Magnetic Disk Rotational latency Example Find the average rotational latency if the disk rotates at 20,000 rpm.

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Presentation transcript:

Magnetic Disk Rotational latency Example Find the average rotational latency if the disk rotates at 20,000 rpm.

Peripheral Devices

Hard Disk

Static Properties

Dynamic Properties

CS501 Advanced Computer Architecture Lecture 32 Dr.Noor Muhammad Sheikh

Magnetic Disk Average latency Example A magnetic disk has an average seek time of 5 ms. The transfer rate is 50 MB/sec. The disk rotates at 10,000 rpm and the controller overhead is 0.2 msec. Find the average time to read or write 1024 bytes.

Magnetic Disk Solution: Average T seek =5ms Average T rot =0.5*60/10,000=3 ms T transfer =1KB/50MB=0.02ms T controller =0.2ms The total time taken= T seek +T rot + T tsfr +T ctr = =8.22 ms

Magnetic Disk: Capacity Example A hard disk with 5 platters has 1024 tracks per platter,512 sectors per track and 512 byte/sectors. What is the total capacity of the disk?

Magnetic Disk: Capacity Solution 512 bytes x 512 sectors=0.2MB/track 0.2MB x 1024 tracks=0.2GB/platter Therefore the hard disk has the total capacity of 5 x 0.2=1GB

Magnetic Disk: Capacity Example How many platters are required for a 40GB disk if there are 1024 bytes/sectors, 2048 sectors per track and 4096 tracks per platter

Magnetic Disk: Capacity Solution The capacity of one platter = 1024 x 2048 x 4096 = 8GB For a 40GB hard disk, we need 40/8 = 5 such platters.

Magnetic Disk transfer time Example Consider a hard disk that rotates every 20 msec. The seek time to move the head between two adjacent tracks is 1 ms. There are 64 sectors per track stored in linear order.

Assume that the read/write head is initially at the start of sector 1 on track 12. a.How long will it take to transfer sector 1 on track 12 to sector 1 on track 13? b.How long will it take to transfer all the sectors on track 12 to corresponding sectors on track 13? Magnetic Disk transfer time

Solution a.Total transfer time=sector read time+head movement time+rotational delay+sector write time Time to read or write on sector=20/64=0.31ms/sector Head movement time=1ms After reading sector 1 on track 12, which takes 0.31 ms, an additional 19.7 ms of rotational delay is needed for the head to line up with sector 1 again. The head movement time of 2 ms gets included in the19.7 ms. Total transfer time=0.31ms+19.7ms+0.31ms=20.3ms Magnetic Disk transfer time

b.The time to transfer all the sectors of track 12 to track 13 can be computed in the similar way. Assume that the memory buffer can hold an entire track. So the time to read or write an entire track is simply the rotational delay for a track, which is 20 ms. The head movement time is 1ms, which is also the time for 1/0.3=3.3  4 sectors to pass under the head. Thus after reading a track and repositioning the head, it is now on track 13, at four sectors past the initial sector that was read on track 12. (Assuming track 13 is written starting at sector 5) therefore total transfer time= =41ms. Magnetic Disk transfer time

Calculate time to read 64 KB (128 sectors) for the following disk parameters. –181.6 GB, 3.5 inch disk –12 platters, 24 surfaces –24,247 cylinders –7,200 RPM; (4.2 ms avg. latency) –7.4/8.2 ms avg. seek (r/w) –65 MB/s (internal) –0.1 ms controller time

Solution Disk latency = average seek time + average rotational delay + transfer time + controller overhead = 7.4 ms x 1/(7200 RPM) /(60000ms/M)) + 64 KB / (65 MB/s) ms = 12.7 ms

Calculate the time to read 64 KB for the previous disk, this time using 1/3 of quoted seek time, 3/4 of internal outer track bandwidth Solution: Disk latency = average seek time + average rotational delay + transfer time + controller overhead = (0.33* 7.4 ms) * 1/(7200 RPM) + 64 KB / (0.75* 64 MB/s) ms = 2.44 ms /(7200 RPM/(60000ms/M)) + 64 KB / (48 KB/ms) ms = ms = 8 ms

Magnetic Disk Example 3: Compare the time taken by a flash memory and a magnetic disk to read or write a 64 KB block. Assume the Flash takes 60 ns to read and 0.001ms to write one byte. 6 ms are required to erase 4 KB. The specifications of magnetic disk are the same as in previous example

Magnetic Disk Solution: The average time to read or write 64KB on a magnetic disk = T seek +T rot + T tsfr +T ctr = =9.48ms Flash read time=64KB/(1B/60ns) =3.83ms Flash write time =64KB/(4KB/6ms)+64KB/(1B/0.001ms) =96 ms +64ms = 160 ms Conclusion: Flash memory is about 2.5 times faster than disk for reading. But it is much slower than disk for writing the same amount of data.