Re: [PATCH 3/8] iomap: add a IOMAP_F_ZONE_APPEND flag

[Christoph Hellwig <hch@lst.de>]

On Thu, Dec 12, 2024 at 10:05:47AM -0800, Darrick J. Wong wrote:
> On Wed, Dec 11, 2024 at 09:53:43AM +0100, Christoph Hellwig wrote:
> > This doesn't much - just always returns the start block number for each
> 
> "This doesn't much" - I don't understand the sentence very well.  How
> about:
> 
> "Add a new IOMAP_F_ZONE_APPEND flag for the filesystem to indicate that
> the storage device must inform us where it wrote the data, so that the
> filesystem can update its own internal mapping metadata.  The filesystem
> should set the starting address of the zone in iomap::addr, and extract
> the LBA address from the bio during ioend processing.  iomap builds
> bios unconstrained by the hardware limits and will split them in the bio
> submission handler."

Sounds reasonable.

> The splitting happens whenever IOMAP_F_BOUNDARY gets set by
> ->map_blocks, right?

The splitting happens when:

 (a) we run out of enough space in one zone and have to switch to another
     one.  That then most likely picks an new zone, in which case
     IOMAP_F_BOUNDARY will be set
 (b) if the hardware I/O constraints require splitting the I/O (typically
     because it's too larger)