CASSANDRA-21632: Bound and chunk the BTI partition index preload (for 6.0) - #5091
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CASSANDRA-21632: Bound and chunk the BTI partition index preload (for 6.0)#5091aviau wants to merge 1 commit into
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driftx
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Sep 2, 2026
Opening a BTI SSTable whose bloom filter is uninformative warms the whole Partitions.db, one byte per page through a reader that follows disk_access_mode. That costs one pread or one readahead-bounded page fault per page: on a 309 GiB index, 81.1M reads at 50 MiB/s regardless of access mode. It also warms far more than the available page cache can hold. Add cassandra.bti.partition_index_preload_size to bound how much is warmed. The tail is warmed because the trie is written bottom-up, so the upper levels traversed by every lookup are at the end of the file, while the bulk at the front is leaf pages that a lookup reads one of. The property takes a human-readable size, e.g. 512MiB. 0B skips warming entirely, which was not previously possible: preload is enabled whenever the bloom filter is uninformative, so the only way to avoid it was to lower bloom_filter_fp_chance below 1.0. A negative value warms the whole index and is the default, so behaviour is unchanged unless the property is set. Also read the preload in chunks: touching one byte per page costs a pread or a page fault per page. Read it in 1 MiB positioned reads on the channel instead, which are unaffected by disk_access_mode; page cache is shared between the buffered and mapped views of a file, so a mapped lookup still finds the data resident. The trade-offs: the whole range is copied into userspace rather than one byte per page, and under a mapped mode the first lookup takes a minor fault the walk had already resolved. Under disk_access_mode: standard the warm no longer populates the chunk cache, nor evicts it; lookups fill it on first touch instead. Assisted-by: Claude Code:claude-opus-5 patch by Alexandre Viau; reviewed by TBD for CASSANDRA-21632
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As requested by @driftx, this is the same as #5089 but for
cassandra-6.0.