week2 - CS 351 -Data Organization and Management ( Section:...

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CS 351 -Data Organization and Management ( Section: 02) LectureNotes, Week 2 Prepared by: Nurettin Mert Aydın /Ahmet Örnek /ArmağanDurusal I/O Time in Disks We assume that there is just a single user for the system and ignore the CPU time (it is negligible). Disk I/O Time Components: Seek Time (s): Time needed to move read/write head from one track to another one. Rotational Latency Time (r): Time needed to come to the right track.It is ½ of the full revolution time. Block Transfer Time (btt): Time needed to read block to memory (buffer) Example: If a harddisk has 3600 revolutions per minute, its rotational latency time (r) can be found in this way: 3600/60 = 60 rev./sec. => 1/60 sec./rev. (It takes 1/60 seconds to complete 1 revolution) 1/60 sec/rev = 1000/60 msec/rev = 16.67msec/rev r = f (revolution time) / 2 => r = 16.67/2 ~ 8.3 msec/rev Units and Specifications of IBM 3380 Size = 2500 MB B (block size)=2400 bytes t (data transfer speed / data rate)=3000 bytes/msec t' (formatted speed) = 2857 bytes/msec btt (block transfer time) = B / t = 0.8 msec c (blocks per cylinder) = 600 ebt (effective block transfer time in seq. reading) = 0.84 msec m (minimum seek time) = 3 msec N (number of cylinders) = 885 r (rotational latency time) = 8.3 msec s (average seek time) = 16 msec Random vs. Sequential Reading With the information of IBM 3380,sequential and random readings can be calculated as in the following table: Read Type 10 Blocks to Read 100 Blocks to Read Sequential s + r + 10 * ebt = 16 + 8.3 + 10 * 0.84 = 32.7 msec s + r + 100 * ebt = 16 + 8.3 + 100 * 0.84 = 108.3 msec
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Random 10 * (s + r + btt) = 10 * (16 + 8.3 + 0.80) = 251 msec 100 * (s + r + btt) = 2510 msec Let"b" denote the number of blocks to be read (b* 25.1) / (24.3 + b * 0.84)= 25.1/ ((24.3 / b) + 0.84) when b goes to infinity (asymptotic case) random read is 30 times SLOWER than sequential read! Random Read : b * ( s + r + btt) Sequential Read (for large values of b) : b * ebt Note: In our calculations we assume that blocks are even multiple of record size Bfr (blocking factor)=B (Block size) / R (Record size) mod (B,R) = 0 b (number of blocks) = n (number ofrecords) / Bfr Example(COBOL):
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This note was uploaded on 12/01/2011 for the course CS 351 taught by Professor Fazlıcan during the Spring '11 term at Bilkent University.

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week2 - CS 351 -Data Organization and Management ( Section:...

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