From d71d1394ec48f89fab50bf8e592e3de6200b5b94 Mon Sep 17 00:00:00 2001 From: Anton Kaliaev Date: Thu, 21 Sep 2017 16:11:28 -0700 Subject: [PATCH] call fsync after flush (Refs #573) short: flushing the bufio buffer is not enough to ensure data consistency. long: Saving an entry to the WAL calls writeLine to append data to the autofile group backing the WAL, then calls group.Flush() to flush that data to persistent storage. group.Flush() in turn proxies to headBuf.flush(), flushing the active bufio.BufferedWriter. However, BufferedWriter wraps a Writer, not another BufferedWriter, and the way it flushes is by calling io.Writer.Write() to clear the BufferedWriter's buffer. The io.Writer we're wrapping here is AutoFile, whose Write method calls os.File.Write(), performing an unbuffered write to the operating system, where, I assume, it sits in the OS buffers awaiting sync. This means that Wal.Save does not, in fact, ensure the saved operation is synced to disk before returning. --- autofile/group.go | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/autofile/group.go b/autofile/group.go index 39f274e04..ce3e30009 100644 --- a/autofile/group.go +++ b/autofile/group.go @@ -153,7 +153,11 @@ func (g *Group) WriteLine(line string) error { func (g *Group) Flush() error { g.mtx.Lock() defer g.mtx.Unlock() - return g.headBuf.Flush() + err := g.headBuf.Flush() + if err == nil { + err = g.Head.Sync() + } + return err } func (g *Group) processTicks() {