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Date: Fri, 28 Mar 2008 15:13:11 -0300
From: Marcelo Tosatti <marcelo@kvack.org>
Subject: Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush
	request
Message-ID: <20080328181311.GA19547@dmt>
References: <20080328150517.GA18077@dmt>
	<200803281640.55185.paul@codesourcery.com>
	<20080328165941.GA19155@dmt>
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To: Paul Brook <paul@codesourcery.com>
Cc: kvm-devel <kvm-devel@lists.sourceforge.net>, qemu-devel@nongnu.org

On Fri, Mar 28, 2008 at 05:00:39PM +0000, Paul Brook wrote:
> > > Surely you should be using the normal aio notification to wait for the
> > > aio_fsync to complete before reporting success to the device.
> >
> > qemu_aio_flush() will wait for all pending AIO requests (including
> > aio_fsync) to complete.
> 
> Then why do you need to separate fdatasync?

Oh, I see what Jamie means now: fdatasync() is redundant with
aio_fsync(O_DSYNC).

How's this? 

Index: kvm-userspace.io/qemu/block-raw-posix.c
===================================================================
--- kvm-userspace.io.orig/qemu/block-raw-posix.c
+++ kvm-userspace.io/qemu/block-raw-posix.c
@@ -557,10 +557,39 @@ static int raw_create(const char *filena
     return 0;
 }
 
+static void raw_aio_flush_complete(void *opaque, int ret)
+{
+    if (ret)
+        printf("WARNING: aio_fsync failed (completion)\n");
+}
+
+static void raw_aio_flush(BlockDriverState *bs)
+{
+    RawAIOCB *acb;
+
+    acb = raw_aio_setup(bs, 0, NULL, 0, raw_aio_flush_complete, NULL);
+    if (!acb)
+        return;
+
+    if (aio_fsync(O_DSYNC, &acb->aiocb) < 0) {
+        qemu_aio_release(acb);
+        perror("aio_fsync");
+        printf("WARNING: aio_fsync failed\n");
+        return;
+    }
+}
+
 static void raw_flush(BlockDriverState *bs)
 {
     BDRVRawState *s = bs->opaque;
-    fsync(s->fd);
+    raw_aio_flush(bs);
+
+    /* We rely on the fact that no other AIO will be submitted
+     * in parallel, but this should be fixed by per-device
+     * AIO queues when allowing multiple CPU's to process IO
+     * in QEMU.
+     */
+    qemu_aio_flush();
 }
 
 BlockDriver bdrv_raw = {