So I did some further investigation on the weird optimal i/o size I got from my usb3/sata adapter/ssd, started by grep'ing the size in the /sys/block/sdx/queue: [tom@localhost ~]$ grep 33553920 /sys/block/sdb/queue/* grep: /sys/block/sdb/queue/iosched: Is a directory /sys/block/sdb/queue/optimal_io_size:33553920 /sys/block/sdb/queue/write_same_max_bytes:33553920 Because of this, for once I thought that "optimal_io_size" is somehow set to match with "write_same_max_bytes", but later I proved that I was wrong, and it is just sort of a coincidence. However, I found out that the "33553920" of "write_same_max_bytes" is actually originated from a "max_write_same_blocks": [tom@localhost ~]$ grep 65535 `find /sys/devices -name max_write_same_blocks` /sys/devices/pci0000:00/0000:00:14.0/usb4/4-2/4-2:1.0/host7/target7:0:0/7:0:0:0/scsi_disk/7:0:0:0/max_write_same_blocks:65535 which is set by: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=5db44863b6ebbb400c5e61d56ebe8f21ef48b1bd but not my hardware: [tom@localhost ~]$ sudo sg_inq -p 0xb0 /dev/sdb | grep same Maximum write same length: 0x0 blocks [tom@localhost ~]$ sudo sg_write_same --10 --num=1 /dev/sdb Write same(10): Illegal request, invalid opcode While it is disabled through the variable "no_write_same" for usb-storage (similarly for ATA and FireWire), for some reason, the uas driver neither uses (everything of) "scsiglue" nor set the variable itself. After setting the variable in uas.c, I found that the "optimal i/o size" is still "33553920". So I keep digging. It's actually supplied by the hardware through something called "VPD": http://people.redhat.com/msnitzer/docs/io-limits.txt But when I use `sg_inq` to look into different drives, I see that one of them supplied an (enormous) "OPTIMAL TRANSFER LENGTH" but it's not used to derive the "optimal_io_size" for it: [tom@localhost ~]$ sudo sg_inq -p 0xb0 /dev/sdc | grep "Optimal transfer length:" Optimal transfer length: 8388607 blocks [tom@localhost ~]$ cat /sys/block/sdc/queue/optimal_io_size 0 So I suspect something is "missing" in the uas driver again, and I am right, which is: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=09b6b51b0b6c1b9bb61815baf205e4d74c89ff04 the "skip_vpd_pages" variable makes sd_try_extended_inquiry() in drivers/scsi/sd.c return 0 for the test in sd_revalidate_disk(), which prevent sd_read_block_limits() and hence blk_queue_io_opt() from running. So finally I solve all the mystery I got and I would like to share a bit here. I hope it is alright. Also, should "no_write_same" and "skip_vpd_pages" actually be set for uas devices as well? -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
