linux-ext4 - Re: [RFCv3 7/7] iomap: Optimize data access patterns for filesystems with indirect mappings

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Date: Fri, 26 Apr 2024 21:47:03 -0700
From: Christoph Hellwig <hch@...radead.org>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
Cc: linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
	"Darrick J . Wong" <djwong@...nel.org>,
	Ojaswin Mujoo <ojaswin@...ux.ibm.com>, Jan Kara <jack@...e.cz>
Subject: Re: [RFCv3 7/7] iomap: Optimize data access patterns for filesystems
 with indirect mappings

On Thu, Apr 25, 2024 at 06:58:51PM +0530, Ritesh Harjani (IBM) wrote:
> Currently the bios for reads within iomap are only submitted at
> 2 places -
> 1. If we cannot merge the new req. with previous bio, only then we
>    submit the previous bio.
> 2. Submit the bio at the end of the entire read processing.
> 
> This means for filesystems with indirect block mapping, we call into
> ->iomap_begin() again w/o submitting the previous bios. That causes
> unoptimized data access patterns for blocks which are of BH_Boundary type.

The same is true for extent mappings.  And it's not ideal there either,
although it only inreases the bio submission latency.