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Re: [Qemu-block] [Qemu-devel] [PATCH v12 1/2] block/backup: fix max_tran
Re: [Qemu-block] [Qemu-devel] [PATCH v12 1/2] block/backup: fix max_transfer handling for copy_range
Thu, 19 Sep 2019 06:50:16 +0000
18.09.2019 22:57, John Snow wrote:
> On 9/17/19 12:07 PM, Vladimir Sementsov-Ogievskiy wrote:
>> Of course, QEMU_ALIGN_UP is a typo, it should be QEMU_ALIGN_DOWN, as we
>> are trying to find aligned size which satisfy both source and target.
>> Also, don't ignore too small max_transfer. In this case seems safer to
>> disable copy_range.
>> Fixes: 9ded4a0114968e
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>> block/backup.c | 12 ++++++++----
>> 1 file changed, 8 insertions(+), 4 deletions(-)
>> diff --git a/block/backup.c b/block/backup.c
>> index 763f0d7ff6..d8fdbfadfe 100644
>> --- a/block/backup.c
>> +++ b/block/backup.c
>> @@ -741,12 +741,16 @@ BlockJob *backup_job_create(const char *job_id,
>> BlockDriverState *bs,
>> job->cluster_size = cluster_size;
>> job->copy_bitmap = copy_bitmap;
>> copy_bitmap = NULL;
>> - job->use_copy_range = !compress; /* compression isn't supported for it
>> job->copy_range_size =
>> - job->copy_range_size = MAX(job->cluster_size,
>> - QEMU_ALIGN_UP(job->copy_range_size,
>> - job->cluster_size));
>> + job->copy_range_size = QEMU_ALIGN_DOWN(job->copy_range_size,
>> + job->cluster_size);
>> + /*
>> + * Compression is not supported for copy_range. Also, we don't want to
>> + * handle small max_transfer for copy_range (which currently don't
>> + * handle max_transfer at all).
>> + */
>> + job->use_copy_range = !compress && job->copy_range_size > 0;
>> /* Required permissions are already taken with target's blk_new() */
>> block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
> I'm clear on the alignment fix, I'm not clear on the comment about
> max_transfer and how it relates to copy_range_size being non-zero.
> "small max transfer" -- what happens when it's zero? we're apparently OK with
> a single cluster, but when it's zero, what happens?
if it zero it means that source or target requires max_transfer less than
cluster_size. It seems not valid to call copy_range in this case.
Still it's OK to use normal read/write, as they handle max_transfer internally
in a loop (copy_range doesn't do it).