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Backport feat/write buffer manager - #596
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Adds a process-wide WriteBufferManager singleton in DBSettings, lazy- created on first DB open and shared across every database in the process. Memtable memory is bounded by a single configurable limit rather than each DB managing its own budget. New `RocksDatabase.config()` options: - `writeBufferManagerSize` — bytes of memtable memory the WBM caps across all DBs (0 disables); runtime-mutable via SetBufferSize - `writeBufferManagerCostToCache` — when true, memtable charges show up in `block-cache-usage` as pinned entries, giving a single observability metric for read cache + memtable footprint - `writeBufferManagerAllowStall` — when true, writes stall once the cap is reached instead of letting memtables grow past it Implementation notes: - DBSettings::getInstance() now uses C++11 magic statics for thread- safe lazy initialization (previously raced on concurrent first-call from libuv worker threads). - Configuration fields are `std::atomic` so JS-thread Config() writes are safe against worker-thread reads in getWriteBufferManager(). - WBM creation is mutex-protected; double-checked locking on the atomic size guards against teardown between load and lock. - `costToCache` and `allowStall` are fixed at first WBM creation (already-open DBs hold shared_ptr refs; the WBM can't be swapped). Config() writes after creation are ignored with an stderr warning. - Setting size=0 at runtime is a "no new attachments" signal, not a teardown — RocksDB asserts SetBufferSize(>0). Verified by 11 tests covering: negative-size rejection, default-off path, costToCache wiring, multi-DB sharing, runtime resize, active memtable reclamation after flush, block-cache-usage charge visibility, the actual structural cap (writeBufferManagerSize, not blockCacheSize), and maintain-window cycling. Co-Authored-By: Claude Sonnet 4.7 <noreply@anthropic.com>
kriszyp
approved these changes
May 27, 2026
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Backport of #584