On Fri, Mar 20, 2020 at 07:40:14AM -0700, Matthew Wilcox wrote: > From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> > > By definition, an extent covers a range of consecutive blocks, so > it would be quite rare to be able to just add pages to the BIO from > a previous range. The only case we can think of is a mapped extent > followed by a hole extent, followed by another mapped extent which has > been allocated immediately after the first extent. We believe this to > be an unlikely layout for a filesystem to choose and, since the queue > is plugged, those two BIOs would be merged by the block layer. > > The reason we care is that ext2/ext4 choose to lay out blocks 0-11 > consecutively, followed by the indirect block, and we want to merge those > two BIOs. If we don't submit the data BIO before asking the filesystem > for the next extent, then the indirect BIO will be submitted first, > and waited for, leading to inefficient I/O patterns. Buffer heads solve > this with the BH_boundary flag, but iomap doesn't need that as long as > we submit the bio here. > > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> Looks good, Reviewed-by: Christoph Hellwig <hch@xxxxxx>
