On Fri, Jul 26, 2024 at 03:29:48PM +0100, John Garry wrote: > I have been considering another approach to solve this problem. > > In this patch - as you know - we zero unwritten parts of a newly allocated > extent. 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. 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. This will also be a big win for COW / out of place overwrites.
