bcache: allow quick writeback when backing idle
If the control system would wait for at least half a second, and there's been no reqs hitting the backing disk for awhile: use an alternate mode where we have at most one contiguous set of writebacks in flight at a time. (But don't otherwise delay). If front-end IO appears, it will still be quick, as it will only have to contend with one real operation in flight. But otherwise, we'll be sending data to the backing disk as quickly as it can accept it (with one op at a time). Signed-off-by: NMichael Lyle <mlyle@lyle.org> Reviewed-by: NTang Junhui <tang.junhui@zte.com.cn> Acked-by: NColy Li <colyli@suse.de> Signed-off-by: NJens Axboe <axboe@kernel.dk>
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