful as a 20-foot python. SAS was designed as the successor to parallel SCSI, which has been running up against fundamental technical limitations. SAS relies on faster processors to pump more data down a single wire faster than parallel SCSI can send down a 32-wire cable. However, SAS keeps the SCSI command set, making it software compatible with parallel SCSI.
Early reports indicate there aren't many implementation issues with SAS, thanks in large part to a concerted effort by the standards committee to avoid the sort of problems that bedeviled SCSI and SANs in their early days and to a series of highly publicized plugfests involving most of the major vendors.
The following points can help you to easily implement SAS:
Understand your choices
Although SAS is highly compatible with regular SCSI, it is quite a bit different because of its flexibility. To get the most out of SAS, you need to take some time to understand what you can do with it. From an administrator's standpoint SAS isn't that complicated; it's just different. A lot of the old rules and restrictions of SCSI simply don't apply to SAS, which means that a lot of automatic assumptions don't apply either.
For example, SAS not only supports far more devices per port, but the devices don't have to be explicitly identified. On power-up, the SAS system searches for and identifies the attached devices using pre-assigned world wide names (WWN) and automatically identifies both attached SAS and serial ATA (SATA) devices.
Choose your topology
One of the major differences between SCSI and SAS is the number of devices which can be connected and the way they are connected. Basically, SAS' broader reach (up to 8 meters), mix-and-match SAS/SATA and much larger fan outs mean that you can fundamentally redesign your storage layout.
There are two types of SAS expanders: the edge expander and the fan-out expander. A SAS edge expander, which plugs into the SAS port on a server, will support up to 128 devices without enhancement over distances of up to eight meters. A fan out expander, which offers more routing capability and other features at a higher price, can also connect up to 128 devices. While only one fan out expander can be used in a domain, edge expanders can be ganged together in edge expander device sets to increase the number of addresses up to the maximum of 16,384. (The expanders use some of those addresses so the maximum number of devices in an SAS domain is 16,256.) The basic speed is 3 gigabits per second (Gbps), the equivalent of 300 megabytes per second (MBps) data pathway, which can be doubled to the equivalent of 600 MBps since the SAS channel is full-duplex. In addition, you can use several pathways per device to multiply the available bandwidth.
Clearly with this many potential device connections, topology is important in a way it never was with SCSI. Because of the flexibility inherent in SAS, the storage system topology can also be laid out in a way that reflects business processes and strategy much more than the restrictions of the storage devices. However, getting the most out of this flexibility means understanding just what you are trying to accomplish from a business standpoint. For example, how do you divide up your storage devices to best support the business' needs?
Choose your mix of SAS and SATA
Because SAS is plug compatible with SATA, SAS and SATA drives can be on the same controller or expander. This works on a hardware level because each SAS connector includes a SATA connector. On a firmware/software level, SAS recognizes when an attached device is SAS or SATA and automatically communicates with it appropriately.
Of course, SAS remains SAS and SATA remains SATA. SAS is best suited for higher speeds (up to 3 MBps) and more demanding applications. SAS is not only faster, SAS drives are dual-ported which increases reliability. SATA drives are slower and less reliable, but are also less expensive than SAS drives.
As a result of this mix-and-match ability, you can add pools of slower, cheaper SATA storage to a SAS system to support jobs like mirroring backups and less-critical databases while keeping the full speed of SAS for the storage devices that need it. You can also upgrade a storage pool at any point by replacing the SATA devices with SAS devices. This is a very powerful capability, however, it will take some careful thought to get the most out of it.
Plan for growth
Storage needs grow not just in capacity but in performance as well. The SATA/SAS compatibility of SAS means you can upgrade storage performance as easily as you upgrade capacity. When planning your SAS system, consider the areas where you are likely to need more speed and reliability and how to upgrade them if need be. For the parts of the storage network which are originally configured as SAS, consider how you can use multiple pathways to increase bandwidth and throughput down the road.
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About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years, he has been a freelance writer specializing in storage and other computer issues.
27 Mar 2006
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