Availability Aware Continuous Replica Placement Problem

Replica placement (RP) intended at producing a set of duplicated data items across the nodes of a distributed system in order to optimize fault tolerance, availability, system performance load balancing. Typically, RP formulations employ dynamic methods to change the replica placement in the system potentially upon user request profile. Continuous Replica Placement Problem (CRPP) is an extension of replica placement problem that takes into consideration the current replication state of the distributed system along with user request profile to define a new replication scheme, subject to optimization criteria and constraints. This paper proposes an alternative technique, named Availability Aware Continuous Replica Placement Problem (AACRPP).AACRPP can be defined as: Given an already defined replica placement scheme, a user request profile, and a node failure profile define a new replication scheme, subject to optimization criteria and constraints. In this effort we use modified greedy heuristics from the CRPP and investigated the proposed mechanism using a trace driven java based simulation

Mirrored and Hybrid Disk Arrays: Organization, Scheduling, Reliability, and Performance

Basic mirroring (BM) classified as RAID level 1 replicates data on two disks, thus doubling disk access bandwidth for read requests. RAID1/0 is an array of BM pairs with balanced loads due to striping. When a disk fails the read load on its pair is doubled, which results in halving the maximum attainable bandwidth. We review RAID1 organizations which attain a balanced load upon disk failure, but as shown by reliability analysis tend to be less reliable than RAID1/0. Hybrid disk arrays which store XORed instead of replicated data tend to have a higher reliability than mirrored disks, but incur a higher overhead in updating data. Read request response time can be improved by processing them at a higher priority than writes, since they have a direct effect on application response time. Shortest seek distance and affinity based routing both shorten seek time. Anticipatory arm placement places arms optimally to minimize the seek distance. The analysis of RAID1 in normal, degraded, and rebuild mode is provided to quantify RAID1/0 performance. We compare the reliability of mirrored disk organizations against each other and hybrid disks and erasure coded disk arrays