qemu-img: Add --target-is-zero to convert
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Message ID 20200117103434.1363985-1-david.edmondson@oracle.com
State New
Headers show
Series
  • qemu-img: Add --target-is-zero to convert
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Commit Message

David Edmondson Jan. 17, 2020, 10:34 a.m. UTC
In many cases the target of a convert operation is a newly provisioned
target that the user knows is blank (filled with zeroes). In this
situation there is no requirement for qemu-img to wastefully zero out
the entire device.

Add a new option, --target-is-zero, allowing the user to indicate that
an existing target device is already zero filled.
---
 qemu-img.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

Comments

no-reply@patchew.org Jan. 17, 2020, 7:44 p.m. UTC | #1
Patchew URL: https://patchew.org/QEMU/20200117103434.1363985-1-david.edmondson@oracle.com/



Hi,

This series seems to have some coding style problems. See output below for
more information:

Message-id: 20200117103434.1363985-1-david.edmondson@oracle.com
Type: series
Subject: [PATCH] qemu-img: Add --target-is-zero to convert

=== TEST SCRIPT BEGIN ===
#!/bin/bash
git rev-parse base > /dev/null || exit 0
git config --local diff.renamelimit 0
git config --local diff.renames True
git config --local diff.algorithm histogram
./scripts/checkpatch.pl --mailback base..
=== TEST SCRIPT END ===

From https://github.com/patchew-project/qemu
 * [new tag]         patchew/20200117103434.1363985-1-david.edmondson@oracle.com -> patchew/20200117103434.1363985-1-david.edmondson@oracle.com
Switched to a new branch 'test'
71f2a28 qemu-img: Add --target-is-zero to convert

=== OUTPUT BEGIN ===
ERROR: Missing Signed-off-by: line(s)

total: 1 errors, 0 warnings, 62 lines checked

Commit 71f2a28bebbe (qemu-img: Add --target-is-zero to convert) has style problems, please review.  If any of these errors
are false positives report them to the maintainer, see
CHECKPATCH in MAINTAINERS.
=== OUTPUT END ===

Test command exited with code: 1


The full log is available at
http://patchew.org/logs/20200117103434.1363985-1-david.edmondson@oracle.com/testing.checkpatch/?type=message.
---
Email generated automatically by Patchew [https://patchew.org/].
Please send your feedback to patchew-devel@redhat.com
John Snow Jan. 21, 2020, 12:33 a.m. UTC | #2
CC qemu-block and block maintainers

On 1/17/20 5:34 AM, David Edmondson wrote:
> In many cases the target of a convert operation is a newly provisioned
> target that the user knows is blank (filled with zeroes). In this
> situation there is no requirement for qemu-img to wastefully zero out
> the entire device.
> 

Is there no way to convince bdrv_has_zero_init to return what we want
already in this case? I cannot recall off hand, but wonder if there's an
advanced syntax method of specifying the target image that can set this
flag already.

> Add a new option, --target-is-zero, allowing the user to indicate that
> an existing target device is already zero filled.
> ---
>  qemu-img.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/qemu-img.c b/qemu-img.c
> index 95a24b9762..56ca727e8c 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -70,6 +70,7 @@ enum {
>      OPTION_PREALLOCATION = 265,
>      OPTION_SHRINK = 266,
>      OPTION_SALVAGE = 267,
> +    OPTION_TARGET_IS_ZERO = 268,
>  };
>  
>  typedef enum OutputFormat {
> @@ -1593,6 +1594,7 @@ typedef struct ImgConvertState {
>      bool copy_range;
>      bool salvage;
>      bool quiet;
> +    bool target_is_zero;
>      int min_sparse;
>      int alignment;
>      size_t cluster_sectors;
> @@ -1984,10 +1986,11 @@ static int convert_do_copy(ImgConvertState *s)
>      int64_t sector_num = 0;
>  
>      /* Check whether we have zero initialisation or can get it efficiently */
> -    if (s->target_is_new && s->min_sparse && !s->target_has_backing) {
> +    s->has_zero_init = s->target_is_zero;
> +
> +    if (!s->has_zero_init && s->target_is_new && s->min_sparse &&
> +        !s->target_has_backing) {
>          s->has_zero_init = bdrv_has_zero_init(blk_bs(s->target));
> -    } else {
> -        s->has_zero_init = false;
>      }
>  
>      if (!s->has_zero_init && !s->target_has_backing &&
> @@ -2076,6 +2079,7 @@ static int img_convert(int argc, char **argv)
>          .buf_sectors        = IO_BUF_SIZE / BDRV_SECTOR_SIZE,
>          .wr_in_order        = true,
>          .num_coroutines     = 8,
> +        .target_is_zero     = false,
>      };
>  
>      for(;;) {
> @@ -2086,6 +2090,7 @@ static int img_convert(int argc, char **argv)
>              {"force-share", no_argument, 0, 'U'},
>              {"target-image-opts", no_argument, 0, OPTION_TARGET_IMAGE_OPTS},
>              {"salvage", no_argument, 0, OPTION_SALVAGE},
> +            {"target-is-zero", no_argument, 0, OPTION_TARGET_IS_ZERO},
>              {0, 0, 0, 0}
>          };
>          c = getopt_long(argc, argv, ":hf:O:B:Cco:l:S:pt:T:qnm:WU",
> @@ -2209,6 +2214,9 @@ static int img_convert(int argc, char **argv)
>          case OPTION_TARGET_IMAGE_OPTS:
>              tgt_image_opts = true;
>              break;
> +        case OPTION_TARGET_IS_ZERO:
> +            s.target_is_zero = true;
> +            break;
>          }
>      }
>  
> @@ -2247,6 +2255,11 @@ static int img_convert(int argc, char **argv)
>          warn_report("This will become an error in future QEMU versions.");
>      }
>  
> +    if (s.target_is_zero && !skip_create) {
> +        error_report("--target-is-zero requires use of -n flag");
> +        goto fail_getopt;
> +    }
> +
>      s.src_num = argc - optind - 1;
>      out_filename = s.src_num >= 1 ? argv[argc - 1] : NULL;
>  
>
David Edmondson Jan. 21, 2020, 1:06 p.m. UTC | #3
On Monday, 2020-01-20 at 19:33:43 -05, John Snow wrote:

> CC qemu-block and block maintainers
>
> On 1/17/20 5:34 AM, David Edmondson wrote:
>> In many cases the target of a convert operation is a newly provisioned
>> target that the user knows is blank (filled with zeroes). In this
>> situation there is no requirement for qemu-img to wastefully zero out
>> the entire device.
>> 
>
> Is there no way to convince bdrv_has_zero_init to return what we want
> already in this case?

In the current HEAD code, bdrv_has_zero_init will never be called for
“convert -n” (skip target volume creation).

If -n is not specified the host_device block driver doesn't provide a
bdrv_has_zero_init function, so it's assumed not supported.

> I cannot recall off hand, but wonder if there's an advanced syntax
> method of specifying the target image that can set this flag already.

I couldn't see one, but I'd be happy to learn of its existence. My first
approach was to add a raw specific option and add it using
--target-image-opts, resulting in something like:

qemu-img convert -n --target-image-opts sparse.qcow2 \
	driver=raw,file.filename=/dev/sdg,assume-blank=on

(assume-blank=on is the new bit). This worked, but is only useful for
raw targets.

The discussion here led me to switch to --target-is-zero.

Mark Kanda sent me some comments suggesting that I get rid of the new
target_is_zero boolean and simply set has_zero_init, which I will do
before sending out another patch if this overall approach is considered
appropriate.

>> Add a new option, --target-is-zero, allowing the user to indicate that
>> an existing target device is already zero filled.
>> ---
>>  qemu-img.c | 19 ++++++++++++++++---
>>  1 file changed, 16 insertions(+), 3 deletions(-)
>> 
>> diff --git a/qemu-img.c b/qemu-img.c
>> index 95a24b9762..56ca727e8c 100644
>> --- a/qemu-img.c
>> +++ b/qemu-img.c
>> @@ -70,6 +70,7 @@ enum {
>>      OPTION_PREALLOCATION = 265,
>>      OPTION_SHRINK = 266,
>>      OPTION_SALVAGE = 267,
>> +    OPTION_TARGET_IS_ZERO = 268,
>>  };
>>  
>>  typedef enum OutputFormat {
>> @@ -1593,6 +1594,7 @@ typedef struct ImgConvertState {
>>      bool copy_range;
>>      bool salvage;
>>      bool quiet;
>> +    bool target_is_zero;
>>      int min_sparse;
>>      int alignment;
>>      size_t cluster_sectors;
>> @@ -1984,10 +1986,11 @@ static int convert_do_copy(ImgConvertState *s)
>>      int64_t sector_num = 0;
>>  
>>      /* Check whether we have zero initialisation or can get it efficiently */
>> -    if (s->target_is_new && s->min_sparse && !s->target_has_backing) {
>> +    s->has_zero_init = s->target_is_zero;
>> +
>> +    if (!s->has_zero_init && s->target_is_new && s->min_sparse &&
>> +        !s->target_has_backing) {
>>          s->has_zero_init = bdrv_has_zero_init(blk_bs(s->target));
>> -    } else {
>> -        s->has_zero_init = false;
>>      }
>>  
>>      if (!s->has_zero_init && !s->target_has_backing &&
>> @@ -2076,6 +2079,7 @@ static int img_convert(int argc, char **argv)
>>          .buf_sectors        = IO_BUF_SIZE / BDRV_SECTOR_SIZE,
>>          .wr_in_order        = true,
>>          .num_coroutines     = 8,
>> +        .target_is_zero     = false,
>>      };
>>  
>>      for(;;) {
>> @@ -2086,6 +2090,7 @@ static int img_convert(int argc, char **argv)
>>              {"force-share", no_argument, 0, 'U'},
>>              {"target-image-opts", no_argument, 0, OPTION_TARGET_IMAGE_OPTS},
>>              {"salvage", no_argument, 0, OPTION_SALVAGE},
>> +            {"target-is-zero", no_argument, 0, OPTION_TARGET_IS_ZERO},
>>              {0, 0, 0, 0}
>>          };
>>          c = getopt_long(argc, argv, ":hf:O:B:Cco:l:S:pt:T:qnm:WU",
>> @@ -2209,6 +2214,9 @@ static int img_convert(int argc, char **argv)
>>          case OPTION_TARGET_IMAGE_OPTS:
>>              tgt_image_opts = true;
>>              break;
>> +        case OPTION_TARGET_IS_ZERO:
>> +            s.target_is_zero = true;
>> +            break;
>>          }
>>      }
>>  
>> @@ -2247,6 +2255,11 @@ static int img_convert(int argc, char **argv)
>>          warn_report("This will become an error in future QEMU versions.");
>>      }
>>  
>> +    if (s.target_is_zero && !skip_create) {
>> +        error_report("--target-is-zero requires use of -n flag");
>> +        goto fail_getopt;
>> +    }
>> +
>>      s.src_num = argc - optind - 1;
>>      out_filename = s.src_num >= 1 ? argv[argc - 1] : NULL;
>>  
>> 

dme.
Max Reitz Jan. 21, 2020, 3:02 p.m. UTC | #4
Hi,

On 17.01.20 11:34, David Edmondson wrote:
> In many cases the target of a convert operation is a newly provisioned
> target that the user knows is blank (filled with zeroes). In this
> situation there is no requirement for qemu-img to wastefully zero out
> the entire device.
> 
> Add a new option, --target-is-zero, allowing the user to indicate that
> an existing target device is already zero filled.
> ---
>  qemu-img.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/qemu-img.c b/qemu-img.c
> index 95a24b9762..56ca727e8c 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -70,6 +70,7 @@ enum {
>      OPTION_PREALLOCATION = 265,
>      OPTION_SHRINK = 266,
>      OPTION_SALVAGE = 267,
> +    OPTION_TARGET_IS_ZERO = 268,
>  };
>  
>  typedef enum OutputFormat {
> @@ -1593,6 +1594,7 @@ typedef struct ImgConvertState {
>      bool copy_range;
>      bool salvage;
>      bool quiet;
> +    bool target_is_zero;

As you already said, we probably don’t need this and it’d be sufficient
to set the has_zero_init value directly.

>      int min_sparse;
>      int alignment;
>      size_t cluster_sectors;
> @@ -1984,10 +1986,11 @@ static int convert_do_copy(ImgConvertState *s)
>      int64_t sector_num = 0;
>  
>      /* Check whether we have zero initialisation or can get it efficiently */
> -    if (s->target_is_new && s->min_sparse && !s->target_has_backing) {
> +    s->has_zero_init = s->target_is_zero;

We cannot has_zero_init to true if the target has a backing file,
because convert_co_write() asserts that the target must not have a
backing file if has_zero_init is true.  (It’s impossible for a file to
be initialized to zeroes if it has a backing file; or at least it
doesn’t make sense then to have a backing file.)

Case in point:

$ ./qemu-img create -f qcow2 src.qcow2 64M
Formatting 'src.qcow2', fmt=qcow2 size=67108864 cluster_size=65536
lazy_refcounts=off refcount_bits=16
$ ./qemu-img create -f qcow2 backing.qcow2 64M
Formatting 'backing.qcow2', fmt=qcow2 size=67108864 cluster_size=65536
lazy_refcounts=off refcount_bits=16
$ ./qemu-img create -f qcow2 -b backing.qcow2 dst.qcow2 64M

Formatting 'dst.qcow2', fmt=qcow2 size=67108864
backing_file=backing.qcow2 cluster_size=65536 lazy_refcounts=off
refcount_bits=16
$ ./qemu-img convert -n -B backing.qcow2 -f qcow2 -O qcow2
--target-is-zero src.qcow2 dst.qcow2
qemu-img: qemu-img.c:1812: convert_co_write: Assertion
`!s->target_has_backing' failed.
[1]    80813 abort (core dumped)  ./qemu-img convert -n -B backing.qcow2
-f qcow2 -O qcow2 --target-is-zero

> +
> +    if (!s->has_zero_init && s->target_is_new && s->min_sparse &&
> +        !s->target_has_backing) {

(This will be irrelevant after target_has_backing is gone, but because
has_zero_init and target_has_backing are equivalent here, there is no
need to check both.)

>          s->has_zero_init = bdrv_has_zero_init(blk_bs(s->target));
> -    } else {
> -        s->has_zero_init = false;
>      }
>  
>      if (!s->has_zero_init && !s->target_has_backing &&
> @@ -2076,6 +2079,7 @@ static int img_convert(int argc, char **argv)
>          .buf_sectors        = IO_BUF_SIZE / BDRV_SECTOR_SIZE,
>          .wr_in_order        = true,
>          .num_coroutines     = 8,
> +        .target_is_zero     = false,
>      };
>  
>      for(;;) {
> @@ -2086,6 +2090,7 @@ static int img_convert(int argc, char **argv)
>              {"force-share", no_argument, 0, 'U'},
>              {"target-image-opts", no_argument, 0, OPTION_TARGET_IMAGE_OPTS},
>              {"salvage", no_argument, 0, OPTION_SALVAGE},
> +            {"target-is-zero", no_argument, 0, OPTION_TARGET_IS_ZERO},
>              {0, 0, 0, 0}
>          };
>          c = getopt_long(argc, argv, ":hf:O:B:Cco:l:S:pt:T:qnm:WU",
> @@ -2209,6 +2214,9 @@ static int img_convert(int argc, char **argv)
>          case OPTION_TARGET_IMAGE_OPTS:
>              tgt_image_opts = true;
>              break;
> +        case OPTION_TARGET_IS_ZERO:
> +            s.target_is_zero = true;
> +            break;
>          }
>      }
>  
> @@ -2247,6 +2255,11 @@ static int img_convert(int argc, char **argv)
>          warn_report("This will become an error in future QEMU versions.");
>      }
>  
> +    if (s.target_is_zero && !skip_create) {
> +        error_report("--target-is-zero requires use of -n flag");

Hm, I could imagine it being useful even without -n, but maybe it’s
safer to forbid this case for now and reconsider if someone were to ask.

> +        goto fail_getopt;
> +    }
> +
>      s.src_num = argc - optind - 1;
>      out_filename = s.src_num >= 1 ? argv[argc - 1] : NULL;

This patch should also add some documentation for the new option (in
qemu-img-cmds.hx and in qemu-img.texi for the man page).

Max
David Edmondson Jan. 23, 2020, 12:17 p.m. UTC | #5
On Tuesday, 2020-01-21 at 16:02:16 +01, Max Reitz wrote:

> Hi,
>
> On 17.01.20 11:34, David Edmondson wrote:
>> In many cases the target of a convert operation is a newly provisioned
>> target that the user knows is blank (filled with zeroes). In this
>> situation there is no requirement for qemu-img to wastefully zero out
>> the entire device.
>> 
>> Add a new option, --target-is-zero, allowing the user to indicate that
>> an existing target device is already zero filled.
>> ---
>>  qemu-img.c | 19 ++++++++++++++++---
>>  1 file changed, 16 insertions(+), 3 deletions(-)
>> 
>> diff --git a/qemu-img.c b/qemu-img.c
>> index 95a24b9762..56ca727e8c 100644
>> --- a/qemu-img.c
>> +++ b/qemu-img.c
>> @@ -70,6 +70,7 @@ enum {
>>      OPTION_PREALLOCATION = 265,
>>      OPTION_SHRINK = 266,
>>      OPTION_SALVAGE = 267,
>> +    OPTION_TARGET_IS_ZERO = 268,
>>  };
>>  
>>  typedef enum OutputFormat {
>> @@ -1593,6 +1594,7 @@ typedef struct ImgConvertState {
>>      bool copy_range;
>>      bool salvage;
>>      bool quiet;
>> +    bool target_is_zero;
>
> As you already said, we probably don’t need this and it’d be sufficient
> to set the has_zero_init value directly.
>
>>      int min_sparse;
>>      int alignment;
>>      size_t cluster_sectors;
>> @@ -1984,10 +1986,11 @@ static int convert_do_copy(ImgConvertState *s)
>>      int64_t sector_num = 0;
>>  
>>      /* Check whether we have zero initialisation or can get it efficiently */
>> -    if (s->target_is_new && s->min_sparse && !s->target_has_backing) {
>> +    s->has_zero_init = s->target_is_zero;
>
> We cannot has_zero_init to true if the target has a backing file,
> because convert_co_write() asserts that the target must not have a
> backing file if has_zero_init is true.  (It’s impossible for a file to
> be initialized to zeroes if it has a backing file; or at least it
> doesn’t make sense then to have a backing file.)

I'll add a check causing (has_zero_init && target_has_backing) to throw
an error after the target_has_backing is determined.

> Case in point:
>
> $ ./qemu-img create -f qcow2 src.qcow2 64M
> Formatting 'src.qcow2', fmt=qcow2 size=67108864 cluster_size=65536
> lazy_refcounts=off refcount_bits=16
> $ ./qemu-img create -f qcow2 backing.qcow2 64M
> Formatting 'backing.qcow2', fmt=qcow2 size=67108864 cluster_size=65536
> lazy_refcounts=off refcount_bits=16
> $ ./qemu-img create -f qcow2 -b backing.qcow2 dst.qcow2 64M
>
> Formatting 'dst.qcow2', fmt=qcow2 size=67108864
> backing_file=backing.qcow2 cluster_size=65536 lazy_refcounts=off
> refcount_bits=16
> $ ./qemu-img convert -n -B backing.qcow2 -f qcow2 -O qcow2
> --target-is-zero src.qcow2 dst.qcow2
> qemu-img: qemu-img.c:1812: convert_co_write: Assertion
> `!s->target_has_backing' failed.
> [1]    80813 abort (core dumped)  ./qemu-img convert -n -B backing.qcow2
> -f qcow2 -O qcow2 --target-is-zero
>
>> +
>> +    if (!s->has_zero_init && s->target_is_new && s->min_sparse &&
>> +        !s->target_has_backing) {
>
> (This will be irrelevant after target_has_backing is gone, but because
> has_zero_init and target_has_backing are equivalent here, there is no
> need to check both.)

I don't understand this comment - I must be missing something.

If both has_zero_init and target_has_backing are false here, we should
go and check bdrv_has_zero_init().

They can't both be true (when the above mentioned test is added) and if
either is true, we don't want to call brv_has_zero_init(), as either the
user has asserted that the target is blank or we have a backing file.

>>          s->has_zero_init = bdrv_has_zero_init(blk_bs(s->target));
>> -    } else {
>> -        s->has_zero_init = false;
>>      }
>>  
>>      if (!s->has_zero_init && !s->target_has_backing &&
>> @@ -2076,6 +2079,7 @@ static int img_convert(int argc, char **argv)
>>          .buf_sectors        = IO_BUF_SIZE / BDRV_SECTOR_SIZE,
>>          .wr_in_order        = true,
>>          .num_coroutines     = 8,
>> +        .target_is_zero     = false,
>>      };
>>  
>>      for(;;) {
>> @@ -2086,6 +2090,7 @@ static int img_convert(int argc, char **argv)
>>              {"force-share", no_argument, 0, 'U'},
>>              {"target-image-opts", no_argument, 0, OPTION_TARGET_IMAGE_OPTS},
>>              {"salvage", no_argument, 0, OPTION_SALVAGE},
>> +            {"target-is-zero", no_argument, 0, OPTION_TARGET_IS_ZERO},
>>              {0, 0, 0, 0}
>>          };
>>          c = getopt_long(argc, argv, ":hf:O:B:Cco:l:S:pt:T:qnm:WU",
>> @@ -2209,6 +2214,9 @@ static int img_convert(int argc, char **argv)
>>          case OPTION_TARGET_IMAGE_OPTS:
>>              tgt_image_opts = true;
>>              break;
>> +        case OPTION_TARGET_IS_ZERO:
>> +            s.target_is_zero = true;
>> +            break;
>>          }
>>      }
>>  
>> @@ -2247,6 +2255,11 @@ static int img_convert(int argc, char **argv)
>>          warn_report("This will become an error in future QEMU versions.");
>>      }
>>  
>> +    if (s.target_is_zero && !skip_create) {
>> +        error_report("--target-is-zero requires use of -n flag");
>
> Hm, I could imagine it being useful even without -n, but maybe it’s
> safer to forbid this case for now and reconsider if someone were to ask.
>
>> +        goto fail_getopt;
>> +    }
>> +
>>      s.src_num = argc - optind - 1;
>>      out_filename = s.src_num >= 1 ? argv[argc - 1] : NULL;
>
> This patch should also add some documentation for the new option (in
> qemu-img-cmds.hx and in qemu-img.texi for the man page).

Will do.

dme.
Max Reitz Jan. 24, 2020, 10:11 a.m. UTC | #6
On 23.01.20 13:17, David Edmondson wrote:
> On Tuesday, 2020-01-21 at 16:02:16 +01, Max Reitz wrote:
> 
>> Hi,
>>
>> On 17.01.20 11:34, David Edmondson wrote:

[...]

>>> +
>>> +    if (!s->has_zero_init && s->target_is_new && s->min_sparse &&
>>> +        !s->target_has_backing) {
>>
>> (This will be irrelevant after target_has_backing is gone, but because
>> has_zero_init and target_has_backing are equivalent here, there is no
>> need to check both.)
> 
> I don't understand this comment - I must be missing something.

Just the fact that for some reason I read “target_has_backing” as
“target_is_zero”.  Sorry for the false alarm. O:-)

Max

Patch
diff mbox series

diff --git a/qemu-img.c b/qemu-img.c
index 95a24b9762..56ca727e8c 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -70,6 +70,7 @@  enum {
     OPTION_PREALLOCATION = 265,
     OPTION_SHRINK = 266,
     OPTION_SALVAGE = 267,
+    OPTION_TARGET_IS_ZERO = 268,
 };
 
 typedef enum OutputFormat {
@@ -1593,6 +1594,7 @@  typedef struct ImgConvertState {
     bool copy_range;
     bool salvage;
     bool quiet;
+    bool target_is_zero;
     int min_sparse;
     int alignment;
     size_t cluster_sectors;
@@ -1984,10 +1986,11 @@  static int convert_do_copy(ImgConvertState *s)
     int64_t sector_num = 0;
 
     /* Check whether we have zero initialisation or can get it efficiently */
-    if (s->target_is_new && s->min_sparse && !s->target_has_backing) {
+    s->has_zero_init = s->target_is_zero;
+
+    if (!s->has_zero_init && s->target_is_new && s->min_sparse &&
+        !s->target_has_backing) {
         s->has_zero_init = bdrv_has_zero_init(blk_bs(s->target));
-    } else {
-        s->has_zero_init = false;
     }
 
     if (!s->has_zero_init && !s->target_has_backing &&
@@ -2076,6 +2079,7 @@  static int img_convert(int argc, char **argv)
         .buf_sectors        = IO_BUF_SIZE / BDRV_SECTOR_SIZE,
         .wr_in_order        = true,
         .num_coroutines     = 8,
+        .target_is_zero     = false,
     };
 
     for(;;) {
@@ -2086,6 +2090,7 @@  static int img_convert(int argc, char **argv)
             {"force-share", no_argument, 0, 'U'},
             {"target-image-opts", no_argument, 0, OPTION_TARGET_IMAGE_OPTS},
             {"salvage", no_argument, 0, OPTION_SALVAGE},
+            {"target-is-zero", no_argument, 0, OPTION_TARGET_IS_ZERO},
             {0, 0, 0, 0}
         };
         c = getopt_long(argc, argv, ":hf:O:B:Cco:l:S:pt:T:qnm:WU",
@@ -2209,6 +2214,9 @@  static int img_convert(int argc, char **argv)
         case OPTION_TARGET_IMAGE_OPTS:
             tgt_image_opts = true;
             break;
+        case OPTION_TARGET_IS_ZERO:
+            s.target_is_zero = true;
+            break;
         }
     }
 
@@ -2247,6 +2255,11 @@  static int img_convert(int argc, char **argv)
         warn_report("This will become an error in future QEMU versions.");
     }
 
+    if (s.target_is_zero && !skip_create) {
+        error_report("--target-is-zero requires use of -n flag");
+        goto fail_getopt;
+    }
+
     s.src_num = argc - optind - 1;
     out_filename = s.src_num >= 1 ? argv[argc - 1] : NULL;