one 16K random read I/O issues 2 scsi I/O (16K and 4K) requests in linux
- by hiroyuki
I noticed weird issue when benchmarking random read I/O for files in linux (2.6.18).
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 by systemtap and
I noticed that one 16KB sysread 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.
Please ignore values except for start-sector and size.)
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