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Destage Algorithms for Disk Arrays with Non-Volatile Caches. Anujan Varma Quinn Jacobson. Presentation Outline. Introduction Write- Cache Purpose of Scheduling algorithm Parameters determining destages Different Scheduling algorithms Performance Evaluation Conclusion. Introduction.

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presentation outline
Presentation Outline
  • Introduction
  • Write- Cache
  • Purpose of Scheduling algorithm
  • Parameters determining destages
  • Different Scheduling algorithms
  • Performance Evaluation
  • Conclusion
introduction
Introduction
  • Parity logging only attempts to reduce the overhead of parity updates.
  • A non-volatile write cache is used to reduce write latency.
  • The process of updating data or parity in the disks from the write cache is called destaging.
non volatile write cache advantages
Non-Volatile Write Cache(Advantages)
  • Lower service time seen by write requests to an array.
  • Locality in writes in the work load can be exploited: temporal & spatial.
  • Lowers the response time for read requests serviced by the disks.
non volatile write cache disadvantages
Non-Volatile Write Cache(Disadvantages)
  • Reliability
  • Data Loss can occur.
  • Scheduling destages.
how a scheduling algorithm improves disk performance
How a scheduling algorithm improves disk performance
  • Can reduce the number of destages by capturing most of the re-writes in the write cache.
  • Can reduce the number of destages by aggregating blocks that lie physically close on a disk & destage them as a large read and/or write.
  • Can reduce the average time for a destage by ordering destage requests to the disk such that the service times in the individual disks are minimized.
parameters to determine the block to destage next
Parameters to determine the block to destage next
  • The probability of the block to be re-written in the near future.
  • Number of blocks to be read/updated on the same track.
  • Service time of the requests in the destage queue.
  • Current level of cache occupancy.
least cost scheduling
Least-Cost Scheduling
  • Modeled after the shortest seek-time first disk scheduling algorithm.
  • Each disk scheduled independently.
  • Request that takes the shortest access time is performed.
  • Exploits spatial locality.
high low mark algorithm
High/Low Mark Algorithm
  • Designed after the cache purging algorithm.
  • Each disk scheduled independently.
  • Two cache-occupancy thresholds are used to enable & disable destages.
  • Least-cost scheduling used to minimize the service time of individual disk accesses.
  • Exploits spatial locality.
linear threshold scheduling
Linear Threshold Scheduling
  • Matches the rate of the destage from the cache to the current level of occupancy for the cache.
  • Each disk scheduled independently.
  • Parity & data destages are treated in a similar manner.
  • Least-cost scheduling used.
  • Trade-off exists in the choice of the threshold.
  • Spatial locality implemented.
  • Temporal Locality is not maximized explicitly.
approximation to linear threshold scheduling
Approximation to Linear threshold scheduling
  • Linear threshold faces the problem of scanning an entire queue of destage request to select the minimum-cost request.
  • Divides the disk into regions.
  • Maintains separate queue of requests in each region.
  • Queues searched from the closest to farthest from the current position of head.
  • Uses function to specify the maximum destage cost for each level of cache occupancy.
  • Regions searched for destages are selected using the function: Th(w) >= cost(i,j)
the model
The Model
  • Read cache has unmodified copies of disk blocks.
  • Write cache holds newer information.
  • Request from host is FCFS.
  • Host request has higher priority than destage request.
  • Write access never bypasses the write cache.
performance metrics
Performance Metrics
  • Response time of host reads

- this is the average delay experienced by a read request from the host.

  • Disk utilization

- it is the fraction of time the disk is busy servicing a request.

  • Burst Tolerance

- the ability to tolerate short bursts in the workload without causing a write cache overflow.

response time of host reads
Response time of host reads
  • Linear Threshold has the best read-to-disk response times for moderate to heavy loads.
  • High/Low drops in performance at high workloads.
  • Linear & Least-Cost converge at high I/O.
burst tolerance
Burst Tolerance
  • FCFS & Least-cost show the best burst-tolerance.
  • Linear threshold recovers from burst tolerance with higher background loads than other algorithms.
disk utilization
Disk Utilization
  • High/Low mark & Linear threshold algorithms perform minimum work for destaging.
conclusion
Conclusion
  • Scheduling algorithm for destaging blocks from the write cache in a RAID –5 are presented.
  • Linear threshold scheduling provides the best read performance and still maintains a high degree of burst tolerance.
  • Approximation to Linear threshold maintains performance of Linear but can be implemented at a lower overhead.
questions
Questions
  • What are the characteristics of the Linear threshold scheduling algorithm?
  • Compare the performance of Linear threshold algorithm with others on the following criterions: Response time of host reads, Burst tolerance, and Disk Utilization?