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We are going to directly use one async block-copy operation for backup job, so we need rate limiter. We want to maintain current backup behavior: only background copying is limited and copy-before-write operations only participate in limit calculation. Therefore we need one rate limiter for block-copy state and boolean flag for block-copy call state for actual limitation. Note, that we can't just calculate each chunk in limiter after successful copying: it will not save us from starting a lot of async sub-requests which will exceed limit too much. Instead let's use the following scheme on sub-request creation: 1. If at the moment limit is not exceeded, create the request and account it immediately. 2. If at the moment limit is already exceeded, drop create sub-request and handle limit instead (by sleep). With this approach we'll never exceed the limit more than by one sub-request (which pretty much matches current backup behavior). Note also, that if there is in-flight block-copy async call, block_copy_kick() should be used after set-speed to apply new setup faster. For that block_copy_kick() published in this patch. Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Message-Id: <20210116214705.822267-7-vsementsov@virtuozzo.com> Signed-off-by: Max Reitz <mreitz@redhat.com>