The cache flush can happen in parallel with other I/O. Forcing specific blocks to persistent media can only slow things down, but they have to get there soon in any case. While the drive is executing the cache flush, it can be satisfying other requests whenever it's convenient. "Cache flush" doesn't mean "write only blocks in the cache" or "don't satisfy outstanding reads while you're at it". It will be far more efficient than a full queue drain.
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