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

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Message-ID: <aIkVHBsC6M5ZHGzQ@casper.infradead.org>
Date: Tue, 29 Jul 2025 19:38:20 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Tony Battersby <tonyb@...ernetics.com>
Cc: Song Liu <song@...nel.org>, Yu Kuai <yukuai3@...wei.com>,
	Christian Brauner <brauner@...nel.org>,
	"Darrick J. Wong" <djwong@...nel.org>, linux-raid@...r.kernel.org,
	linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iomap: align writeback to RAID stripe boundaries

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?