近年來因電腦硬體技術的進展，使得處理機速度已經急遽加快，但是縱使磁碟容量每年提高60-80%，儲存設備的存取時間並沒有加快多少。因此，處理器的性能與磁碟效能兩者間的鴻溝越來越深，而電腦系統的效能瓶頸也逐漸轉移到儲存系統上。
對於時間相關的多媒體應用而言，即時性的磁碟排程是一個重要的研究議題。對即時磁碟排程而言，我們不僅要減少每個行程的硬碟存取時間，同時也必須符合行程的即時限制。一些早期著名的研究結果，如SCAN-EDF和DM-SCAN （DM-SCAN方法，是應用修改期限的技術，並且修改過後的期限必須比之前的還小），利用SCAN演算法來重排輸入排程，以減少行程的服務時間。然而，這些方法只有當行程有相同的期限或是在某一有限的區域內，才可以利用SCAN重排，因此所得到的效能改進程度有限。雖然DM-SCAN法提出了一個更改期限的演算法，將輸入的不符合最早期限優先的排程，轉換成最早期限優先(EDF)的排程，但是，因為更改過後的期限會小於原本的期限，所以，可被重排的行程將逐漸減少，系統的效能和資料的流量也都因此受到影響。
在本論文中，為了解決上述DM-SCAN的缺點，我們提出了一個新的即時性磁碟排程演算法，使用了以較小的工作先作的修改期限概念，我們應用了這個修改期限的方式仔細的去改善整個系統的效能，如實驗結果所示，我們的方法的確有很好的效能，並且能讓更多的工作在規定的期限內完成，除此之外，我們也給了一個正式的證明，顯示一般的磁碟排程問題在線性耗費函式的模型下是NP-Hard。

Although the advances in hardware technology have dramatically increased processor speeds recently, the access time of storage devices does not decrease rapidly when disk capacity is improved at 60－80% annually. As a result, the gap between the performance of processors and the performance of disks is enlarged significantly and the bottleneck of the performance in a computer system is shifted to the storage subsystem.
Real-time disk scheduling is an important research topic for time-critical multimedia applications. For real-time disk scheduling, our goal is to minimize disk access time while guaranteeing tasks’ timing constraints. Some well-known research results, such as SCAN-EDF (SCAN-Earliest-Deadline-First) and DM-SCAN (Deadline-Modification-SCAN), in the DM-SCAN approach, a deadline modification scheme was applied, and the modified deadlines were smaller than the original one, applied the SCAN scheme to reschedule service sequence of input tasks to reduce tasks' service time. The performance is limited as the seek-optimizing SCAN scheme is only applied to tasks with the same deadline or within a confined group. For example, in DM-SCAN, groups of tasks that can be successfully rescheduled by SCAN under specified real-time requirements are selected. In addition, input schedule must be in Earliest-Deadline-First (EDF) sequence. In DM-SCAN, a deadline modification scheme was employed to obtain a pseudo EDF sequence from non-EDF ordered input tasks. Because the modified deadlines were earlier than the original ones, the number of tasks that could be rescheduled is decreased and thus data throughput is reduced.
In this paper, to resolve the drawback of DM-SCAN, we propose a new real-time disk-scheduling algorithm that uses the concept of Shortest-Task-First with deadline modification. We applied the DM scheme carefully to improve the overall performance. As shown in experimental results, our approach indeed has good performance and makes more tasks to meet their deadlines. In addition, we also give a formal proof to show that the general Disk-scheduling problem with linear cost-function is NP-Hard.

Andrews, M., M.A. Bender, L. Zhang, 1996. ”New algorithms for the disk
scheduling problem”. 37th Annual Symposium on Foundations of Computer
Science, p.550
Azer Bestavros, 1996. ”AIDA-based real-time fault-tolerant broadcast disks”.
Proceedings of 1996 IEEE Real-Time Technology and Applications Symposium,
May, Boston, Massachusetts.
Chang,R.I., Shih,W.K., Chang,R.C., 1998. ”Deadline-modification-SCAN with
maximum scannable-groups for multimedia real-time disk scheduling”.
Proceedings of the 19th IEEE Real-Time Systems Symposium, pp. 40–49.
Chang,H.P., Chang,R.I., Shih,W.K., Chang,R.C., 2000a. ”Enlarged-maximumscannable-
groups for real-time disk scheduling in a multimedia system”.
Proceedings of the 24th IEEE International Computer Software and Application
Conference (COMPSAC), pp. 383–388.
Chang,R.I., Shih,W.K., Chang,R.C., 2000b. ”Real-Time Disk Scheduling for
Multimedia Applications with Deadline-Modification-Scan Scheme”. The
International Journal of Time-Critical Computing Systems, 19, pp. 149–168
Chen,T.S., Yang,W.P., 1992. ”Amortized analysis of disk scheduling algorithm V
(R)”. Journal of Information Science and Engineering 8 (2), pp. 223–242.
Chen,T.S., Yang,W.P., Lee,R.C.T., 1992. ”Amortized analysis of some
Disk-Scheduling algorithms: SSTF, SCAN, and N-Step SCAN”. BIT 32, pp.
546–558.
Denning,P.L., 1967. ”Effects of scheduling on file memory operations”. Proceedings
of AFIPS SJCC, pp. 9–21.
Dan,A., Sitaram,D., Shahabuddin,P., 1994. ”Scheduling policies for an on-demand
video server with batching”. Proceedings of 2nd ACM Multimedia Conference, pp.
5–22.
Gemmell,D.J., Christodoulakis,S., 1992. ”Principles of delay sensitive multimedia
data storage and retrieval”. ACM Transaction on Information Systems 10 (1), pp.
51–90.
Gemmell,D.J., Vin,H.M., Kaudlur,D.D., Rangan,P.V., Rowe,L.A., 1995. ”
Multimedia storage servers: a tutorial”. IEEE Computer 28 (5), pp. 40–49.
Jellay,K., Stanat,D.F., Martel,C.U., 1991. ”On nonpreemptive scheduling of periodic
and sporadic tasks”. Proceedings of Twelfth IEEE Real-Time Systems Symposium,
pp. 129–139.
Lougher,P., Shepherd,D., 1993. ”The design of a storage server for continuous
media”. The Computer Journal 36 (1), pp. 32–42.
Lehoczky,J.P., 1990. ”Fixed priority scheduling of periodic task sets with arbitrary
deadlines”. Proceedings of 11th IEEE Real-Time Systems Symposium, pp.
201–212.
Lin,T.H., Tarng,W., 1991. ”Scheduling period and aperiodic tasks in hard real-time
computing systems”. Proceedings of ACM SIGMETRICS, pp. 31–38.
Liu,C.L., Layland,J.W., 1973. ”Scheduling algorithms for multiprogramming in a
hard real-time environment”. Journal of ACM 20 (1), pp. 46–61.
Nitin,H.V., Sohail,H., 1999. ”Scheduling data broadcast in asymmetric
communication environments”. Wireless Networks May Volume 5 Issue 2.
Pitoura,E., Samaras,G., 1997. ” Data Management for Mobile Computing”. Kluwer
Academic Publishers.
Reddy,A.L.N., Wyllie,J.C., 1993. ”Disk scheduling in a multimedia I/ O system”.
Proceedings of 1st ACM Multimedia Conference, pp. 225–233.
Reddy,A.L.N., Wyllie,J.C., 1994. ”I/O issues in a multimedia system”. IEEE
Computer 27 (3), pp. 69–76.
Ruemmler,C., Wyllie,J.C., 1994. ”An introduction to disk drive modeling”. IEEE
Computer 27 (3), pp. 17–28.
Stankovic,J.A., Buttazzo,G.C., 1995. ”Implications of classical scheduling results for
real-time systems”. IEEE Computer 28 (6), pp. 16–25.
Steinmetz,R., 1995. ”Multimedia file systems survey: approaches for continuous
media disk scheduling”. Computer Communication 18 (3), pp. 133–144.
Swarup Acharya, Michael Franklin, Stanley Zonik, 1995. “Dissemination-based Data
Delivery using Broadcast Disks”. IEEE Personal Communications, December.
Lim,S.H., Kim,J.H., 2000. “Real-time broadcast algorithm for mobile computing”.
the journal of systems and software, Volume 26, pp. 173-181,.
Wang,Y.C., Tsao,S.L., Chang,R.I., Chen,M.C., Ho,J.M., Ko,M.T., 1997. ”A fast data
placement scheme for video server with zoned-disks”. Proceedings of SPIE
VVDC, pp. 92–102.
Wong,C.K., 1980. ”Minimizing expected head movement in one dimension and two
dimensions mass storage system”. Computer Survey 12 (2), pp. 167–178.
Xuan,P., Gonzalez,J., Fernandez,R.K., 1997. ”Broadcast on demand: efficient and
timely dissemination of data in mobile environment”. Proceeding of Third IEEE
Real-Time Technology Application Symposium.