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Petersen" , Nitesh Shetty , Javier Gonzalez , Matthew Wilcox , Keith Busch , Christoph Hellwig , Keith Busch , "linux-block@vger.kernel.org" , "linux-nvme@lists.infradead.org" , "linux-scsi@vger.kernel.org" , "io-uring@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "joshi.k@samsung.com" Subject: Re: [PATCHv10 0/9] write hints with nvme fdp, scsi streams From: "Martin K. 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And, based on my experience implementing support for both types of copy offload in Linux, what made things elegant was treating the operation as a read followed by a write throughout the stack. Exactly like the token-based offload specification describes. > Token-based copy offloading (called ODX by Microsoft) could be > implemented by maintaining a state machine in the SCSI sd driver I suspect the SCSI maintainer would object strongly to the idea of maintaining cross-device copy offload state and associated object lifetime issues in the sd driver. > I'm assuming that the IMMED bit will be set to zero in the WRITE USING > TOKEN command. Otherwise one or more additional RECEIVE ROD TOKEN > INFORMATION commands would be required to poll for the WRITE USING TOKEN > completion status. What would the benefit of making WRITE USING TOKEN be a background operation? That seems like a completely unnecessary complication. > I guess that the block layer maintainer wouldn't be happy if all block > drivers would have to deal with three or four phases for copy > offloading just because ODX is this complicated. Last I looked, EXTENDED COPY consumed something like 70 pages in the spec. Token-based copy is trivially simple and elegant by comparison. -- Martin K. Petersen Oracle Linux Engineering