HBAs and RAID Controllers
| An HBA is a generic name for bus interface card. A $25 PCI card that adds FireWire or USB 2.0 is called an HBA. From a computer storage standpoint, an HBA is something that can recognize and communicate with a storage device, and therefore all HBAs are "named entities" in the bus or channel type that they participate in. That is to say, every HBA takes an address. Because an HBA is a network interface, there are all manner of HBAs on the market. Your NIC is an HBA, and so are the storage HBAs described in this section and the ones that follow. An HBA offloads I/O processing from the CPUs and contains not only a command set but often specialized algorithms and intelligent responses. In Ethernet networking, the fastest cards are now the ones that come with TCP/IP offload engines. The intelligence added to storage HBAs is RAID and management technologies. Depending on the technology, vendor groups coin their own terms for their HBAs (for example, TCA or HCA for InfiniBand). Each of the technologies described so far requires its own particular HBA. The chipset on the HBA contains the commands, logic, timing circuits, and so on that are all part of the bus's specification. HBAs require their own device drivers. For widely available adapters, you will find support for them in your operating system, but for many specialized drivers, you need to get the current software from the adapter's vendor. Form FactorsHBAs are almost always sold as add-in PCI cards, although you can expect to see more and more HBAs as PCI-X or PCI-Express cardsparticularly high-throughput cards like the ones we talk about in a moment. Occasionally, you will find high-performance interfaces that are externally housed or use nonstandard form factors, but these alternate forms are rare for the newer and more advanced HBAs. SCSI has been around for 20 years, and it is possible to find SCSI controllers for all kinds of buses, including ISA, EISA, MCA, PCI, VLB, PCMCIA, and CardBus. From the standpoint of server-to-storage connections, adding an EIDE card or a SATA card that is simply a drive interface connection with no multidisk array features almost never happens. That's a PC upgrade, not a server upgrade. HBAs sold to server manufacturers or as after-market products almost always have a RAID function. RAID is so inexpensive to add to a product that you will find RAID on your server's motherboard and even on higher-performing PC and workstation boards. You may also find multiple types of RAIDusually EIDE RAID, SATA RAID, and SCSI RAIDon the same board. Usually, if your server has a motherboard with multiple RAID interfaces, you set up the drive connections and RAID features in your server's BIOS. After a drive has been recognized or an array has been created, you can proceed to partition and format your drive(s), and with the operating system loaded, you can apply different storage management and configuration utilities to your storage devices. Most onboard RAID systems offer limited RAID capabilities: RAID 0, 1, and 10 or 1+0 are most common. Keep in mind that onboard RAID is not "free." You use up some processor cycles to manage onboard RAID, and the effect is small but noticeable. Because you don't want hard drive I/O to be a bottleneck (and it commonly is, especially in server environments), a high-efficiency HBA on a fast bus is preferable. There is a good reason that servers come with HBAs that often cost as much as a good desktop or laptop PC, and there is a good reason for all the activity with and interest in new forms of interfaces in servers such as PCI-X, PCI-Express, InfiniBand, and others. Many HBAs offer various levels of RAID or no RAID at all. If your system uses a disk array, but with no RAID implemented, you have a system commonly referred to a JBOD (just a bunch of disks). You use JBOD when storage capacity is your only concern and you don't care about either performance or data protection. We will return to a more complete description of RAID later in this chapter, but the point for our purposes here is that the terms HBA and RAID controller are almost synonymous in the industry for servers (and the technologies described in this chapter), and they differentiate only in describing upgrades to older PCs.
PATA HBAsPATA connectors (or EIDE, if you prefer) are included on every server motherboard you can buy, and they will be for the near future. There are changes on the horizon, though. Intel, for one, is intent on replacing parallel ATA with serial ATA in future server motherboards but has not done so yet. People add HBAs when they need more channels for parallel ATA drives. They do so to create larger JBODs or to build very large-capacity volumes using low-cost drives. ATA RAID can offer you faster performance and fault tolerance, so you sometimes find an EIDE HBA deployed in that manner. Most of the time people purchase an EIDE RAID HBA because of "RAID lock-in." Say you choose to implement RAID on a captive drive set, using the onboard ATA RAID function that your motherboard offers. That feature might even be one of the reasons you selected the motherboard in the first place. Onboard ATA RAID chips come from companies such as Adaptec, Highpoint, Intel, and Promise, and their inclusion adds very little to the cost of the motherboard relative to the dramatic improvement in performance they provide to a system. The RAID chips found on good motherboards are the same chips used to power the HBAs sold by these manufacturers, although very often you are buying an onboard RAID chip that has a newer, faster, or more capable brother on the way. Including older chip technology on motherboard chips while providing their latest and greatest chips on separate HBAs is one way for RAID vendors to improve volume or reduce inventory. This methodology may seem innocuous until one day your server's motherboard or onboard RAID chip fails. Your disk volume contains data you want to use, and although you could reinstall from backup, it is just easier to replace the function. Unfortunately, vendors can charge a premium to provide HBAs based on previous iterations of their chipsets, and at this point, you're locked into whatever cost they want to set. The moral is that when you implement a RAID volume, you may be marrying yourself to a specific RAID HBA vendor. Aside from RAID lock-in, the other reason that server administrators install RAID adapters is that they give you access to a faster standard for EIDE (perhaps Ultra ATA/133), allowing them to unlock disk performance for the EIDE drives they already possess. EIDE cards are the least expensive cards you can buy with RAID functionality, and they range from costing as little as $50 for a two-channel ATA/100 or ATA/133 RAID HBA from a company such as Startech or SIIG to as much as an absurd $400 for a four-channel Adaptec 2400A HBA with some nice features but only ATA/66 or ATA/100 support. You should consider using PATA RAID for servers only if you need to keep an investment in legacy equipment. With SATA RAID rapidly displacing EIDE, that network interface would be a better and higher-performing choice for little extra money. SATA ControllersMost of the development effort these days for high-capacity, low-cost disk array HBA is in the area of SATA RAID. If you want to build a storage server dedicated to media of any kind, this is the standard to look at. Why? The disks are cheap, arrays are reliable enough to be acceptable for servers, and SATA RAID offers good, albeit not outstanding, performance, especially for the price. SATA RAID controllers are available from a wide variety of vendors, including 3Ware, Adaptec, Broadcom, Highpoint, Intel, LSI Logic, Promise, and many others. You can choose from PCI, PCI-X, and PCI-Express cards offering a variety of RAID levels. Let's consider for a moment the application of a SATA RAID card in practice. As an example, let's look at Adaptec's 2810SA eight-port SATA card (shown in Figure 11.2). This card retails for around $600 and is targeted for use by departmental and workgroup servers running video, backup, and web servicesall high-capacity applications. Figure 11.2. The Adaptec eight-port 2810SA PCI-X SATA RAID host adapter. These are some of the features of this host adapter:
These features go well beyond what you would find on most EIDE controllers and are quite similar to the features you would find in an enterprise SCSI or Fibre Channel controller. The industry has a keen interest in SATA HBAs because an eight-port SATA HBA can attach to cheap capacious hard drives. At today's prices, you could purchase eight 300GB HBAs for about $200 a piece. Your total investment of HBA and disk would run about $2,200, and you would have 2.4 terabytes (TB) of storage to work with. Clearly, you would need an enclosure, a processor(s), a motherboard, and other system components to complete your storage server, but even allowing another $1,500 for those components, you would have a massive amount of storage for less than $4,000. SCSI or Fibre Channel storage servers offer you only a small fraction of this storage at that price level. Although the performance of a SATA HBA won't be as fast as those other two technologies, having that many drives in play makes SATA arrays plenty fast for many applications. SCSI HBAsSCSI HBAs were among the first high-performance HBAs sold in large numbers. Originally used for desktop PCs, mass production lowered their prices to the point where the technology became mainstream for servers of all kinds. As discussed earlier in this chapter, SCSI has gone through a range of changes, improving in both speed and performance. But through it all, SCSI has remained a parallel bus, and the principles under which a SCSI bus operates have remained largely the same. Recently, several new technologies have leveraged companies' investments in SCSI, in particular serial attached SCSI (SAS) and iSCSI. Before looking at the alternative technologies and where you would want to use them, let's look at some of the specifics about SCSI HBAs. A SCSI HBA connects your computer with the SCSI bus. A SCSI HBA that is in a device on the SCSI daisy chain is called a target controller. Target controllers can be either internal to the device or created by adding a SCSI card to the device. There can be more than one host adapter on a SCSI bus, which permits peripheral sharing between systems. High-speed SCSI is often used, for example, as the connection between nodes in a cluster and shared storage, as well as the connection to whatever device in a cluster is maintaining the heartbeat of the cluster. A SCSI bus is a peer network where all devices can initiate and target data requests. From a storage standpoint, the important logical addressing on the SCSI chain is called a logical unit number (LUN). LUNs are the connection to a SCSI device (not just disk), and because storage structures on disks may or may not bear any relationship to the physical disk, LUNs are used to indicate the actual disks in use, regardless of how the data is written. SCSI uses an LBA scheme, which allows applications such as backup programs to copy data using the address on the disk, without regard to what kind of information it is. If you want a simple explanation that defines the difference between a SAN device and a network attached storage (NAS) device, it is this: SANs move data primarily as blocks, and NAS moves data as files. Almost all SCSI HBAs come with built-in RAID controllers, powerful RISC (reduced instruction set computer) processors, and lots of cache. RAID controllers are configured in software, which is found in the adapter's BIOS. As your server performs a POST and the bus with the SCSI HBA is enumerated, you see the software load, with a prompt on how to access it. As the SCSI BIOS loads, it communicates with each possible target and enumerates them onscreen.
A vast number of vendors sell SCSI HBAs into the marketplace. Chief among them are AMI, ATTO, Adaptec, Emulex, FalconStor, Intel, LSI Logic, Mylex, Parlan, and others, as well as all the server vendors, such as Hewlett-Packard, Dell, IBM, and Sun. Emulex is the largest supplier selling to OEMs. Adaptec is the largest supplier selling to the general public, and it most certainly has the widest range of offerings of any vendor. There are many different SCSI HBAs on the market today, but as far as servers are concerned, only the two highest-performing standards really matter: Ultra160 and Ultra320. An Ultra SCSI board is fine for a workstation or very small server, but unless cost is the primary concern, it is preferable to go with one of the later standards. Let's consider some of the characteristics of one of the current crop of SCSI Ultra320 HBAs, the Adaptec SCSI RAID 2130SLP (see Figure 11.3), a $500 board that Adaptec aims at entry-level to high-density servers. This is a single-channel 16-bit PCI-X board that can attach to up to 15 disk drives. The primary features of this board are as follows:
Figure 11.3. The Adaptec SCSI RAID 2130SLP PCI-X HBA. Many of these features are also available on the Adaptec SATA RAID board, but the Adaptec SCSI RAID 2130SLP PCI-X HBA is a much higher-performing adapter. Key features of the board are not only its performance but its RAID support and software portfolio. Most people who don't have experience in SCSI make a purchase based solely on price and speed, but the management software is a critical feature that is often overlooked. What should be obvious is that Ultra320 controllers stand alone; that is, they don't offer backward compatibility with previous forms of SCSI. This becomes obvious when you consider the wide variety of shapes and sizes that exists with regard to the various types of SCSI ports and cables that the technology has employed over the years. The Ultra160 Adaptec SCSI controller card comes with a 68-pin LVD SCSI (one internal and one external), a 68-pin Wide Ultra SCSI (internal), and a 50-pin Ultra SCSI connection (internal). The 50-pin Ultra SCSI connection gives this board the ability to preserve your investment in legacy SCSI devices. When purchasing a modern SCSI adapter, you should look for one of the following connection types:
Most other connector types, in particular the 25-pin connector, are obsolete. You can find more information on SCSI connectors, including illustrations of each type, in Chapter 7. Both of the aforementioned SCSI adapters are single-channel devices. Because it is 16-bit, that channel supports up to 15 devices. It is also possible to buy dual-channel SCSI boards, which support up to 30 SCSI devices. For onboard or captive drives inside a server, single-channel SCSI HBAs are desirable since they are both less expensive and easier to implement. When storage server vendors build dense arrays based on the SCSI interface, the preference is to us an HBA that supports a larger number of devices. In those instances, installation of a separate dual-channel board is the way to go. There is yet one more RAID implementation you might find in low-end servers: a Zero Channel RAID (ZCR) card. This small credit cardsized card goes into a specially designed EMRL (embedded RAID logic, usually seen on Adaptec systems) slot in low form factor 1U or blade servers and lets you add RAID to a server without having to use a PCI slot to do so. Using ZCR is akin to adding a RAID ASIC (application-specific integrated circuit) to your server's motherboard. |
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