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The main measures of the qualities of a disk are capacity, access time, data-transfer rate, and reliability Access time is the time from when a read or write request is issued to when data transfer begins To access (that is, to read or write) data on a given sector of a disk, the arm rst must move so that it is positioned over the correct track, and then must wait for the sector to appear under it as the disk rotates The time for repositioning the arm is called the seek time, and it increases with the distance that the arm must move Typical seek times range from 2 to 30 milliseconds, depending on how far the track is from the initial arm position Smaller disks tend to have lower seek times since the head has to travel a smaller distance The average seek time is the average of the seek times, measured over a sequence of (uniformly distributed) random requests If all tracks have the same number of sectors, and we disregard the time required for the head to start moving and to stop moving, we can show that the average seek time is one-third the worst case seek time Taking these factors into account, the average seek time is around one-half of the maximum seek time Average seek times currently range between 4 milliseconds and 10 milliseconds, depending on the disk model Once the seek has started, the time spent waiting for the sector to be accessed to appear under the head is called the rotational latency time Rotational speeds of disks today range from 5400 rotations per minute (90 rotations per second) up to 15,000 rotations per minute (250 rotations per second), or, equivalently, 4 milliseconds to 111 milliseconds per rotation On an average, one-half of a rotation of the disk is required for the beginning of the desired sector to appear under the head Thus, the average latency time of the disk is one-half the time for a full rotation of the disk The access time is then the sum of the seek time and the latency, and ranges from 8 to 20 milliseconds Once the rst sector of the data to be accessed has come under
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the head, data transfer begins The data-transfer rate is the rate at which data can be retrieved from or stored to the disk Current disk systems claim to support maximum transfer rates of about 25 to 40 megabytes per second, although actual transfer rates may be signi cantly less, at about 4 to 8 megabytes per second The nal commonly used measure of a disk is the mean time to failure (MTTF), which is a measure of the reliability of the disk The mean time to failure of a disk (or of any other system) is the amount of time that, on average, we can expect the system to run continuously without any failure According to vendors claims, the mean time to failure of disks today ranges from 30,000 to 1,200,000 hours about 34 to 136 years In practice the claimed mean time to failure is computed on the probability of failure when the disk is new the gure means that given 1000 relatively new disks, if the MTTF is 1,200,000 hours, on an average one of them will fail in 1200 hours A mean time to failure of 1,200,000 hours does not imply that the disk can be expected to function for 136 years! Most disks have an expected life span of about 5 years, and have signi cantly higher rates of failure once they become more than a few years old There may be multiple disks sharing a disk interface The widely used ATA-4 interface standard (also called Ultra-DMA) supports 33 megabytes per second transfer rates, while ATA-5 supports 66 megabytes per second SCSI-3 (Ultra2 wide SCSI) supports 40 megabytes per second, while the more expensive Fibre Channel interface supports up to 256 megabytes per second The transfer rate of the interface is shared between all disks attached to the interface
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