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.
--
Please forgive me if I act a little strange, for I know not what I do.
