Although SCSI hard disks have traditionally been used in midrange and high-end server platforms, ATA-based hard disks continue to be popular in entry-level servers. The superior performance and easier configuration offered by SATA hard disks make them a suitable choice for both entry-level and many midrange server implementations. The following sections provide recommendations for using ATA drives in a server platform. ATA/ATAPI Configurations for Server PlatformsIf you decide to use PATA hard disks in an entry-level server, you need to configure your server as follows:
Note that most commercial entry-level servers no longer use PATA hard disks, although ATAPI CD and DVD drives continue to be popular. SATA Configurations for Server PlatformsAs noted earlier in this chapter, SATA drives have largely replaced PATA hard disks in entry-level servers, and they have made inroads into the midrange server market as well. To improve reliability and performance, you should consider using SATA hard disks that meet the following standards:
Each SATA hard disk uses a separate host adapter. ATA/SATA RAID Configurations for Server PlatformsRAID, which stands for redundant array of independent (or inexpensive) disks, was designed to improve the fault tolerance and performance of computer storage systems. RAID was first developed at the University of California at Berkeley in 1987, and it was designed so that a group of smaller, less expensive drives could be interconnected with special hardware and software to make them appear as a single, larger drive to the system. By using multiple drives to act as one drive, fault tolerance and performance could be increased. Initially, RAID was conceived to simply enable all the individual drives in the array to work together as a single, larger drive with the combined storage space of all the individual drives added together. However, this actually reduced reliability and didn't do much for performance, either. For example, if you had four drives connected in an array acting as one drive, you would be four times as likely to experience a drive failure than if you used just a single, larger drive. To improve its reliability and performance, the Berkeley scientists proposed six levels (corresponding to different methods) of RAID. These levels provide varying emphasis on fault tolerance (reliability), storage capacity, or performanceor a combination of the three. An organization called the RAID Advisory Board (RAB) was formed in July 1992 to standardize, classify, and educate on the subject of RAID. RAB has developed specifications for RAID, a conformance program for the various RAID levels, and a classification program for RAID hardware. Currently, RAB defines seven standard RAID levels, RAID 06. RAID is typically implemented by a RAID controller board, although software-only implementations are possible (but not recommended). The levels are as follows:
Additional RAID levels exist that are not supported by RAB but that are instead custom implementations by specific companies. Note Note that a higher RAID level number doesn't necessarily mean increased performance or fault tolerance; the numbered order of the RAID levels is entirely arbitrary. At one time, virtually all RAID controllers were SCSI based, meaning that they used SCSI drives. For a professional setup, SCSI RAID is definitely the best choice because it combines the advantages of RAID with the advantages of SCSIan interface that was already designed to support multiple drives. Now, however, ATA RAID controllers are available that allow for even less expensive RAID implementations. These ATA RAID controllers are typically used in single-user systems for performance rather than reliability increases. Most ATA RAID implementations are much simpler than the professional SCSI RAID adapters used on network file servers. ATA RAID is designed more for an individual who is seeking performance or simple drive mirroring for redundancy. When they're set up for performance, ATA RAID adapters run RAID Level 0, which incorporates data striping. Unfortunately, RAID 0 also sacrifices reliability, such that if one drive fails, all data is lost. With RAID 0, performance scales up with the number of drives you add to the array. If you use four drives, you don't necessarily have four times the performance of a single drive, but you can get close to that for sustained transfers. Some overhead is still involved in the controller performing the striping, and issues still exist with latencythat is, how long it takes to find the databut performance with RAID is higher than what any single drive could normally achieve. When they're set up for reliability, ATA RAID adapters generally run RAID Level 1, which is simple drive mirroring. All data written to one drive is written to the other. If one drive fails, the system can continue to work on the other drive. Unfortunately, this does not increase performance at all, and it also means you get to use only half of the available drive capacity. In other words, you must install two drives, but you get to use only one (the other is the mirror). However, in an era of high capacities and low drive prices, this is not a significant issue. If you want to eliminate a lot of bulky cable, consider SATA RAID, which uses the narrow SATA cables shown in Figure 6.1, earlier in this chapter. Combining performance with fault tolerance requires using one of the other RAID levels, such as RAID Level 3 or RAID Level 5. For example, virtually all professional RAID controllers used in network file servers are designed to use RAID Level 5. Controllers that implement RAID Level 5 are more expensive, and at least three drives must be connected. To improve reliability, but at a lower cost, many of the ATA RAID controllers enable combinations of the RAID levelssuch as 0 and 1 combined (also known as RAID 10). This usually requires four drives, two of which are striped together in a RAID Level 0 arrangement, which is then redundantly written to a second set of two drives in a RAID Level 1 arrangement. This enables you to have approximately double the performance of a single drive, and you have a backup set, in case one of the primary sets fails. Many recent servers include four SATA host adapters with RAID functionality, enabling SATA RAID 0+1 implementations. Today, you can get PATA or SATA RAID controllers from companies such as Arco Computer Products, Iwill, Promise Technology, and HighPoint. A typical low-cost ATA RAID controller enables up to four drives to be attached, and you can run them in RAID Level 0, 1, or 0+1 mode. Remember that performance suffers somewhat when you run two drives (master/slave) on a single PATA channel because only one drive can transfer on the cable at a time, which cuts performance in half. Four-channel PATA RAID cards are available, but most new RAID cards are moving to SATA, which doesn't have the master/slave channel sharing problems of PATA. SATA RAID cards use a separate SATA data channel (cable) for each drive, allowing maximum performance. You should use SATA RAID cards for best performance. If you are looking for an ATA RAID controller (or a motherboard with an integrated ATA RAID controller), you should look for the following:
If you want to experiment with RAID inexpensively, you can implement RAID without a custom controller when using certain higher-end (often server-based) operating systems. For example, the Windows NT/2000 and Windows XP or Windows Server 2003 operating systems provide a software implementation for RAID, using both striping and mirroring. In these operating systems, you use the Disk Administrator tool to set up and control the RAID functions, as well as to reconstruct the volume after a failure occurs. Normally, though, if you are building a server and want the ultimate in performance and reliability, you should look for SATA or SCSI RAID controllers that support RAID Level 3 or Level 5. Tip The best solutions for RAID are SATA and SATA RAID implementations that are native to the motherboard chipset's South Bridge or I/O controller hub chip. When Windows 2000 Server or Windows Server 2003 is installed on a system with native SATA or SATA RAID support, there is no need to press the F6 key when you're prompted to install a driver from a floppy disk. With add-on SATA and ATA/SATA RAID add-on cards or motherboard implementations using a separate SATA host adapter chip, you must press F6 when prompted to install the appropriate driver from a floppy disk. The latest type of RAID array, RAIDn, developed by Inostor, a division of Tandberg Data, combines special software that can specify the number of drives used for replacements. A RAIDn array designed to protect against the failure of up to four drives uses only 7 drives (4 for data and 3 for parity), compared to 10 drives for a RAID 5+1 array (4 data, 1 parity; 4 mirrored data; 1 mirrored parity). Currently, RAIDn arrays are available only in Inostor network attached storage (NAS) devices, but RAIDn software is designed to be portable to other operating systems.
Using NASAs an alternative to expanding internal server storage by adding ATA, SATA, or SCSI hard disks to a server (a concept often referred to as direct attached storage [DAS]), you can connect a network attached storage (NAS) device to your network. Each NAS device has its own IP address, and an NAS device contains one or more hard disks that can be accessed directly from the network. The first company to develop an NAS device was Auspex, which introduced the first NAS shortly after being founded in 1987. Although the company went bankrupt in June 2003, its technology was sold to other firms. Using NAS devices continues to be a popular method of expanding network storage, and many companies now build NAS devices. In addition to hard disks, an NAS device contains one or more processors to handle I/O, network, and data storage and retrieval tasks. Although a given NAS device might contain a processor similar to that used by a desktop PC or server, an NAS device is not a PC or a server. Instead, it is a specialized storage appliance. The benefits of expanding network storage by using an NAS device include the following:
The following are the shortcomings of NAS storage:
If you need speeds significantly faster than NAS can provide (about 32MBps sustained transfer rate with a Gigabit Ethernet network) and prefer more security options, you should consider a storage area network (SAN) installation.
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