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Either it fits in the back of that request, or it fits at the front. That is called either a back merge candidate or a front merge candidate. Due to the way files are typically laid out, back merges are much more common than front merges. For some work loads, you may even know that it is a waste of time to spend any time attempting to front merge requests. Setting front_merges to 0 disables this functionality. Front merges may still occur due to the cached last_merge hint, but since that comes at basically 0 cost we leave that on. We simply disable the rbtree front sector lookup when the io scheduler merge function is called.”h]”hXõSometimes it happens that a request enters the io scheduler that is contiguous with a request that is already on the queue. Either it fits in the back of that request, or it fits at the front. That is called either a back merge candidate or a front merge candidate. 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