Re: [PATCH] qemu-img: Add --target-is-zero to convert

[David Edmondson]

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.
-- 
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