CASSANDRA-19988: Direct IO support for background reads - #5143
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In Cassandra 5.0, Direct I/O support was added for background writes (background_write_disk_access_mode) and compaction reads (compaction_read_disk_access_mode). However, other sequential background read operations (Scrub, Verify, SAI and Accord index builds, streaming) still hardcoded buffered I/O, causing massive Page Cache pollution and kicking out hot cached user data when scanning multi-gigabyte SSTables. This patch: 1. Config: introduces background_read_disk_access_mode with @Replaces for backward-compatibility with compaction_read_disk_access_mode. 2. DatabaseDescriptor: resolves background_read_disk_access_mode and preserves deprecated compaction_read_disk_access_mode accessors as delegates. 3. SSTableReader: adds openDataReader(DiskAccessMode, RateLimiter) overload. 4. SortedTableScrubber & SortedTableVerifier: plumb background_read_disk_access_mode into data reader creation so nodetool scrub and verify respect Direct I/O. 5. SecondaryIndexBuilders: plumb background_read_disk_access_mode into SAI and Accord index rebuild scans. Fixes: CASSANDRA-19988
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Status: changes requested — not ready to merge
The published setting does not reach streaming readers despite advertising streaming coverage.
Verification correction
Earlier descriptions overstated correctness and/or test coverage. Those claims are withdrawn. AI-assisted source review has been performed; this is not maintainer approval. Previously mixed build artifacts are not accepted as verification evidence. Corrective changes and clean, targeted verification are in progress; the published head has not yet been replaced.
Published head under review:
cd631b47c77a05f8663d60db0ac62640c1265735.