[PATCH v7 10/10] qcow2: Forward ZERO_WRITE flag for full preallocation

[Kevin Wolf <kwolf@redhat.com>]

The BDRV_REQ_ZERO_WRITE is currently implemented in a way that first the
image is possibly preallocated and then the zero flag is added to all
clusters. This means that a copy-on-write operation may be needed when
writing to these clusters, despite having used preallocation, negating
one of the major benefits of preallocation.

Instead, try to forward the BDRV_REQ_ZERO_WRITE to the protocol driver,
and if the protocol driver can ensure that the new area reads as zeros,
we can skip setting the zero flag in the qcow2 layer.

Unfortunately, the same approach doesn't work for metadata
preallocation, so we'll still set the zero flag there.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2.c              | 22 +++++++++++++++++++---
 tests/qemu-iotests/274.out |  4 ++--
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 98065d7808..2ba0b17c39 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -4170,9 +4170,25 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
         /* Allocate the data area */
         new_file_size = allocation_start +
                         nb_new_data_clusters * s->cluster_size;
-        /* Image file grows, so @exact does not matter */
-        ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc, 0,
-                               errp);
+        /*
+         * Image file grows, so @exact does not matter.
+         *
+         * If we need to zero out the new area, try first whether the protocol
+         * driver can already take care of this.
+         */
+        if (flags & BDRV_REQ_ZERO_WRITE) {
+            ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc,
+                                   BDRV_REQ_ZERO_WRITE, NULL);
+            if (ret >= 0) {
+                flags &= ~BDRV_REQ_ZERO_WRITE;
+            }
+        } else {
+            ret = -1;
+        }
+        if (ret < 0) {
+            ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc, 0,
+                                   errp);
+        }
         if (ret < 0) {
             error_prepend(errp, "Failed to resize underlying file: ");
             qcow2_free_clusters(bs, allocation_start,
diff --git a/tests/qemu-iotests/274.out b/tests/qemu-iotests/274.out
index 1a796fd07c..9d6fdeb1f7 100644
--- a/tests/qemu-iotests/274.out
+++ b/tests/qemu-iotests/274.out
@@ -187,7 +187,7 @@ read 65536/65536 bytes at offset 9437184
 10 MiB (0xa00000) bytes     allocated at offset 5 MiB (0x500000)
 
 [{ "start": 0, "length": 5242880, "depth": 1, "zero": true, "data": false},
-{ "start": 5242880, "length": 10485760, "depth": 0, "zero": true, "data": false, "offset": 327680}]
+{ "start": 5242880, "length": 10485760, "depth": 0, "zero": false, "data": true, "offset": 327680}]
 
 === preallocation=full ===
 Formatting 'TEST_DIR/PID-base', fmt=qcow2 size=16777216 cluster_size=65536 lazy_refcounts=off refcount_bits=16
@@ -206,7 +206,7 @@ read 65536/65536 bytes at offset 11534336
 4 MiB (0x400000) bytes     allocated at offset 8 MiB (0x800000)
 
 [{ "start": 0, "length": 8388608, "depth": 1, "zero": true, "data": false},
-{ "start": 8388608, "length": 4194304, "depth": 0, "zero": true, "data": false, "offset": 327680}]
+{ "start": 8388608, "length": 4194304, "depth": 0, "zero": false, "data": true, "offset": 327680}]
 
 === preallocation=off ===
 Formatting 'TEST_DIR/PID-base', fmt=qcow2 size=393216 cluster_size=65536 lazy_refcounts=off refcount_bits=16
-- 
2.25.3