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Wong" , Dave Chinner Cc: brauner@kernel.org, cem@kernel.org, dchinner@redhat.com, hch@lst.de, ritesh.list@gmail.com, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, martin.petersen@oracle.com References: <20241204154344.3034362-1-john.g.garry@oracle.com> <20241204154344.3034362-2-john.g.garry@oracle.com> <3ab6000e-030d-435a-88c3-9026171ae9f1@oracle.com> <20241212013433.GC6678@frogsfrogsfrogs> <20250114235726.GA3566461@frogsfrogsfrogs> Content-Language: en-US From: John Garry Organization: Oracle Corporation In-Reply-To: <20250114235726.GA3566461@frogsfrogsfrogs> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: LO2P123CA0075.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:138::8) To DM6PR10MB4313.namprd10.prod.outlook.com (2603:10b6:5:212::20) Precedence: bulk X-Mailing-List: linux-xfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM6PR10MB4313:EE_|SJ2PR10MB7787:EE_ X-MS-Office365-Filtering-Correlation-Id: 1468e16f-156e-4a4b-d9ef-08dd36e16c4f X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|366016; 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Wong wrote: >> i.e. RWF_ATOMIC as implemented by a COW capable filesystem should >> always be able to succeed regardless of IO alignment. In these >> situations, the REQ_ATOMIC block layer offload to the hardware is a >> fast path that is enabled when the user IO and filesystem extent >> alignment matches the constraints needed to do a hardware atomic >> write. >> >> In all other cases, we implement RWF_ATOMIC something like >> always-cow or prealloc-beyond-eof-then-xchg-range-on-io-completion >> for anything that doesn't correctly align to hardware REQ_ATOMIC. >> >> That said, there is nothing that prevents us from first implementing >> RWF_ATOMIC constraints as "must match hardware requirements exactly" >> and then relaxing them to be less stringent as filesystems >> implementations improve. We've relaxed the direct IO hardware >> alignment constraints multiple times over the years, so there's >> nothing that really prevents us from doing so with RWF_ATOMIC, >> either. Especially as we have statx to tell the application exactly >> what alignment will get fast hardware offloads... > Ok, let's do that then. Just to be clear -- for any RWF_ATOMIC direct > write that's correctly aligned and targets a single mapping in the > correct state, we can build the untorn bio and submit it. For > everything else, prealloc some post EOF blocks, write them there, and > exchange-range them. I have some doubt about this, but I may be misunderstanding the concept: So is there any guarantee that what we write into is aligned (after the exchange-range routine)? If not, surely every subsequent write with RWF_ATOMIC to that logical range will require this exchange-range routine until we get something aligned (and correct granularity) - correct? I know that getting unaligned blocks continuously is unlikely, unless a heavily fragmented disk. However, databases prefer guaranteed performance (which HW offload gives). We can use extszhint to hint at granularity, but that does not help with alignment (AFAIK). > > Tricky questions: How do we avoid collisions between overlapping writes? > I guess we find a free file range at the top of the file that is long > enough to stage the write, and put it there? And purge it later? > > Also, does this imply that the maximum file size is less than the usual > 8EB? > > (There's also the question about how to do this with buffered writes, > but I guess we could skip that for now.)