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a common lock. This change avoids a deadlock between snapshots when separate requests cause them to deadlock checking each other for a need to copy blocks that are close enough together that they fall into the same indirect block. Although I had anticipated a slowdown from contention for the single lock, my filesystem benchmarks show no measurable change in throughput on a uniprocessor system with three active snapshots. I conjecture that this result is because every copy-on-write fault must check all the active snapshots, so the process was inherently serial already. This change removes the last of the deadlocks of which I am aware in snapshots. Sponsored by: DARPA & NAI Labs.
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