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There are two subvol scan algorithms implemented so far. The two modes are unimaginatively named 0 and 1. 0: sorts extents by (inode, subvol, offset), 1: scans extents round-robin from all subvols. Algorithm 0 scans references to the same extent at close to the same time, which is good for performance; however, whenever a snapshot is created, the scan of the entire filesystem restarts at the beginning of the new snapshot. Algorithm 1 makes continuous forward progress even when new snapshots are created, but it does not benefit from caching and will force the kernel to reread data multiple times when there are snapshots. The algorithm can be selected at run-time using the -m or --scan-mode option. We can collect some field data on these before replacing them with an extent-tree-based scanner. Alternatively, for pre-4.14 kernels, we can keep these two modes as non-default options. Currently these algorithms have terrible names. TODO: fix that, but also TODO: delete all that code and do scans directly from the extent tree instead. Augment the scan algorithms relative to their earlier implementation by batching multiple extents to scan from each subvol before switching to a different subvol. Sprinkle some BEESNOTEs on the Task objects so that they don't disappear from the thread status output. Adjust some timing constants to deal with the increased latency from competing threads. Signed-off-by: Zygo Blaxell <bees@furryterror.org>