Re: [PATCH v3 14/21] iomap: Sub-extent zeroing

[Christoph Hellwig <hch@lst.de>]

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.