High Availability in Clustered Multimedia Servers Renu Tewari Daniel M. Dias Rajat Mukherjee Harrick M. Vin

Topics Problem: high availability in clustered multimedia servers Schemes for high availability Details of some schemes for high availability Simulation Cost performance analysis Conclusion

The problem the author addressed High availability in clustered multimedia servers Clustered multimedia servers consists of a set of processing node, each with a local disk array, connected by a high bandwidth switch or network. High availability requires the servers can provide continuous delivery in the presence of failures, including disk failure and node failure.

Architecture of clustered multimedia servers Front end performs delivery. Back end provides storage.

Architecture of clustered multimedia servers Front end fails Stream can be resumed from another delivery node. Back end fails Has system-wide effect. It is the major concern in this paper.

Schemes for high availability Mirroring – Disk level mirroring – Block level mirroring

Schemes for high availability Twin-tailing/Multi Tailing Used to handle node failure. A buddy node can access the others disks in case of failure.

Schemes for high availability Software raid – Sequential parity placement – Random parity placement

Sequential parity placement Disk numbered i, is attached to a node numbered i mod N, N=number of nodes Parity group, if size=5, DDDDPDDDDP …

Sequential parity placement

Random parity placement Constraint Blocks belonging to the same parity group are not placed on the same disk or on any disk on the same node.

Random parity placement

Some simulation results

Comparison between spp and rpp According to the simulation, RPP ’ s performance is better than SPP. RPP needs much more meta-data. Increasing the read-ahead buffer size,with RPP, the loss can be reduced substantially. To decrease the amount of meta data of RPP, SPP with multiple strides can be used.

Cost analytical model Queue theory Each disk behaves like an M/D/1 queue.

Conclusions Sequential placement of parity can only balance the space and bandwidth utilization of all disks during normal operation. Balanced random placement of parity can achieve during failure and normal operation. Mirroring is cost effective only when missing the real time constraints during failure is more expensive than the extra disk capacity. Larger parity group sizes have smaller disk space overhead but larger memory costs for buffering. Tighter loss criteria require small parity group size.

Questions Name 3 schemes for high availability. In sequential parity placement, why should you choose the parity group size to be relatively prime to the number of disks?