On Tue, 6 Dec 2011 20:04:33 -0800 Yucong Sun (叶雨飞) <sunyucong@xxxxxxxxx> wrote: > The problem with using page-flush as a write cache here is that write > to MD don't go through IO scheduler, which is a very big problem, > because when flush thread decide to write to MD, it's impossible to > control the write speed, or prioritize them with read, every requests > basically is a fifo, and when flush size is big, no read can be > served. > I'm not sure I understand.... Requests don't go through an IO scheduler before they hit md, but they do after md sends them on down, so they can be re-ordered there. There was a bug where raid10 would allow an arbitrary number of writes to queue up so that flushing code didn't know when to stop. This was fixed by commit 34db0cd60f8a1f4ab73d118a8be3797c20388223 nearly 2 months ago :-) NeilBrown
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