Hard Disk Features
| To make the best decision in purchasing a hard disk for your system or to understand what distinguishes one brand of hard disk from another, you must consider many features. This section examines some of the issues you should consider when you evaluate drives :
CapacityAs stated earlier, a corollary of Parkinson's famous "law" can be applied to hard drives: Data expands so as to fill the space available for its storage. This, of course, means that no matter how big a drive you get, you will find a way to fill it. If you've exhausted the space on your current hard disk, you might be wondering, "How much storage space is enough?" Because it is much more likely that you will run out of space rather than have too much, generally it is best to aim high and go for the largest drive that will fit within your budget. Modern systems are used to store many space-hungry file types, including digital photos, music, video, newer operating systems, applications, and games. As an example, according to hard drive manufacturer Western Digital, storing 600 high-res photos (500KB each), 12 hours of digital music, five games , 20 applications, and just 90 minutes of digital video would require an estimated 43GB of space. Running out of space causes numerous problems in a modern system, mainly due to the fact that Windows (as well as many newer applications) uses a large amount of drive space for temporary files and virtual memory. When Windows runs out of room, system instability, crashes, and data loss are inevitable. Capacity LimitationsHow big a hard drive you can use depends somewhat on the interface you choose. Although the ATA interface is by far the most popular interface for hard drives, SCSI interface drives are also available. Each has different limitations, but those of ATA have always been lower than those of SCSI. When ATA was first created in 1986, it had a maximum capacity limitation of 137GB (65,536x16x255 sectors). BIOS issues further limited capacity to 8.4GB in systems earlier than 1998, and 528MB in systems earlier than 1994. Even after the BIOS problems were resolved, however, the 137GB limit of ATA remained. Fortunately, this was recently broken in the ATA-6 specification drafted in 2001. ATA-6 augments the addressing scheme used by ATA to allow drive capacity to grow to 144PB (petabytes, or quadrillion bytes), which is 2 48 sectors. This has opened the door for ATA drives over 137GB to be released. Obviously, any drives larger than 137GB would by nature conform to ATA-6; however, if you are installing a drive larger than that, you should ensure that your motherboard BIOS has ATA-6 support as well. BIOS LimitationsMotherboard ROM BIOSes have been updated throughout the years to support larger and larger drives. The following shows the most important relative dates where drive capacity limits were changed:
These are when the limits were broken, such that BIOSes older than August 1994 will generally be limited to drives of up to 528MB, whereas BIOSes older than January 1998 will generally be limited to 8.4GB. Most BIOSes dated 1998 or newer will support drives up to 137GB, and those dated September 2002 or newer should support drives larger than 137GB. These are only general guidelines. To accurately determine this for a specific system, you should check with your motherboard manufacturer. You can also use the BIOS Wizard utility from www.unicore.com/bioswiz/index2.html. It will tell you the BIOS date from your system and specifically whether your system supports the "Enhanced Hard Disk Drive" specification, which means drives over 8.4GB. If your BIOS does not support EDD (drives over 8.4 GB), you have two possible solutions:
Of these, the first one is the most desirable because it is normally free and works more seamlessly in the system. Visit your motherboard manufacturer's Web site to see whether it has any newer BIOS available for your motherboard that will support large drives. I almost never recommend the software patch solution because it merely installs a program in the boot sector area of the hard drive, which can result in numerous problems when booting from different drives, installing new drives, or recovering data. BIOS LimitationsIf your current hard drive is 8GB or smaller, your system might not be able to handle a larger drive without a BIOS upgrade, because many older (pre-1998) BIOSes can't handle drives above the 8.4GB limit, and others (pre-2002) have other limits, such as 137GB. Although most ATA hard drives ship with a setup disk containing a software BIOS substitute such as OnTrack's Disk Manager or Phoenix Technologies' EZ-Drive (Phoenix purchased EZ-Drive creator StorageSoft in January 2002), I don't recommend using a software BIOS replacement. EZ-Drive, Disk Manager, and their OEM offshoots (Drive Guide, MAXBlast, Data Lifeguard, and others) can cause problems if you need to boot from floppy or CD media or if you need to repair the nonstandard master boot record these products use. If your motherboard ROM BIOS dates before 1998 and is limited to 8.4GB, or dates before 2002 and is limited to 137GB, and you wish to install a larger drive, I recommend you first contact your motherboard (or system) manufacturer to see if an update is available. Virtually all motherboards incorporate a Flash ROM, which allows for easy updates via a utility program. Operating System LimitationsNewer operating systems such as Windows Me as well as Windows 2000 and XP fortunately don't have any problems with larger drives; however, older operating systems may have limitations when it comes to using large drives. DOS will generally not recognize drives larger than 8.4GB because those drives are accessed using LBA (logical block addressing), and DOS versions 6.x and lower only use CHS (cylinder, head, sector) addressing. Windows 95 has a 32GB hard disk capacity limit, and there is no way around it other than upgrading to Windows 98 or a newer version. Additionally, the retail or upgrade versions of Windows 95 (also called Windows 95 OSR 1 or Windows 95a) are further limited to using only the FAT16 (16-bit file allocation table) file system, which carries a maximum partition size limitation of 2GB. This means if you had a 30GB drive you would be forced to divide it into 15 2GB partitions, each appearing as a separate drive letter (drives C: through Q: in this example). Windows 95B and 95C can use the FAT32 file system, which allows partition sizes up to 2TB (terabytes). Note that due to internal limitations, no version of FDISK can create partitions larger than 512MB. Windows 98 supports large drives, but a bug in the FDISK program included with Windows 98 reduces the reported drive capacity by 64GB for drives over that capacity. The solution is an updated version of FDISK that can be downloaded from Microsoft. Another bug appears in the FORMAT command with Windows 98. If you run FORMAT from a command prompt on a partition over 64GB, the size isn't reported correctly, although the entire partition will be formatted. PerformanceWhen you select a hard disk drive, one of the important features you should consider is the performance (speed) of the drive. Hard drives can have a wide range of performance capabilities. As is true of many things, one of the best indicators of a drive's relative performance is its price. An old saying from the automobile-racing industry is appropriate here: "Speed costs money. How fast do you want to go?" Normally the speed of a disk drive is measured in several ways:
Transfer RatesThe transfer rate is probably more important to overall system performance than any other statistic, but it is also one of the most misunderstood specifications. The problem stems from the fact that several transfer rates can be specified for a given drive; however, the most important of these is usually overlooked. A great deal of confusion arises from the fact that drive manufacturers can report up to seven different transfer rates for a given drive. Perhaps the least important of these (but the one people seem to focus on the most) is the raw interface transfer rate, which for the 2.5-inch ATA drives used in portable systems is 100MBps. Unfortunately, few people seem to realize that the drives actually read and write data much slower than that. The most important transfer rate specifications are the media (or internal) transfer rates, which express how fast a drive can actually read or write data. Media transfer rates can be expressed as a raw maximum, raw minimum, formatted maximum, formatted minimum, or averages of any of these. Few report the averages, but they can be easily calculated. The media transfer rate is far more important than the interface transfer rate because it is the true rate at which data can be read from (or written to) the disk. In other words, it tells how fast data can be moved to and from the drive platters (media). It is the rate that any sustained transfer can hope to achieve. This rate will normally be reported as a minimum and maximum figure, although many drive manufacturers report the maximum only. Media transfer rates have minimum and maximum figures because drives today use zoned recording with fewer sectors per track on the inner cylinders than the outer cylinders. Typically, a drive is divided into 16 or more zones, with the inner zone having about half the sectors per track (and therefore about half the transfer rate) of the outer 0zone. Because the drive spins at a constant rate, data can be read twice as fast from the outer cylinders than from the inner cylinders . Two primary factors contribute to transfer rate performance: rotational speed and the linear recording density or sector-per-track figures. When two drives with the same number of sectors per track are being compared, the drive that spins more quickly will transfer data more quickly. Likewise, when two drives with identical rotational speeds are being compared, the drive with the higher recording density (more sectors per track) will be faster. A higher-density drive can be faster than one that spins faster ”both factors have to be taken into account to know the true score. To find the transfer specifications for a given drive, look in the data sheet or preferably the documentation or manual for the drive. These can usually be downloaded from the drive manufacturer's Web site. This documentation will often report the maximum and minimum sector-per-track specifications, which ”combined with the rotational speed ”can be used to calculate true formatted media performance. Note that you would be looking for the true number of physical sectors per track for the outer and inner zones. Be aware that many drives ( especially zoned-bit recording drives) are configured with sector translation, so the number of sectors per track reported by the BIOS has little to do with the actual physical characteristics of the drive. You must know the drive's true physical parameters rather than the values the BIOS uses. When you know the true sector per track (SPT) and rotational speed figures, you can use the following formula to determine the true media data transfer rate in millions of bytes per second (MBps):
For example, the Hitachi/IBM Travelstar 7K60 drive spins at 7,200rpm and has an average of 540 sectors per track. The average media transfer rate for this drive is figured as follows :
Some drive manufacturers don't give the sector per track values for the outer and inner zones, instead offering only the raw unformatted transfer rates in Mbps (megabits per second). To convert raw megabits per second to formatted megabytes per second in a modern drive, divide the figure by 11. For example, Toshiba reports transfer rates of 373Mbps maximum and 203Mbps minimum for its MK6022GAX 60GB drive. This is an average of 288Mbps, which equates to an average formatted transfer rate of about 26MBps. Using these formulas (or slight variations thereof), you can calculate the media transfer rate of any drive. Table 9.5 shows the transfer rate specifications of the biggest and fastest 2.5-inch drives, listed in order from fastest to slowest. Table 9.5. 60GB+ 2.5-inch Hard Disk Transfer Rates
As you can see from the table, even though all these drives have a theoretical interface transfer rate of 100MBps, the fastest 2.5-inch drive has an average true media transfer rate of just over 33MBps. As an analogy, think of the drive as a tiny water faucet, and the ATA interface as a huge firehouse connected to the faucet that is being used to fill a swimming pool. No matter how much water can theoretically flow through the hose, you will only be able to fill the pool at the rate the faucet can flow water. The cache in a drive allows for burst transfers at the full interface rate. In our analogy, the cache is like a bucket that, once filled, can be dumped at full speed into the pool. The only problem is that the bucket is also filled by the faucet, so any data transfer larger than the size of the bucket can proceed only at the rate that the faucet can flow water. When you study drive specifications, it is true that larger caches and faster interface transfer rates are nice, but in the end, they are limited by the true transfer rate, which is the rate at which data can be read from or written to the actual drive media. In general, the media (also called internal or true ) transfer rate is the most important specification for a drive. Note There is a price to pay for portability, both in actual cost and performance. In general, the smaller a drive, the more expensive it is, and the slower the transfer rate performance. For example, a 2.5-inch notebook drive generally costs twice as much as the same capacity 3.5-inch desktop drive. Also, it will be slower. The fastest of the larger 3.5-inch drives normally used in desktop systems has an average transfer rate of between 44MBps and 50MBps, which is significantly faster than the 20MBps to 33MBps average rates of the fastest 2.5-inch notebook drives. Very small drives such as the Hitachi MicroDrive cost even more. A 1GB MicroDrive costs more than an 80GB 2.5-inch drive or a 250GB 3.5-inch drive, and it transfers at an average of just under 5MBps, significantly slower than any other drive. Average Seek TimeAverage seek time, normally measured in milliseconds (ms), is the average amount of time it takes to move the heads from one cylinder to another a random distance away. One way to measure this specification is to run many random track-seek operations and then divide the timed results by the number of seeks performed. This method provides an average time for a single seek. The standard method used by many drive manufacturers when reporting average seek times is to measure the time it takes the heads to move across one-third of the total cylinders. Average seek time depends only on the drive itself; the type of interface or controller has little effect on this specification. The average seek rating is primarily a gauge of the capabilities of the head actuator mechanism. Note Be wary of benchmarks that claim to measure drive seek performance. Most ATA and SCSI drives use a scheme called sector translation , so any commands the drive receives to move the heads to a specific cylinder might not actually result in the intended physical movement. This situation renders some benchmarks meaningless for those types of drives. SCSI drives also require an additional step because the commands first must be sent to the drive over the SCSI bus. These drives might seem to have the fastest access times because the command overhead is not factored in by most benchmarks. However, when this overhead is factored in by benchmark programs, these drives receive poor performance figures. LatencyLatency is the average time (in milliseconds) it takes for a sector to be available after the heads have reached a track. On average, this figure is half the time it takes for the disk to rotate once. A drive that spins twice as fast would have half the latency. Latency is a factor in disk read and write performance. Decreasing the latency increases the speed of access to data or files and is accomplished only by spinning the drive platters more quickly. Latency figures for the most popular drive rotational speeds are shown in Table 9.6. Table 9.6. Hard Disk Rotation Speeds and Their Latencies
Many 3.5-inch drives today spin at 7,200rpm, resulting in a latency time of only 4.17ms, whereas others spin at 10,000rpm and even 15,000rpm, resulting in incredible 3.00ms and 2.00ms latency figures. Most 2.5-inch drives spin at either 4,200 or 5,400rpm. However, several high-performance 7,200rpm 2.5-inch drives are now on the market. In addition to increasing performance where real-world access to data is concerned , spinning the platters more quickly also increases the data-transfer rate after the heads arrive at the desired sectors. Average Access TimeA measurement of a drive's average access time is the sum of its average seek time plus latency . The average access time is normally expressed in milliseconds (ms), which are thousandths of a second. A measurement of a drive's average access time (average seek time plus latency) provides the average total amount of time required for the drive to access a randomly requested sector. Cache Programs and Caching ControllersAt the software level, disk cache programs, such as SMARTDRV (DOS) and VCACHE (Windows), can have a major effect on disk drive performance. These cache programs hook into the BIOS hard drive and interrupt and intercept the read and write calls to the disk BIOS from application programs and device drivers. When an application program wants to read data from a hard drive, the cache program intercepts the read request, passes the read request to the hard drive controller in the usual way, saves the data read from the disk in its cache memory buffer, and then passes the data back to the application program. Depending on the size of the cache buffer, data from numerous sectors can be read into and saved in the buffer. When the application wants to read more data, the cache program again intercepts the request and examines its buffers to see whether the requested data is still in the cache. If so, the program passes the data back from the cache to the application immediately, without another hard drive operation. Because the cached data is stored in memory, this method speeds access tremendously and can greatly affect disk drive performance measurements. Most controllers now have some form of built-in hardware buffer or cache that doesn't intercept or use any BIOS interrupts. Instead, the drive caches data at the hardware level, which is invisible to normal performance-measurement software. Manufacturers originally included track read-ahead buffers in controllers to permit 1:1 interleave performance. Some manufacturers now increase the size of these read-ahead buffers in the controller, whereas others add intelligence by using a cache instead of a simple buffer. Most ATA and SCSI drives have cache memory directly built in to the drive's onboard controller. Most newer ATA drives have between 2MB through 16MB of built-in cache, whereas many SCSI drives have 16MB or more. I remember the days when 1 or 2MB of RAM was a lot of memory for an entire system. Nowadays, some 2.5-inch hard disk drives can have up to 16MB of cache memory built right in! Although software and hardware caches can make a drive faster for routine or repetitive data transfer operations, a cache will not affect the true maximum transfer rate the drive can sustain. SMARTSMART (Self-Monitoring, Analysis, and Reporting Technology) is an industry standard providing failure prediction for disk drives. When SMART is enabled for a given drive, the drive monitors predetermined attributes that are susceptible to or indicative of drive degradation. Based on changes in the monitored attributes, a failure prediction can be made. If a failure is deemed likely to occur, SMART makes a status report available so the system BIOS or driver software can notify the user of the impending problems, perhaps enabling the user to back up the data on the drive before any real problems occur. Predictable failures are the types of failures SMART attempts to detect. These failures result from the gradual degradation of the drive's performance. According to Seagate, 60% of drive failures are mechanical, which is exactly the type of failures SMART is designed to predict. Of course, not all failures are predictable, and SMART cannot help with unpredictable failures that occur without any advance warning. These can be caused by static electricity, improper handling or sudden shock , or circuit failure, such as thermal- related solder problems or component failure. SMART originated in technology that was created by IBM in 1992. That year IBM began shipping 3.5-inch hard disk drives equipped with Predictive Failure Analysis (PFA), an IBM-developed technology that periodically measures selected drive attributes and sends a warning message when a predefined threshold is exceeded. IBM turned this technology over to the ANSI organization, and it subsequently became the ANSI-standard SMART protocol for SCSI drives, as defined in the ANSI-SCSI Informational Exception Control (IEC) document X3T10/94-190. Interest in extending this technology to ATA drives led to the creation of the SMART Working Group in 1995. Besides IBM, other companies represented in the original group were Seagate Technology, Conner Peripherals (now a part of Seagate), Fujitsu, Hewlett-Packard, Maxtor, Quantum, and Western Digital. The SMART specification produced by this group and placed in the public domain covers both ATA and SCSI hard disk drives and can be found in most of the more recently produced drives on the market. The SMART design of attributes and thresholds is similar in ATA and SCSI environments, but the reporting of information differs . In an ATA environment, driver software on the system interprets the alarm signal from the drive generated by the SMART "report status" command. The driver polls the drive on a regular basis to check the status of this command and, if it signals imminent failure, sends an alarm to the operating system, where it will be passed on via an error message to the end user. This structure also enables future enhancements, which might allow reporting of information other than drive failure conditions. The system can read and evaluate the attributes and alarms reported in addition to the basic "report status" command. SCSI drives with SMART communicate a reliability condition only as either good or failing. In a SCSI environment, the failure decision occurs at the disk drive, and the host notifies the user for action. The SCSI specification provides for a sense bit to be flagged if the drive determines that a reliability issue exists. The system then alerts the end user via a message. The basic requirements for SMART to function in a system are simple. All you need are a SMART-capable hard disk drive and a SMART-aware BIOS or hard disk driver for your particular operating system. If your BIOS does not support SMART, utility programs are available that can support SMART on a given system. These include Norton Disk Doctor from Symantec, EZ-Drive from StorageSoft, and Data Advisor from Ontrack Data International. Note that traditional disk diagnostics, such as Scandisk and Norton Disk Doctor, work only on the data sectors of the disk surface and do not monitor all the drive functions that are monitored by SMART. Most modern disk drives keep spare sectors available to use as substitutes for sectors that have errors. When one of these spares is reallocated, the drive reports the activity to the SMART counter but still looks completely "defect free" to a surface analysis utility, such as Scandisk. Drives with SMART monitor a variety of attributes that vary from one manufacturer to another. Attributes are selected by the device manufacturer based on their capability to contribute to the prediction of degrading or fault conditions for that particular drive. Most drive manufacturers consider the specific set of attributes being used and the identity of those attributes as vendor specific and proprietary. Some drives monitor the floating height of the head above the magnetic media. If this height changes from a nominal figure, the drive could fail. Other drives can monitor different attributes, such as ECC (error-correction code) circuitry that indicates whether soft errors are occurring when reading or writing data. Some of the attributes monitored on various drives include the following:
Each attribute has a threshold limit that is used to determine the existence of a degrading or fault condition. These thresholds are set by the drive manufacturer, can vary among manufacturers and models, and cannot be changed. The basic requirements for SMART to function in a system are simple. All you need is a SMART-capable hard disk drive and a SMART-aware BIOS or hard disk driver for your particular operating system. If your BIOS does not support SMART, utility programs are available that can support SMART on a given system. These include Norton Utilities from Symantec, EZ Drive from StorageSoft, and Data Advisor from Ontrack. An excellent free utility called SMARTDefender can be downloaded from Hitachi Global Storage (formerly a part of IBM) at www.hgst.com/hdd/support/download.htm. This program monitors the SMART status of drives in the background and can also be manually run to check the SMART status of a drive. It includes a SMART Monitor system tray application that performs SMART tests and capacity monitoring based on SMARTDefender settings. A SMARTDefender icon appears in the system tray when the monitor is running. If you like, you can disable the background monitoring (SMART Monitor) and run SMARTDefender manually to check the current health of a hard disk drive as well as run the following tests:
Any drives reporting a SMART failure should be considered likely to fail at any time. Of course, you should back up the data on such a drive immediately, and you might consider replacing the drive before any actual data loss occurs. When sufficient changes occur in the monitored attributes to trigger a SMART alert, the drive sends an alert message via an ATA or a SCSI command (depending on the type of hard disk drive you have) to the hard disk driver in the system BIOS, which then forwards the message to the operating system. The operating system then displays a warning message as follows: Immediately back up your data and replace your hard disk drive. A failure may be imminent. The message might contain additional information, such as which physical device initiated the alert, a list of the logical drives (partitions) that correspond to the physical device, and even the type, manufacturer, and serial number of the device. The first thing to do when you receive such an alert is to heed the warning and back up all the data on the drive. It also is wise to back up to new media and not overwrite any previous good backups you might have, just in case the drive fails before the backup is complete. After backing up your data, what should you do? SMART warnings can be caused by an external source and might not actually indicate that the drive itself is going to fail. For example, environmental changes, such as high or low ambient temperatures , can trigger a SMART alert, as can excessive vibration in the drive caused by an external source. Additionally, electrical interference from motors or other devices on the same circuit as your PC can induce these alerts. If the alert was not caused by an external source, a drive replacement might be indicated. If the drive is under warranty, contact the vendor and ask whether they will replace it. If no further alerts occur, the problem might have been an anomaly, and you might not need to replace the drive. If you receive further alerts, replacing the drive is recommended. If you can connect both the new and existing (failing) drive to the same system, you might be able to copy the entire contents of the existing drive to the new one, saving you from having to install or reload all the applications and data from your backup. CostThe cost of hard disk storage is continually falling. You can now purchase a 2.5-inch 60GB ATA drive for around $150, which is about one-quarter of a cent per megabyte. A drive I bought in 1983 had a maximum capacity of 10MB and cost me $1,800 at the time. At current 2.5-inch drive pricing (0.25 cents per megabyte or less), that drive is worth just over 2 cents ! Of course, the cost of drives continues to fall, and eventually, even half a cent per megabyte will seem expensive. Because of the low costs of disk storage today, not many 2.5-inch drives with capacities of less than 20GB are even being manufactured. |
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