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RestoreSink: start writeback while restoring large files - #765
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With fsync-store-paths, RestoreSink started writeback of a regular file (sync_file_range(SYNC_FILE_RANGE_WRITE)) only when the file was closed, so the fsync before registering the path waited for the whole file to reach the disk. Start writeback of each 32 MiB as it is written instead, so the disk writes while the rest of the NAR is still being downloaded and unpacked. The fsync before registration is unchanged; sync_file_range is only a hint and its errors are still ignored.
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Motivation
With
fsync-store-paths = true,RestoreSinkstarts writeback of a restored regular file only when the file is closed (AutoCloseFD::startFsync(), i.e.sync_file_range(fd, 0, 0, SYNC_FILE_RANGE_WRITE)). The fsync before registration then waits for the whole file to reach the disk, and every path that refers to it waits too. While a large file streams in from a substituter the disk sits idle, then has to write all of it at the end: for a 731 MiB file streamed at 230 MB/s the final fsync took 1.21 s, versus 0.01 s with writeback started as the file is written.Change
AutoCloseFD::startFsync()takes an optional byte range (default: the whole file, as before).fsync-store-pathsis on,RestoreRegularFilestarts writeback of each 32 MiB as it is written, and of the remaining tail at close.sync_file_range()stays Linux-only and advisory, and its errors are still ignored. Withfsync-store-paths = falsenothing changes.Testing
restore-large-file.sh: substitutes a 100 MiB file withfsync-store-paths = trueand checks its registration and contents. (That writeback now starts per chunk shows up understrace -e sync_file_rangeas ranged calls; there's no cheap way to assert that in a test.)restore-large-file,binary-cache,binary-cache-compression,nars,nar-access,substitute-with-invalid-ca,multiple-outputs-substitute-failure,nix-util-testsandnix-util-tests-without-new-syscallspass.fsync-store-paths = true, paired interleaved runs (n = 34 each): install time −14% ± 5.3% (95% CI) on a closure dominated by a few multi-hundred-MiB files, −6.9% on another. To reproduce without our closures: substitute any path containing a file of several hundred MiB and time the final fsync (strace -T -e fsync,sync_file_range).