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Wed, 27 Jan 2021 03:50:09 +0000 To: Changheun Lee Cc: martin.petersen@oracle.com, Damien.LeMoal@wdc.com, arnd@arndb.de, hch@lst.de, jejb@linux.ibm.com, jisoo2146.oh@samsung.com, junho89.kim@samsung.com, linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org, michael.christie@oracle.com, mj0123.lee@samsung.com, oneukum@suse.com, seunghwan.hyun@samsung.com, sookwan7.kim@samsung.com, woosung2.lee@samsung.com, yt0928.kim@samsung.com Subject: Re: [PATCH 1/1] scsi: sd: use max_xfer_blocks for set rw_max if max_xfer_blocks is available From: "Martin K. 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For example, device reports opt_xfer_blocks is 512KB and > 1MB as a max_xfer_blocks too. Currently rw_max is set with 512KB only. Because that's what the device asks for. If a device explicitly requests us to use 512 KB I/Os we should not be sending it 1 MB requests. The spec is very clear. It says that if you send a command *larger* than opt_xfer_blocks, you should expect *slower* performance. That makes max_xfer_blocks a particularly poor choice for setting the default I/O size. In addition to being slower, max_xfer_blocks could potentially also be much, much larger than opt_xfer_blocks. I understand your 512 KB vs. 1 MB example. But if the max_xfer_blocks limit is reported as 1 GB, is that then the right value to use instead of 512 KB? Probably not. If a device does not report an opt_xfer_blocks value that suits your workload, just override the resulting max_sectors_kb in sysfs. This is intentionally a soft limit so it can be adjusted by the user without having to change the kernel. -- Martin K. Petersen Oracle Linux Engineering