DirectIO WAL Write does not honor DBOptions.use_fsync

[jsygit]

In my test using direct io for WAL write (with ext4 in the kernel), I noticed direct io WAL writes do not do metadata sync even when I have DBOptions.use_fsync = true. I understand that the actual data is persistent with direct IO, but the metadata is not flushed. I think we may be exposed to metadata loss in the case of sudden power outage? Please correct me if otherwise.

In the code, the function WritableFileWriter::Sync(bool use_fsync) is excluding SyncInternal for directio.

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Expected behavior

Actual behavior

Steps to reproduce the behavior

[jsygit]

I tried a quick fix by appending the O_DSYNC flag in env_posix.cc's OpenWritableFile() for direct IO. I was able to see the sync happening via kernel tracing and blkrace.

[cbi42]

We don't use direct I/O for WAL files: see https://github.com/facebook/rocksdb/wiki/Direct-IO and https://github.com/facebook/rocksdb/issues/12136.

I noticed direct io WAL writes do not do metadata sync even when I have DBOptions.use_fsync = true.

Did you set WriteOptions::sync to true or call SyncWAL()? Setting DBOptions.use_fsync to true only means we use fsync instead of fdatasync when sync is needed.