[Qemu-devel] [RFC PATCH 07/17] block: make high level discard operation always zero

[Paolo Bonzini]

While the formats/protocols are free to implement a discard operation
that does *not* zero the data, providing a common view to the guests is
the only way to avoid configuration and migration nightmares.  (We don't
want the admin to manually set up discard_zeroes_data/discard_granularity!)

This has a couple of drawbacks:

1) QEMU effectively will not try to use discard unless discard_zeroes_data
is true.  To do this we could add a flag such as BDRV_ALWAYS_DISCARD,
which the device models could use if their discard_zeroes_data property is
set to false.  However, I haven't done this, estimating that no strictly
positive integer could represent the number of people using it.

2) it may turn a thin-provisioning operation into a full preallocation,
which is not nice.

3) it may cost memory for a bounce buffer that only needs to be filled
with zeroes.

While (3) can be worked around, the only way around the other two,
unfortunately, is support in the formats and protocols.  We will still
provide device options to opt out of this, but with raw and qed covered
(+ qcow2 without backing file, and qcow3 in the future) it should not
be too bad.

Signed-off-by: Paolo Bonzini <address@hidden>
---
 block.c |   14 +++++++++++---
 1 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/block.c b/block.c
index 34db914..30b137f 100644
--- a/block.c
+++ b/block.c
@@ -74,6 +74,8 @@ static int coroutine_fn bdrv_co_do_writev(BlockDriverState 
*bs,
     BdrvRequestFlags flags);
 static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
     int64_t sector_num, int nb_sectors, QEMUIOVector *qiov);
+static int coroutine_fn bdrv_co_do_discard(BlockDriverState *bs,
+    int64_t sector_num, int nb_sectors);
 static BlockDriverAIOCB *bdrv_co_aio_rw_vector(BlockDriverState *bs,
                                                int64_t sector_num,
                                                QEMUIOVector *qiov,
@@ -1797,7 +1799,7 @@ static int coroutine_fn 
bdrv_co_do_write_zeroes(BlockDriverState *bs,
         bdi.discard_granularity &&
         (sector_num & (bdi.discard_granularity - 1)) == 0 &&
         (nb_sectors & (bdi.discard_granularity - 1)) == 0) {
-        return bdrv_co_discard(bs, sector_num, nb_sectors);
+        return bdrv_co_do_discard(bs, sector_num, nb_sectors);
     }
 
     if (qiov) {
@@ -3621,8 +3623,8 @@ static void coroutine_fn bdrv_discard_co_entry(void 
*opaque)
     rwco->ret = bdrv_co_discard(rwco->bs, rwco->sector_num, rwco->nb_sectors);
 }
 
-int coroutine_fn bdrv_co_discard(BlockDriverState *bs, int64_t sector_num,
-                                 int nb_sectors)
+static int coroutine_fn bdrv_co_do_discard(BlockDriverState *bs,
+                                           int64_t sector_num, int nb_sectors)
 {
     if (!bs->drv) {
         return -ENOMEDIUM;
@@ -3651,6 +3653,12 @@ int coroutine_fn bdrv_co_discard(BlockDriverState *bs, 
int64_t sector_num,
     }
 }
 
+int coroutine_fn bdrv_co_discard(BlockDriverState *bs,
+    int64_t sector_num, int nb_sectors)
+{
+    return bdrv_co_do_write_zeroes(bs, sector_num, nb_sectors, NULL);
+}
+
 int bdrv_discard(BlockDriverState *bs, int64_t sector_num, int nb_sectors)
 {
     Coroutine *co;
-- 
1.7.7.6