I noticed weird issue when benchmarking random read I/O for files in linux (2.6.18-274). The Benchmarking program is my own program and it simply keeps reading 16KB of a file from a random offset. I traced I/O behavior at system call level and scsi level with systemtap and I noticed that one 16KB pread issues 2 scsi I/Os as following. ============================================= SYSPREAD random(8472) 3, 0x16fc5200, 16384, 128137183232 SCSI random(8472) 0 1 0 0 start-sector: 226321183 size: 4096 bufflen 4096 FROM_DEVICE 1354354008068009 SCSI random(8472) 0 1 0 0 start-sector: 226323431 size: 16384 bufflen 16384 FROM_DEVICE 1354354008075927 SYSPREAD random(8472) 3, 0x16fc5200, 16384, 21807710208 SCSI random(8472) 0 1 0 0 start-sector: 1889888935 size: 4096 bufflen 4096 FROM_DEVICE 1354354008085128 SCSI random(8472) 0 1 0 0 start-sector: 1889891823 size: 16384 bufflen 16384 FROM_DEVICE 1354354008097161 SYSPREAD random(8472) 3, 0x16fc5200, 16384, 139365318656 SCSI random(8472) 0 1 0 0 start-sector: 254092663 size: 4096 bufflen 4096 FROM_DEVICE 1354354008100633 SCSI random(8472) 0 1 0 0 start-sector: 254094879 size: 16384 bufflen 16384 FROM_DEVICE 1354354008111723 SYSPREAD random(8472) 3, 0x16fc5200, 16384, 60304424960 SCSI random(8472) 0 1 0 0 start-sector: 58119807 size: 4096 bufflen 4096 FROM_DEVICE 1354354008120469 SCSI random(8472) 0 1 0 0 start-sector: 58125415 size: 16384 bufflen 16384 FROM_DEVICE 1354354008126343 ============================================= As shown above, one 16KB pread issues 2 scsi I/Os. (I traced scsi io dispatching with probe scsi.iodispatching) One scsi I/O is 16KB I/O as requested from the application and it's OK. The thing is the other 4KB I/O which I don't know why linux issues that I/O. Of course, I/O performance is degraded by the weired 4KB I/O and I am having trouble. I also use fio (famous I/O benchmark tool) and noticed the same issue, so it's not from the application. Does anybody know what is going on ? Any comments or advices are appreciated. Thanks -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
