Date: 2026-05-29
Time: 06:23
No problem — I have enough from the observations to give a thorough explanation.
write-ahead-log/wal.py:checkpoint — The checkpoint record (line 169) could serve as a durable truncation watermark, eliminating the need for in-place rewrites entirelylog-structured-merge-tree/lsm.py:truncate — Compare the LSM WAL's even more aggressive truncation (line 56) which opens with "wb" and instantly destroys all contenthash-index-storage/bitcask.py — The Bitcask compaction at line 297 uses os.rename() for segment rotation — contrast this safe approach with the WAL truncationfsync-directory-durability — Even with atomic rename, the directory entry must be fsynced separately on Linux for the rename to survive a power loss; none of the implementations in this repo do thiswrite-ahead-log/wal.py:readrecord — The CRC validation (lines 43-54) detects post-crash corruption but doesn't prevent data loss — understanding the difference between detection and prevention is keywal-truncate-not-crash-safe — WriteAheadLog.truncate() rewrites WAL files in place without write-to-temp-then-rename, creating a window where a crash loses both old and new recordslsm-wal-truncate-destroys-immediately — LSMTree's WAL.truncate() opens the file with "wb" which zeroes it instantly, with no intermediate durable state to recover fromcrc-detects-but-does-not-prevent — CRC32 checksums in readrecord detect partial writes after a crash but cannot recover the lost data; they turn silent corruption into detected data losscheckpoint-record-exists-but-unused-for-truncation — The WAL supports OP_CHECKPOINT records (line 169) that could serve as a durable truncation marker, but truncate() physically removes records instead of using this mechanismno-directory-fsync-anywhere — No implementation in the repo calls fsync on a directory file descriptor after file creation or rename, meaning even correct atomic-rename patterns would not be fully crash-safe on Linux