a0e66849d7
Count consecutive read requests as blocking in CTL for files and ZVOLs. Technically read requests can be executed in any order or simultaneously since they are not changing any data. But ZFS prefetcher goes crasy when it receives consecutive requests from different threads. Since prefetcher works on level of separate blocks, instead of two consecutive 128K requests it may receive 32 8K requests in mixed order. This patch is more workaround then a real fix, and it does not fix all of prefetcher problems, but it improves sequential read speed by 3-4x times in some configurations. On the other side it may hurt performance if some backing store has no prefetch, that is why it is disabled by default for raw devices.
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