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This is so that when we later issue an atomic write, we would not >> have the problem of unwritten extents and how the iomap iterator will >> create multiple BIOs (which is not permitted). >> >> How about an alternate approach like this: >> - no sub-extent zeroing >> - iomap iter is changed to allocate a single BIO for an atomic write in >> first iteration >> - each iomap extent iteration appends data to that same BIO >> - when finished iterating, we submit the BIO >> >> Obviously that will mean many changes to the iomap bio iterator, but is >> quite self-contained. > > Yes, I also suggested that during the zeroing fix discussion. Maybe missed that. I did notice https://lore.kernel.org/linux-xfs/ZmwJuiMHQ8qgkJDS@infradead.org, but got a different impression of your idea there (to this one). > There > is generally no good reason to start a new direct I/O bio if the > write is contiguous on disk and only the state of the srcmap is different. Sure, so we don't need to worry about partially-completed writes, if that was a concern; and it would also mean dropping that unpleasant code change in xfs_iomap_write_unwritten() where the start/count fsb were being rounded out to the extent granule boundary. > This will also be a big win for COW / out of place overwrites. >