Hi Elliott, Thank you for the comments. > 1. All the starting-sector values are unaligned. Are the files on an unaligned partition (e.g. on an MBR disk starting at LBA 63)? That would cause extra accesses. I think the partition is fine. I access the partition itself with DIRECT_IO and the issue never happens. (Requested bytes and scsi drivers dispatching bytes match.) > 2. Files might be fragmented. I re-created filesystem several times and I still get the same issue. (I re-created filesystem and create a dummy file with dd and I access the file as I said.) so, files are not fragmented in that case. I noticed that when I keep reading files randomly, unknown 4KB I/O gradually disappears. (only 16KB I/O is issued.) After I flushed the page cache, I again get 4KB I/O and requested 16KB I/O. (the file is big enough for the DRAM size, so basically most of the I/O does not hit the page cache and goes to scsi driver level.) On Sun, Dec 2, 2012 at 5:18 AM, Elliott, Robert (Server Storage) <Elliott@xxxxxx> wrote: > Two things to consider: > 1. All the starting-sector values are unaligned. Are the files on an unaligned partition (e.g. on an MBR disk starting at LBA 63)? That would cause extra accesses. > > It is important that all I/Os be aligned nowadays, due to: > - SSDs (with 4 KiB or larger page sizes) > - 512e Advanced Format HDDs (with 4 KiB or larger physical sector sizes); > - RAID logical drives (with 16 KiB or larger strip sizes) > > 2. Files might be fragmented. > > > -----Original Message----- > From: linux-scsi-owner@xxxxxxxxxxxxxxx [mailto:linux-scsi-owner@xxxxxxxxxxxxxxx] On Behalf Of Hiroyuki Yamada > Sent: Saturday, December 01, 2012 3:45 AM > To: linux-scsi@xxxxxxxxxxxxxxx > Subject: one 16K random read I/O issues 2 scsi I/O (16K and 4K) requests > > 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 -- 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
