DBMS-Secondary Storage Devices
Some characteristics of magnetic disk storage devices.
Hardware Description of Disk Devices
Magnetic disks are used for storing large amounts of data. The most basic unit of data on the disk is a single bit of information. By magnetizing an area on disk in certain ways, one can make it represent abit value of either 0 (zero) or 1 (one). To code information, bits are grouped into bytes (or characters). Byte sizes are typically 4 to 8 bits, depending on the computer and the device. We assume that one character is stored in a single byte, and we use the terms byte and character interchangeably. The capacity of a disk is the number of bytes it can store, which is usually very large. Small floppy disks used with microcomputers typically hold from 400 Kbytes to 1.5 Mbytes; hard disks for micros typically hold from several hundred Mbytes up to a few Gbytes; and large disk packs used with minicomputers and mainframes have capacities that range up to a few tens or hundreds of Gbytes. Disk capacities continue to grow as technology improves.Whatever their capacity, disks are all made of magnetic material shaped as a thin circular disk (Figure 05.01a) and protected by a plastic or acrylic cover. A disk is single-sided if it stores information on only one of its surfaces and double-sided if both surfaces are used. To increase storage capacity, disks are assembled into a disk pack (Figure 05.01b), which may include many disks and hence many surfaces. Information is stored on a disk surface in concentric circles of small width,each having a distinct diameter. Each circle is called a track. For disk packs, the tracks with the same diameter on the various surfaces are called a cylinder because of the shape they would form if connected in space. The concept of a cylinder is important because data stored on one cylinder can be retrieved much faster than if it were distributed among different cylinders.
The number of tracks on a disk ranges from a few hundred to a few thousand, and the capacity of each track typically ranges from tens of Kbytes to 150 Kbytes. Because a track usually contains a large amount of information, it is divided into smaller blocks or sectors. The division of a track into sectors is hard-coded on the disk surface and cannot be changed. One type of sector organization calls a portion of a track that subtends a fixed angle at the center as a sector (Figure 05.02a). Several other sector organizations are possible, one of which is to have the sectors subtend smaller angles at the center as one moves away, thus maintaining a uniform density of recording (Figure 05.02b). Not all disks have their tracks divided into sectors.
The division of a track into equal-sized disk blocks (or pages) is set by the operating system during disk formatting (or initialization). Block size is fixed during initialization and cannot be changed dynamically. Typical disk block sizes range from 512 to 4096 bytes. A disk with hard-coded sectors often has the sectors subdivided into blocks during initialization. Blocks are separated by fixed-size interblock gaps, which include specially coded control information written during disk initialization. This information is used to determine which block on the track follows each interblock gap.
There is a continuous improvement in the storage capacity and transfer rates associated with disks; they are also progressively getting cheaper—currently costing only a fraction of a dollar per megabyte of disk storage. Costs are going down so rapidly that costs as low as one cent per megabyte or $10K per terabyte by the year 2001 are being forecast.
A disk is a random access addressable device. Transfer of data between main memory and disk takes place in units of disk blocks. The hardware address of a block—a combination of a surface number, track number (within the surface), and block number (within the track)—is supplied to the disk input/output (I/O) hardware. The address of a buffer—a contiguous reserved area in main storage that holds one block—is also provided. For a read command, the block from disk is copied into the buffer; whereas for a write command, the contents of the buffer are copied into the disk block. Sometimes several contiguous blocks, called a cluster, may be transferred as a unit. In this case the buffer size is adjusted to match the number of bytes in the cluster.
The actual hardware mechanism that reads or writes a block is the disk read/write head, which is part of a system called a disk drive. A disk or disk pack is mounted in the disk drive, which includes a motor that rotates the disks. A read/write head includes an electronic component attached to a mechanical arm. Disk packs with multiple surfaces are controlled by several read/write heads—one for each surface (see Figure 05.01b). All arms are connected to an actuator attached to another electrical motor, which moves the read/write heads in unison and positions them precisely over the cylinder of tracks specified in a block address.
Disk drives for hard disks rotate the disk pack continuously at a constant speed (typically ranging between 3600 and 7200 rpm). For a floppy disk, the disk drive begins to rotate the disk whenever a particular read or write request is initiated and ceases rotation soon after the data transfer is completed. Once the read/write head is positioned on the right track and the block specified in the block address moves under the read/write head, the electronic component of the read/write head is activated to transfer the data. Some disk units have fixed read/write heads, with as many heads as there are tracks. These are called fixed-head disks, whereas disk units with an actuator are called movable-head disks. For fixed-head disks, a track or cylinder is selected by electronically switching to the appropriate read/write head rather than by actual mechanical movement; consequently, it is much faster. However, the cost of the additional read/write heads is quite high, so fixed-head disks are not commonly used.
A disk controller, typically embedded in the disk drive, controls the disk drive and interfaces it to the computer system. One of the standard interfaces used today for disk drives on PC and workstations is called SCSI (Small Computer Storage Interface). The controller accepts high-level I/O commands and takes appropriate action to position the arm and causes the read/write action to take place. To transfer a disk block, given its address, the disk controller must first mechanically position the read/write head on the correct track. The time required to do this is called the seek time. Typical seek times are 12 to 14 msec on desktops and 8 or 9 msecs on servers. Following that, there is another delay—called the rotational delay or latency—while the beginning of the desired block rotates into position under the read/write head. Finally, some additional time is needed to transfer the data; this is called the block is the sum of the seek time, rotational delay, and block transfer time. The seek time and rotational delay are usually much larger than the block transfer time. To make the transfer of multiple blocks more efficient, it is common to transfer several consecutive blocks on the same track or cylinder. This eliminates the seek time and rotational delay for all but the first block and can result in a substantial saving of time when numerous contiguous blocks are transferred. Usually, the disk manufacturer provides a bulk transfer rate for calculating the time required to transfer consecutive blocks. Appendix B contains a discussion of these and other disk parameters.
The time needed to locate and transfer a disk block is in the order of milliseconds, usually ranging from 12 to 60 msec. For contiguous blocks, locating the first block takes from 12 to 60 msec, but transferring subsequent blocks may take only 1 to 2 msec each. Many search techniques take advantage of consecutive retrieval of blocks when searching for data on disk. In any case, a transfer time in the order of milliseconds is considered quite high compared with the time required to process data in main memory by current CPUs. Hence, locating data on disk is a major bottleneck in database applications. The file structures we discuss here and in Chapter 6 attempt to minimize the number of block transfers needed to locate and transfer the required data from disk to main memory.
