[Qemu-devel] [PATCH] Add cache=volatile parameter to -drive

[Alexander Graf <agraf@suse.de>]

Usually the guest can tell the host to flush data to disk. In some cases we
don't want to flush though, but try to keep everything in cache.

So let's add a new cache value to -drive that allows us to set the cache
policy to most aggressive, disabling flushes. We call this mode "volatile",
as guest data is not guaranteed to survive host crashes anymore.

This patch also adds a noop function for aio, so we can do nothing in AIO
fashion.

Signed-off-by: Alexander Graf <agraf@suse.de>

---

v2 -> v3:

  - Add description of cache=volatile
  - Squash aio noop noop patch into this one
---
 block.c         |   28 ++++++++++++++++++++++++++++
 block.h         |    1 +
 qemu-config.c   |    2 +-
 qemu-options.hx |   13 ++++++++++---
 vl.c            |    3 +++
 5 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/block.c b/block.c
index 48305b7..b742965 100644
--- a/block.c
+++ b/block.c
@@ -50,6 +50,8 @@ static BlockDriverAIOCB *bdrv_aio_writev_em(BlockDriverState *bs,
         BlockDriverCompletionFunc *cb, void *opaque);
 static BlockDriverAIOCB *bdrv_aio_flush_em(BlockDriverState *bs,
         BlockDriverCompletionFunc *cb, void *opaque);
+static BlockDriverAIOCB *bdrv_aio_noop_em(BlockDriverState *bs,
+        BlockDriverCompletionFunc *cb, void *opaque);
 static int bdrv_read_em(BlockDriverState *bs, int64_t sector_num,
                         uint8_t *buf, int nb_sectors);
 static int bdrv_write_em(BlockDriverState *bs, int64_t sector_num,
@@ -1306,6 +1308,10 @@ const char *bdrv_get_device_name(BlockDriverState *bs)
 
 void bdrv_flush(BlockDriverState *bs)
 {
+    if (bs->open_flags & BDRV_O_NO_FLUSH) {
+        return;
+    }
+
     if (bs->drv && bs->drv->bdrv_flush)
         bs->drv->bdrv_flush(bs);
 }
@@ -2082,6 +2088,10 @@ BlockDriverAIOCB *bdrv_aio_flush(BlockDriverState *bs,
 {
     BlockDriver *drv = bs->drv;
 
+    if (bs->open_flags & BDRV_O_NO_FLUSH) {
+        return bdrv_aio_noop_em(bs, cb, opaque);
+    }
+
     if (!drv)
         return NULL;
     return drv->bdrv_aio_flush(bs, cb, opaque);
@@ -2196,6 +2206,24 @@ static BlockDriverAIOCB *bdrv_aio_flush_em(BlockDriverState *bs,
     return &acb->common;
 }
 
+static BlockDriverAIOCB *bdrv_aio_noop_em(BlockDriverState *bs,
+        BlockDriverCompletionFunc *cb, void *opaque)
+{
+    BlockDriverAIOCBSync *acb;
+
+    acb = qemu_aio_get(&bdrv_em_aio_pool, bs, cb, opaque);
+    acb->is_write = 1; /* don't bounce in the completion hadler */
+    acb->qiov = NULL;
+    acb->bounce = NULL;
+    acb->ret = 0;
+
+    if (!acb->bh)
+        acb->bh = qemu_bh_new(bdrv_aio_bh_cb, acb);
+
+    qemu_bh_schedule(acb->bh);
+    return &acb->common;
+}
+
 /**************************************************************/
 /* sync block device emulation */
 
diff --git a/block.h b/block.h
index f87d24e..8032b6b 100644
--- a/block.h
+++ b/block.h
@@ -33,6 +33,7 @@ typedef struct QEMUSnapshotInfo {
 #define BDRV_O_CACHE_WB    0x0040 /* use write-back caching */
 #define BDRV_O_NATIVE_AIO  0x0080 /* use native AIO instead of the thread pool */
 #define BDRV_O_NO_BACKING  0x0100 /* don't open the backing file */
+#define BDRV_O_NO_FLUSH    0x0200 /* disable flushing on this disk */
 
 #define BDRV_O_CACHE_MASK  (BDRV_O_NOCACHE | BDRV_O_CACHE_WB)
 
diff --git a/qemu-config.c b/qemu-config.c
index d500885..bf3d493 100644
--- a/qemu-config.c
+++ b/qemu-config.c
@@ -54,7 +54,7 @@ QemuOptsList qemu_drive_opts = {
         },{
             .name = "cache",
             .type = QEMU_OPT_STRING,
-            .help = "host cache usage (none, writeback, writethrough)",
+            .help = "host cache usage (none, writeback, writethrough, volatile)",
         },{
             .name = "aio",
             .type = QEMU_OPT_STRING,
diff --git a/qemu-options.hx b/qemu-options.hx
index 12f6b51..6dedb4a 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -118,8 +118,9 @@ ETEXI
 DEF("drive", HAS_ARG, QEMU_OPTION_drive,
     "-drive [file=file][,if=type][,bus=n][,unit=m][,media=d][,index=i]\n"
     "       [,cyls=c,heads=h,secs=s[,trans=t]][,snapshot=on|off]\n"
-    "       [,cache=writethrough|writeback|none][,format=f][,serial=s]\n"
-    "       [,addr=A][,id=name][,aio=threads|native][,readonly=on|off]\n"
+    "       [,cache=writethrough|writeback|volatile|none][,format=f]\n"
+    "       [,serial=s][,addr=A][,id=name][,aio=threads|native]\n"
+    "       [,readonly=on|off]\n"
     "                use 'file' as a drive image\n", QEMU_ARCH_ALL)
 STEXI
 @item -drive @var{option}[,@var{option}[,@var{option}[,...]]]
@@ -148,7 +149,7 @@ 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 "none", "writeback", or "writethrough" and controls how the host cache is used to access block data.
+@var{cache} is "none", "writeback", "volatile", or "writethrough" and controls how the host cache is used to access block data.
 @item aio=@var{aio}
 @var{aio} is "threads", or "native" and selects between pthread based disk I/O and native Linux AIO.
 @item format=@var{format}
@@ -180,6 +181,12 @@ Some block drivers perform badly with @option{cache=writethrough}, most notably,
 qcow2.  If performance is more important than correctness,
 @option{cache=writeback} should be used with qcow2.
 
+In case you don't care about data integrity over host failures, use
+cache=volatile. This option tells qemu that it never needs to write any data
+to the disk but can instead keeps things in cache. If anything goes wrong,
+like your host losing power, the disk storage getting disconnected accidently,
+etc. you're image will most probably be rendered unusable.
+
 Instead of @option{-cdrom} you can use:
 @example
 qemu -drive file=file,index=2,media=cdrom
diff --git a/vl.c b/vl.c
index 85bcc84..c8abce6 100644
--- a/vl.c
+++ b/vl.c
@@ -913,6 +913,9 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque,
             bdrv_flags |= BDRV_O_NOCACHE;
         } else if (!strcmp(buf, "writeback")) {
             bdrv_flags |= BDRV_O_CACHE_WB;
+        } else if (!strcmp(buf, "volatile")) {
+            bdrv_flags |= BDRV_O_CACHE_WB;
+            bdrv_flags |= BDRV_O_NO_FLUSH;
         } else if (!strcmp(buf, "writethrough")) {
             /* this is the default */
         } else {
-- 
1.6.0.2