373 Disk Software In this section we will look at some issues related to disk

373 disk software in this section we will look at

This preview shows page 87 - 89 out of 140 pages.

is four times as fast as one drive, and no downtime. 3.7.3. Disk Software 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. The seek time (the time to move the arm to the proper cylinder). 2. The rotational delay (the time for the proper sector to rotate under the head). 3. The 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. Disk devices are prone to errors. Some kind of error check, a checksum or a cyclic redundancy check, is always recorded along with the data in each sector on a disk. Even the sector addresses recorded when the disk is formatted have check data. Floppy disk controller hardware can usually report when an error is detected, but the software must then decide what to do about it. Hard disk controllers often take on much of this burden. Particularly with hard disks, the transfer time for consecutive sectors within a track can be very fast. Thus reading more data than requested and caching it in memory can be very effective in speeding disk access. Disk Arm Scheduling Algorithms If the disk driver accepts requests one at a time and carries them out in that order, that is, First-Come, First-Served (FCFS), 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 pending requests for each cylinder chained together in a linked list headed by the table entries. [Page 282]
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Given this kind of data structure, we can improve upon the first-come, first-served scheduling algorithm. To see how, consider a disk with 40 cylinders.A request comes in to read a block on cylinder 11. While the seek to cylinder 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 linked list for each cylinder. The requests are shown in Fig. 3-21. Figure 3-21. Shortest Seek First (SSF) disk scheduling algorithm. [View full size image] A slight modification of this algorithm that has a smaller variance in response times is to always scan in the same direction (Teory, 1972). When the highest numbered cylinder with a pending request has been serviced, the arm goes to the lowest-numbered cylinder with a pending request and then continues moving in an upward direction. In effect, the lowest-numbered cylinder is thought of as being just above the highest-numbered cylinder. Some disk controllers provide a way for the software to inspect the current sector number under the head. With such a controller, another optimization is possible. If two or more requests for the same cylinder are pending, the driver can issue a request for the sector that will pass under the head next. Note that when multiple tracks are present in a
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