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How many segments to clean at a time which segments

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Unformatted text preview: y are reused Issues Issues Issues: when when to run cleaner? how many segments to clean at a time? *which segments to clean? *how to re-write the live blocks? Segment Segment cleaning Old segments contain • live data • “dead data” belonging to files that were deleted or overwritten Segment cleaning involves writing out the live data Segment summary block identifies each piece of information in the segment (for data blocks to which inodes are they associated) Segment Segment cleaning (cont’d) Segment cleaning process involves 1. reading a number of segments into memory 2. identifying the live data 3. writing them back to a smaller number of clean segments Key issue is where to write these live data Write Write cost u = utilization utilization (fraction of live data) Segment Segment Cleaning Policies: which Greedy policy: always cleans the least-utilized segments Cost-benefit policy: selects segments with the highest benefitto-cost ratio older data – more stable newer data – more likely to be modified or deleted – cleaning wastes time 1 to read, u to copy Copying Copying life blocks: where Age sort: • sorts the blocks by the time they were last modified • groups blocks of similar age together into new segments Age of a block is good predictor of its survival Supports cost-benefit policy Simulation Simulation results Consider two file access patterns • Uniform • Hot-and-cold: 90% of the accesses involve 10% of the files 10% of the accesses involves 90% of the files Greedy Greedy policy Write cost is very sensitive to disk utilization • higher disk utilizations result in more frequent segment cleanings • will also clean segments that contain more live data Using U...
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