There are some optimizations that require knowing if an image starts
out as reading all zeroes, such as making blockdev-mirror faster by
skipping the copying of source zeroes to the destination. The
existing bdrv_co_is_zero_fast() is a good building block for answering
this question, but it tends to give an answer of 0 for a file we just
created via QMP 'blockdev-create' or similar (such as 'qemu-img create
-f raw'). Why? Because file-posix.c insists on allocating a tiny
header to any file rather than leaving it 100% sparse, due to some
filesystems that are unable to answer alignment probes on a hole. But
teaching file-posix.c to read the tiny header doesn't scale - the
problem of a small header is also visible when libvirt sets up an NBD
client to a just-created file on a migration destination host.
So, we need a wrapper function that handles a bit more complexity in a
common manner for all block devices - when the BDS is mostly a hole,
but has a small non-hole header, it is still worth the time to read
that header and check if it reads as all zeroes before giving up and
returning a pessimistic answer.
Signed-off-by: Eric Blake <ebl...@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
---
v3: Use constant 128k as maximum data header size to read [Stefan]
---
include/block/block-io.h | 2 ++
block/io.c | 62 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 64 insertions(+)
diff --git a/include/block/block-io.h b/include/block/block-io.h
index b49e0537dd4..b99cc98d265 100644
--- a/include/block/block-io.h
+++ b/include/block/block-io.h
@@ -161,6 +161,8 @@ bdrv_is_allocated_above(BlockDriverState *bs,
BlockDriverState *base,
int coroutine_fn GRAPH_RDLOCK
bdrv_co_is_zero_fast(BlockDriverState *bs, int64_t offset, int64_t bytes);
+int coroutine_fn GRAPH_RDLOCK
+bdrv_co_is_all_zeroes(BlockDriverState *bs);
int GRAPH_RDLOCK
bdrv_apply_auto_read_only(BlockDriverState *bs, const char *errmsg,
diff --git a/block/io.c b/block/io.c
index 50dc0e193f0..4fd7768f9cd 100644
--- a/block/io.c
+++ b/block/io.c
@@ -38,10 +38,14 @@
#include "qemu/error-report.h"
#include "qemu/main-loop.h"
#include "system/replay.h"
+#include "qemu/units.h"
/* Maximum bounce buffer for copy-on-read and write zeroes, in bytes */
#define MAX_BOUNCE_BUFFER (32768 << BDRV_SECTOR_BITS)
+/* Maximum read size for checking if data reads as zero, in bytes */
+#define MAX_ZERO_CHECK_BUFFER (128 * KiB)
+
static void coroutine_fn GRAPH_RDLOCK
bdrv_parent_cb_resize(BlockDriverState *bs);
@@ -2778,6 +2782,64 @@ int coroutine_fn bdrv_co_is_zero_fast(BlockDriverState
*bs, int64_t offset,
return 1;
}
+/*
+ * Check @bs (and its backing chain) to see if the entire image is known
+ * to read as zeroes.
+ * Return 1 if that is the case, 0 otherwise and -errno on error.
+ * This test is meant to be fast rather than accurate so returning 0
+ * does not guarantee non-zero data; however, a return of 1 is reliable,
+ * and this function can report 1 in more cases than bdrv_co_is_zero_fast.
+ */
+int coroutine_fn bdrv_co_is_all_zeroes(BlockDriverState *bs)
+{
+ int ret;
+ int64_t pnum, bytes;
+ char *buf;
+ QEMUIOVector local_qiov;
+ IO_CODE();
+
+ bytes = bdrv_co_getlength(bs);
+ if (bytes < 0) {
+ return bytes;
+ }
+
+ /* First probe - see if the entire image reads as zero */
+ ret = bdrv_co_common_block_status_above(bs, NULL, false, BDRV_WANT_ZERO,
+ 0, bytes, &pnum, NULL, NULL,
+ NULL);
+ if (ret < 0) {
+ return ret;
+ }
+ if (ret & BDRV_BLOCK_ZERO) {
+ return bdrv_co_is_zero_fast(bs, pnum, bytes - pnum);
+ }
+
+ /*
+ * Because of the way 'blockdev-create' works, raw files tend to
+ * be created with a non-sparse region at the front to make
+ * alignment probing easier. If the block starts with only a
+ * small allocated region, it is still worth the effort to see if
+ * the rest of the image is still sparse, coupled with manually
+ * reading the first region to see if it reads zero after all.
+ */
+ if (pnum > MAX_ZERO_CHECK_BUFFER) {
+ return 0;
+ }
+ ret = bdrv_co_is_zero_fast(bs, pnum, bytes - pnum);
+ if (ret <= 0) {
+ return ret;
+ }
+ /* Only the head of the image is unknown, and it's small. Read it. */
+ buf = qemu_blockalign(bs, pnum);
+ qemu_iovec_init_buf(&local_qiov, buf, pnum);
+ ret = bdrv_driver_preadv(bs, 0, pnum, &local_qiov, 0, 0);
+ if (ret >= 0) {
+ ret = buffer_is_zero(buf, pnum);
+ }
+ qemu_vfree(buf);
+ return ret;
+}
+
int coroutine_fn bdrv_co_is_allocated(BlockDriverState *bs, int64_t offset,
int64_t bytes, int64_t *pnum)
{
--
2.49.0
