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[49.180.91.142]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2909930fd3csm21322745ad.12.2025.10.16.01.23.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Oct 2025 01:23:18 -0700 (PDT) Received: from dave by dread.disaster.area with local (Exim 4.98.2) (envelope-from ) id 1v9JGV-0000000FW7S-1kMH; Thu, 16 Oct 2025 19:23:15 +1100 Date: Thu, 16 Oct 2025 19:23:15 +1100 From: Dave Chinner To: Christoph Hellwig Cc: Christian Brauner , Jan Kara , Carlos Maiolino , Andrew Morton , willy@infradead.org, dlemoal@kernel.org, hans.holmberg@wdc.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org Subject: Re: [PATCH 2/3] writeback: allow the file system to override MIN_WRITEBACK_PAGES Message-ID: References: <20251015062728.60104-1-hch@lst.de> <20251015062728.60104-3-hch@lst.de> <20251016043958.GC29905@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251016043958.GC29905@lst.de> X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 557C314000F X-Stat-Signature: ph4cyawkyhr9n8mk7yt8zqcioztysxcp X-Rspam-User: X-HE-Tag: 1760603000-922 X-HE-Meta: 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 IGck25Gq 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Oct 16, 2025 at 06:39:58AM +0200, Christoph Hellwig wrote: > On Thu, Oct 16, 2025 at 07:49:20AM +1100, Dave Chinner wrote: > > On Wed, Oct 15, 2025 at 03:27:15PM +0900, Christoph Hellwig wrote: > > > The relatively low minimal writeback size of 4MiB leads means that > > > written back inodes on rotational media are switched a lot. Besides > > > introducing additional seeks, this also can lead to extreme file > > > fragmentation on zoned devices when a lot of files are cached relative > > > to the available writeback bandwidth. > > > > > > Add a superblock field that allows the file system to override the > > > default size. > > > > Hmmm - won't changing this for the zoned rtdev also change behaviour > > for writeback on the data device? i.e. upping the minimum for the > > normal data device on XFS will mean writeback bandwidth sharing is a > > lot less "fair" and higher latency when we have a mix of different > > file sizes than it currently is... > > In theory it is. In practice with a zoned file system the main device > is: > > a) typically only used for metadata > b) a fast SSD when not actually on the same device > > So I think these concerns are valid, but not really worth replacing the > simple superblock field with a method to query the value. But I'll write > a comment documenting these assumptions as that is useful for future > readers of the code. That sounds reasonable to me. Eventually we might want to explore per-device BDIs, but for the moment documenting the trade-off being made is good enough. -Dave. -- Dave Chinner david@fromorbit.com