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The problem of supporting degraded mode of operation in media servers has been investigated by a number of researchers [1 8]. One approach makes use of data replications such as mirroring to sustain disk failure. The idea is to place two or more replicas in different
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disks so that at least one copy is available after a disk failure. Examples include the rotational mirrored declustering scheme proposed by Chen et al. [4], the doubly striped mirroring scheme proposed by Mourad [6], and the random duplicated assignment proposed by Korst [8]. Another approach makes use of parity encoding for data redundancy. A parity block together with a number of data blocks forms a parity group. The entire parity group can be reconstructed even if one of the blocks in the parity group is lost in a disk failure. Compared to replication, parity encoding generally requires less redundancy overhead, but higher buffer requirement for data reconstruction. This approach has been investigated by Tobagi et al. [1] in their Streaming RAID architecture, by Cohen et al. [3] in their pipelined disk array, by Berson et al. [2] in their non-clustered scheme, and by Ozden et al. [5] in their declustered parity scheme and prefetch scheme. In another work by Cohen and Burkhard [7], a segmented information dispersal (SID) scheme was proposed to allow ne grain trade-off between the two extremes of mirroring and RAID-5 parity encoding. Reconstruction reads under SID are contiguous, leading to better disk ef ciency. The authors showed that the SID schemes match the performance of RAID-5 and schemes based on balanced incomplete block designs under normal mode, and outperforms them under degraded mode of operation. The previous studies all focus on the normal mode and degraded mode of operation. The problem of rebuilding data in a failed disk to a spare disk in a media server has received little attention. While there are many existing studies on disk rebuild, they have all focused on data applications such as online transaction processing (OLTP) servers. Some examples are the work by Menon and Mattson [10 11], Hou et al. [12 13], Thomasian and Menon [14 16], Mogi and Kitsuregawa [17], and so on. Disk rebuild in media server applications, however, differs from that of OLTP applications in two major ways. First, OLTP applications generally do not have the stringent performance requirement of a media server. In particular, performance of OLTP applications is commonly measured using response time. While shorter response time is desirable, it is not a condition for correct operation. Therefore, in disk rebuild, the focus is to balance service response time with rebuild time. For example, one can use priority scheduling in OLTP applications to give higher priority to normal requests to minimize their response time and to serve rebuild requests with the unused disk time. By contrast, a media server has to guarantee the retrieval of media data according to a xed schedule. Even a small delay beyond the schedule will result in service disruption. Consequently, the rebuild process can take place only if normal media data retrievals can still be completed on time. This requires detailed disk modeling and the use of worst-case analysis to determine exactly how much disk time can be spent on the rebuild process. Unlike rebuild algorithms for OLTP applications, the amount of disk time to spend on rebuild is determined a priori, given the disk parameters. Moreover, retrievals for playback data and rebuild data are scheduled to minimize disk-seek time instead of according to priority as in the OLTP case. Second, OLTP applications commonly employ the RAID-5 striping scheme to maximize I/O concurrency [9]. On the other hand, media server applications commonly employ the RAID-3 striping scheme for reasons to be discussed in Section 5.3.1. This fundamental difference in the striping scheme, together with the inherently round-based disk scheduling algorithm employed in media servers, requires different designs for the rebuild algorithm.
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