From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8A34156E075 for ; Fri, 16 Oct 2026 17:27:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792171630; cv=none; b=B7DkHjTZvwrYkJniwG61vXAXCG5yEB1YgSXdtGSAWRfY5lp1dE8lVS/FBNLPxAiC236YNvZkgvpA4GGnS+Bt0gbAM+9B/FUaiE1UFyl882plXk4TbKu3Nre171Hl8RrsfNvAagXXfDNMLMdWAvIVw+kE1AwCO/BmA4np3S9jPoc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792171630; c=relaxed/simple; bh=M6y25uchHd3aFk6BqQyUpNdCKs6UPwOw3X1Wzcxltv0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Nm8jQfhr81VemWjGIL0CIkN14YL2jIrh3ReQjbHOT+5839+LEGiI1SiioPzfXwsVjUSqisGWia1Dd70HEef1VBXggyaf0UlfwS6dahVrNbc1U77CNrYaF6zFEAFXMu6ifDc5Q/bfMDYL9ydpfT7uKhdF1XCTglzcFE0F5ts1Ud4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=KnyCjP78; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="KnyCjP78" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 315231F000C5; Fri, 16 Oct 2026 17:27:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792171629; bh=KBEGJr9zQZy95V5Zr1A1ig2Oaq2yE9CW2KUEvun7wpA=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=KnyCjP78vlFuxveebZ9MofPRrsYmdol4uKYwfMmoiuiHuM+wbJ8/YceLl6b6b4r1s XP59RMcjHLCO9sACz7vfhn0nAqdRgKm15yCQ1Oakt5D7Zy4w64V5ETAUN/JeNOe0uQ KNMZCR/vVn4+BSVbHFGy3EqlEb+YKal89csVJTirWnfnBDxB/c8cAqX4SUEGigostS ifGzr+0u/3HODTylWhsY4UmgLfx+TWMQr83gjwI5ihsnPNC/2oqlxtyMOos5yYHsnD wrduJ4vdPCS5itMFeKxZvM2xSykFoY4tkVSWMOlI1BIzYAreDsUzXN0WwmK21W8D9C i1Dok6Nf1el1g== Date: Fri, 16 Oct 2026 11:27:07 -0600 From: Keith Busch To: Erwan Velu Cc: linux-nvme@lists.infradead.org, axboe@kernel.dk, hch@lst.de, sagi@grimberg.me, linux-kernel@vger.kernel.org, Erwan Velu Subject: Re: [PATCH] nvme: honor the volatile write cache enable state Message-ID: References: <20261016141208.2641974-1-e.velu@criteo.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Oct 16, 2026 at 07:15:20PM +0200, Erwan Velu wrote: > Le ven. 16 oct. 2026 à 18:57, Keith Busch a écrit : > > > > On Fri, Oct 16, 2026 at 04:12:08PM +0200, Erwan Velu wrote: > > > job | cache | IOPS | fsync avg | flushes | gain > > > ----------------------+---------+--------+-----------+---------+------ > > > psync buffered, QD1 | back | 3885 | 247.6us | 116552 | > > > | through | 48325 | 15.7us | 0 | x12.4 > > > ----------------------+---------+--------+-----------+---------+------ > > > io_uring direct, QD1 | back | 3886 | 238.2us | 116593 | > > > | through | 41924 | 5.7us | 0 | x10.8 > > > ----------------------+---------+--------+-----------+---------+------ > > > io_uring direct, QD32 | back | 5088 | 6173.5us | 117793 | > > > | through | 140196 | 107.0us | 0 | x27.6 > > > > > > From NVMe 1.4c (sections 5.21.1.6 and 6.8) up to the NVMe Base > > > Specification 2.4 (sections 5.2.30.1.4 and 7.2), a Flush command has no > > > effect if the volatile write cache is not enabled. > > > > Yeah, the NVMe Flush operation should be a no-op in the device, so the > > dramatic performance difference is a bit alarming. Surely the block > > stack's flush machinery is not the reason for the difference, right? > > > > If the device's flush handling is this bad when VWC is disabled, what > > happens when you have it enabled? Can you re-run the same test with that > > setting? > > > > I'm not necessarily against what you're proposing here, but your numbers > > are unexpected. > > Maybe my table here isn't probably clear enough. > The numbers shown in the "write back" mode were done with VWC enabled. But you said the table came from tests "with WCE cleared" in the VWC feature. ?