Re:Lightning Memory-Mapped Database Manager (LMDB) 1.0
by senderista on Saturday July 04, 2026 @03:40PM
(#48788281)
How is pwrite/fsync any better than mmap/msync? Both go through the page cache and combine asynchronous writeback with forced flush. One theoretical advantage of pwrite might be that you can handle I/O errors, but I’d like to see a case where recovering from an I/O error makes sense (rather than just crashing the database, which SIGBUS would do anyway by default).
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Re:Lightning Memory-Mapped Database Manager (LMDB) 1.0
by hyc_symas on Sunday July 05, 2026 @09:33AM
(#48794217)
write/fsync can be faster in a large dataset because writes let the filesystem know an explicit list of dirty pages, so fsync only needs to deal with them.
mmap/msync gives no hints about which pages are dirty (unless the app tracks them itself and msyncs them individually, which would completely defeat any reduced syscall advantage of using a writable mmap in the first place) so the entire map must be scanned for dirty pages.
In practice, the expected performance advantages of using a writable mmap just aren't there, and coupled with the ease of silent corruption, it's best to never use that approach.