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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 410
SEC. 5.4
DISKS
379
The final step in preparing a disk for use is to perform a
high-level format
of
each partition (separately).
This operation lays down a boot block, the free storage
administration (free list or bitmap), root directory, and an empty file system.
It
also puts a code in the partition table entry telling which file system is used in the
partition because many operating systems support multiple incompatible file sys-
tems (for historical reasons).
At this point the system can be booted.
When the power is turned on, the BIOS runs initially and then reads in the
master boot record and jumps to it. This boot program then checks to see which
partition is active.
Then it reads in the boot sector from that partition and runs it.
The boot sector contains a small program that generally loads a larger bootstrap
loader that searches the file system to find the operating system kernel. That pro-
gram is loaded into memory and executed.
5.4.3 Disk Arm Scheduling Algorithms
In this section we will look at some issues related to disk drivers in general.
First, consider how long it takes to read or write a disk block. The time required is
determined by three factors:
1.
Seek time (the time to move the arm to the proper cylinder).
2.
Rotational delay (how long for the proper sector to appear under the
reading head).
3. Actual data transfer time.
For most disks, the seek time dominates the other two times, so reducing the mean
seek time can improve system performance substantially.
If the disk driver accepts requests one at a time and carries them out in that
order, that is,
FCFS
(
First-Come, First-Served
), little can be done to optimize
seek time. However, another strategy is possible when the disk is heavily loaded. It
is likely that while the arm is seeking on behalf of one request, other disk requests
may be generated by other processes. Many disk drivers maintain a table, indexed
by cylinder number, with all the pending requests for each cylinder chained toget-
her in a linked list headed by the table entries.
Given this kind of data structure, we can improve upon the first-come, first-
served scheduling algorithm. To see how, consider an imaginary disk with 40 cyl-
inders. A request comes in to read a block on cylinder 11.
While the seek to cylin-
der 11 is in progress, new requests come in for cylinders 1, 36, 16, 34, 9, and 12, in
that order. They are entered into the table of pending requests, with a separate link-
ed list for each cylinder. The requests are shown in Fig. 5-24.
When the current request (for cylinder 11) is finished, the disk driver has a
choice of which request to handle next. Using FCFS, it would go next to cylinder
1, then to 36, and so on. This algorithm would require arm motions of 10, 35, 20,
18, 25, and 3, respectively, for a total of 111 cylinders.
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