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Previous patch (raid5: only dispatch IO from raid5d for harddisk raid) defers IO dispatching. The goal is to create better IO pattern. At that time, we don't sort the deffered IO and hope the block layer can do IO merge and sort. Now the raid5-cache writeback could create large amount of bios. And if we enable muti-thread for stripe handling, we can't control when to dispatch IO to raid disks. In a lot of time, we are dispatching IO which block layer can't do merge effectively. This patch moves further for the IO dispatching defer. We accumulate bios, but we don't dispatch all the bios after a threshold is met. This 'dispatch partial portion of bios' stragety allows bios coming in a large time window are sent to disks together. At the dispatching time, there is large chance the block layer can merge the bios. To make this more effective, we dispatch IO in ascending order. This increases request merge chance and reduces disk seek. Signed-off-by: Shaohua Li <shli@fb.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.