Re: [PATCH 2/2] iomap: align writeback to RAID stripe boundaries

[Matthew Wilcox]

On Tue, Jul 29, 2025 at 12:13:42PM -0400, Tony Battersby wrote:
> Improve writeback performance to RAID-4/5/6 by aligning writes to stripe
> boundaries.  This relies on io_opt being set to the stripe size (or
> a multiple) when BLK_FEAT_RAID_PARTIAL_STRIPES_EXPENSIVE is set.

When you say "aligning writes to stripe boundaries", what you actually
seem to be doing here is sending writes down once we hit a write stripe
boundary, instead of accumulating writes that cross stripe boundaries.
Do I understand correctly?

If so, the performance gain we see here is presumably from the DM/MD
driver not having to split bios that cross boundaries?

Further, wouldn't it be simpler to just put a new condition in
iomap_can_add_to_ioend() rather than turning iomap_add_to_ioend()
into a nested loop?