Zygo Blaxell 24b08ef7b7 scan_one_extent: eliminate nuisance dedupes, drop caches after reading data ...

A laundry list of problems fixed:

 * Track which physical blocks have been read recently without making
 any changes, and don't read them again.

 * Separate dedupe, split, and hole-punching operations into distinct
 planning and execution phases.

 * Keep the longest dedupe from overlapping dedupe matches, and flatten
 them into non-overlapping operations.

 * Don't scan extents that have blocks already in the hash table.
 We can't (yet) touch such an extent without making unreachable space.
 Let them go.

 * Give better information in the scan summary visualization:  show dedupe
 range start and end points (<ddd>), matching blocks (=), copy blocks
 (+), zero blocks (0), inserted blocks (.), unresolved match blocks
 (M), should-have-been-inserted-but-for-some-reason-wasn't blocks (i),
 and there's-a-bug-we-didn't-do-this-one blocks (#).

 * Drop cached data from extents that have been inserted into the hash
 table without modification.

 * Rewrite the hole punching for uncompressed extents, which apparently
 hasn't worked properly since the beginning.

Nuisance dedupe elimination:

 * Don't do more than 100 dedupe, copy, or hole-punch operations per
 extent ref.

 * Don't split an extent or punch a hole unless dedupe would save at
 least half of the extent ref's size.

 * Write a "skip:" summary showing the planned work when nuisance
 dedupe elimination decides to skip an extent.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

2024-11-30 23:30:33 -05:00

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