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The user DATA field consists of all 512 bytes of data stored in the sector. This field is followed by a CRC field to verify the data. Although many controllers use two bytes of CRC here, the controller may implement a longer Error Correction Code (ECC) that requires more than two CRC bytes to store. The ECC data stored here provides the possibility of DATA-field read correction as well as read error detection. The correction/detection capabilities depend on the ECC code chosen and on the controller implementation. A WRITE TURN-OFF GAP is a pad to allow the ECC (CRC) bytes to be fully recovered.
The INTER-RECORD GAP provides a means to accommodate variances in drive spindle speeds. A track may have been formatted while the disk was running slower than normal and then write-updated while the disk was running faster than normal. In such cases, this gap prevents accidental overwriting of any information in the next sector. The actual size of this padding varies, depending on the speed of disk rotation when the track was formatted and each time the DATA field is updated.
The PRE-INDEX GAP allows for speed tolerance over the entire track. This gap varies in size, depending on the variances in disk-rotation speed and write-frequency tolerance at the time of formatting.
This sector prefix information is extremely important because it contains the numbering information that defines the cylinder, head, and sector. All this information except the DATA field, DATA CRC bytes, and WRITE TURN-OFF GAP is written only during a low-level format. On a typical nonservo–guided (stepper-motor actuator) hard disk on which thermal gradients cause mistracking, the data updates that rewrite the 512-byte DATA area and the CRC that follows may not be placed exactly in line with the sector header information. This situation eventually causes read or write failures of the Abort, Retry, Fail, and Ignore variety. You can often correct this problem by redoing the Low Level Formatting (LLF) of the disk; this process rewrites the header and data information together at the current track positions. Then when you restore the data to the disk, the DATA areas are written in alignment with the new sector headers.
One way to increase the capacity of a hard drive is to format more sectors on the outer cylinders than on the inner ones. Because they have a larger circumference, the outer cylinders can hold more data. Drives without zoned recording store the same amount of data on every cylinder, even though the outer cylinders may be twice as long as the inner cylinders. The result is wasted storage capacity, because the disk media must be capable of storing data reliably at the same density as on the inner cylinders. With older ST-506/412 and ESDI (Enhanced Small Device Interface) controllers, unfortunately, the number of sectors per track was fixed. Drive capacity, therefore, was limited by the density capability of the innermost (shortest) track.
In a zoned recording, the cylinders are split into groups called zones, with each successive zone having more and more sectors per track as you move out from the inner radius of the disk. All the cylinders in a particular zone have the same number of sectors per track. The number of zones varies with specific drives, but most drives have 10 or more zones.
Another effect of zoned recording is that transfer speeds vary depending on what zone the heads are in. Because there are more sectors in the outer zones, but the rotational speed is always the same, the transfer rate will be highest.
Drives with separate controllers could not handle zoned recordings because there was no standard way to communicate information about the zones from the drive to the controller. With SCSI and IDE disks, it became possible to format individual tracks with different numbers of sectors due to the fact that these drives have the disk controller built in. The built-in controllers on these drives can be made fully aware of the zoning that is used. These built-in controllers must then also translate the physical cylinder, head, and sector numbers to logical cylinder, head, and sector numbers so that the drive has the appearance of having the same number of sectors on each track. The PC BIOS was designed to only handle a single number of specific sectors per track throughout the entire drive, meaning that zoned drives always must run under a sector translation scheme.
The use of zoned recording has allowed drive manufacturers to increase the capacity of their hard drives by between 20 percent and 50 percent compared with a fixed-sector-per-track arrangement.
Virtually all IDE and SCSI drives today use zoned recording. Because of limitations in the PC BIOS, these drives still have to act as though they have a fixed number of sectors per track. This situation is handled by translation algorithms that are implemented in the controller.
Many types of hard disks are on the market, but nearly all drives share the same basic physical components. Some differences may exist in the implementation of these components (and in the quality of materials used to make them), but the operational characteristics of most drives are similar. Following are the components of a typical hard disk drive (see Figure 1-1):
The platters, spindle motor, heads, and head actuator mechanisms usually are contained in a sealed chamber called the Head Disk Assembly (HDA). The HDA usually is treated as a single component; it rarely is opened. Other parts external to the drive’s HDA -- such as the logic boards, bezel, and other configuration or mounting hardware -- can be disassembled from the drive.
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