[Qemu-devel] [RFC] Use O_DSYNC by default and update documentation to explain IO integrity in QEMU

[Anthony Liguori <anthony@codemonkey.ws>]

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We'll have some benchmarks later this afternoon.

Regards,

Anthony Liguori

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diff --git a/block-raw-posix.c b/block-raw-posix.c
index 83a358c..e58f191 100644
--- a/block-raw-posix.c
+++ b/block-raw-posix.c
@@ -120,7 +120,7 @@ static int raw_open(BlockDriverState *bs, const char *filename, int flags)
 
     s->lseek_err_cnt = 0;
 
-    open_flags = O_BINARY;
+    open_flags = O_BINARY | O_DSYNC;
     if ((flags & BDRV_O_ACCESS) == O_RDWR) {
         open_flags |= O_RDWR;
     } else {
@@ -996,7 +996,7 @@ static int hdev_open(BlockDriverState *bs, const char *filename, int flags)
             IOObjectRelease( mediaIterator );
     }
 #endif
-    open_flags = O_BINARY;
+    open_flags = O_BINARY | O_DSYNC;
     if ((flags & BDRV_O_ACCESS) == O_RDWR) {
         open_flags |= O_RDWR;
     } else {
diff --git a/qemu-doc.texi b/qemu-doc.texi
index adf270b..2e859ff 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -267,13 +267,56 @@ These options have the same definition as they have in @option{-hdachs}.
 @item snapshot=@var{snapshot}
 @var{snapshot} is "on" or "off" and allows to enable snapshot for given drive (see @option{-snapshot}).
 @item cache=@var{cache}
-@var{cache} is "on" or "off" and allows to disable host cache to access data.
+@var{cache} is "on" or "off" and allows to the use of the host page cache.
 @item format=@var{format}
 Specify which disk @var{format} will be used rather than detecting
 the format.  Can be used to specifiy format=raw to avoid interpreting
 an untrusted format header.
 @end table
 
+By default, QEMU accesses all disk data through the host's page cache.
+This allows the host to perform read-ahead and to avoid duplicating IO
+requests unnecessarily increasing disk performance.  You may notice that
+certain benchmarks in the guest perform better than they do in the host
+(for read) because of this.  This is primarily because the benchmark is
+unaware of the extra level of caching that is occurring when running in
+a virtual environment.
+
+The cache=off option can be used to disable the use of the host's page
+cache.  Disabling the use of the host's page cache will likely reduce
+performance since the host is unable to perform read-ahead and unable
+to avoid duplicating IO requests.  At this time, QEMU will copy data
+internally so the cost of copying data into the host's page cache is
+unlikely to be statistically significant.
+
+The use of cache=off may make a benchmark appear to have results that
+are closer to the results in the host.  This does not imply that data
+integrity is not preserved when using cache=on, it is simply an artifact
+of the fact that the benchmark is not aware that it is in a virtual machine.
+It also does not imply that cache=off should be used for general workloads.
+
+In the future, QEMU will be able to avoid copying data internally and
+under certain workloads, disabling the use of the host's page cache may
+increase performance provided that the guest is actively working to avoid
+bringing data into the CPU's cache.  This can only be achieved when using
+things like sendfile() in the guest or other forms of direct-io.  An example
+of a workload that may benefit from avoiding the host's page cache is a
+static web server that is serving entirely unique data and has a relatively
+large amount of memory relative to the host.  This documentation will be
+updated when this change is made.  For now, cache=off is mostly useful for
+development purposes and for benchmarks that are not virtualization aware.
+
+Write requests are only reported completed to the guest when they have
+been reported completed by the disk regardless of whether the host's
+page cache is used for access so the use of the host's page cache is
+orthogonal to data integrity.
+
+If the host's disk drive has write-back caching enabled and the disk does
+not have a battery-backed cache, then data loss can occur regardless of
+whether write-back caching is disabled in the guest.
+
+If in doubt, do not change the default value (which is cache=on).
+
 Instead of @option{-cdrom} you can use:
 @example
 qemu -drive file=file,index=2,media=cdrom