1.Define the following for a disk system: ts = seek time; average time to positi
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Question
1.Define the following for a disk system: ts = seek time; average time to position head over track r = rotation speed of the disk, in revolutions per second n = number of bits per sector N = capacity of a track, in bits tsector = time to access a sector Develop a formula for tsector as a function of the other parameters.
2.Consider a magnetic disk drive with 8 surfaces, 512 tracks per surface, and 64 sectors per track. Sector size is 1 kB. The average seek time is 8 ms, the track-to-track access time is 1.5 ms, and the drive rotates at 3600 rpm. Successive tracks in a cylinder can be read without head movement.
a. What is the disk capacity?
b. What is the average access time? Assume this file is stored in successive sectors and tracks of successive cylinders, starting at sector 0, track 0, of cylinder i.
c. Estimate the time required to transfer a 5-MB file.
d. What is the burst transfer rate?
3.Consider a disk that rotates at 3600 rpm. The seek time to move the head between adjacent tracks is 2 ms. There are 32 sectors per track, which are stored in linear order from sector 0 through sector 31. The head sees the sectors in ascending order. Assume the read/write head is positioned at the start of sector 1 on track 8. There is a main memory buffer large enough to hold an entire track. Data is transferred between disk locations by reading from the source track into the main memory buffer and then writing the data from the buffer to the target track.
a.How long will it take to transfer sector 1 on track 8 to sector 1 on track 9?
b.How long will it take to transfer all the sectors of track 8 to the corresponding sectors of track 9?
4.It should be clear that disk striping can improve data transfer rate when the strip size is small compared to the I/O request size. It should also be clear that RAID 0 provides improved performance relative to a single large disk, because multiple I/O requests can be handled in parallel. However, in this latter case, is disk striping necessary? That is, does disk striping improve I/O request rate performance compared to a comparable disk array without striping?
Explanation / Answer
2.) a)
1024bytes / sector.
64 sectors per track,
512 tracks per serface
8 surfaces
65536 bytes / track
512 * 65536 = 33554432 bytes per surface (32 mB per surface)
33554432 bytes * 8 = 268435456 bytes / 1024 / 1024 = 256 mB.
The disk can hold 256 megabytes.
b) Assume this file is stored in successive sectors and tracks of successive cylinders, starting at sector 0, track 0, of cylinder
Seek time + rotational delay + track to track. 8ms + 8.3 + 1.5ms = 17.8ms
c) Estimate the time required to transfer a 5-MB file.
5MB = 5242880 bytes.
5242880 / 65536 = 80 tracks.
80 * 64 = 5120 sectors.
So 80 tracks, 5120 sectors, 10 cylinders (80 tracks / 8 surfaces).
D) Bytes/track / time for one rotation. 66536/8.3 ms = 8016.4 bytes / ms.
3)
a) Read/write speed = 16.67ms / 32 = .52ms a sector (16.67 ms being the time for one rotation).
There is .52ms to read the sector, 2ms to switch from track 8 to track 9 along with 16.15ms
After which another 0.52ms to write the sector on track 9. 0.52 + 16.15 + 0.52 = 17.19ms.
b) 16.15 rotational delay (need to get to sector 0) + 16.67ms to read all the sectors + 16.67ms rotational delay which includes 2ms head move + 16.67ms to write all the sectors on track 9. 16.15 + 16.67 + 16.67 + 16.67 = 66.16 ms.
4)
RAID is a set of physical disk drives viewed by the operating system as a single logical drive and data are distributed across the physical drives of an array in a scheme known as striping, described subsequently. Data is spread across the array round-robin style in strips. A set of logically consecutive strips that maps exactly one strip to each array member is referred to as a stripe.
It is achieved through the use of parity bits, or, as with RAID 2, hamming code. This information can be spread across all the disks, or like RAID 4, there could be a single disk. RAID 1 uses mirroring to protect data.
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