Hey, On Mon, Apr 08, 2013 at 07:19:19AM +0300, Tanya Brokhman wrote: > completed after the FLUSH. But perhaps there is a correct way to > move the implementation of "ordering around the flush" to the block > layer? Not that it would work better, it just feels that logically - > block layer is the place to do it at. > What do you think? It used to be implemented that way - REQ_BARRIER. The problem was that filesystems wanted multiple dependency streams - ie. fdatasync() of a file doesn't need to drain all other IOs in progress. We could do coloring of IOs - ie. give IOs which may need flushing later but belong to different dependency streams different colors and let block layer figure out partial drainining, which would work but at the same time be pretty nasty and complex. The thing was that most filesystems were already drainings IOs and didn't need any ordering guarantee from block layer. hch took care of the outliers which were depending on REQ_BARRIER ordering guarantees and we just stripped the ordering mechanism which immediately improved performance noticeably in certain workloads. It was done that way mostly out of convenience at that time but now I think of it it's the correct thing to do. It just is too much information to communicate downwards and the extra communication overhead doesn't really buy anything. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
