I'm guessing that I'm limited by CPU on this IBM JS21 blade's LPAR (MPV, 2 cores max) with an SVC (virtualized SAN) backend. topas showed kernel mode CPU >95% for most of the tests. /oracle $ time dd if=S.dbf of=/dev/zero bs=1024k 16384+1 records in 16384+1 records out real 1m3.044s user 0m0.271s sys 0m42.458s /oracle $ time dd if=S.dbf of=/dev/zero bs=1024k 16384+1 records in 16384+1 records out real 1m2.923s user 0m0.249s sys 0m42.325s t/oracle $ time dd if=S.dbf of=/dev/zero bs=128k 131072+1 records in 131072+1 records out real 1m2.789s user 0m0.709s sys 0m41.665s /oracle $ time dd if=S.dbf of=/dev/zero bs=128k 131072+1 records in 131072+1 records out real 1m2.497s user 0m0.688s sys 0m41.422s /oracle $ time dd if=S.dbf of=/dev/zero bs=8192k 2048+1 records in 2048+1 records out real 1m2.601s user 0m0.106s sys 0m41.666s /oracle $ time dd if=S.dbf of=/dev/zero bs=8192k 2048+1 records in 2048+1 records out real 1m2.628s user 0m0.099s sys 0m41.621s Rich ________________________________ From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Kevin Closson Sent: Monday, December 11, 2006 3:19 PM To: oracle-l Subject: RE: I/O and db_file_multiblock_read_count OK, we should all throw our dd(1) microbenchmark results out there... This is a DL-585 with 2Gb FCP to a PolyServe CFS monted in direct I/O mode. The single LUN is RAID 1+0 st_width 1MB striped across 65 15K RPM drives (hey, I get to play with nice toys...) The file is 16GB $ time dd if=f1 of=/dev/zero bs=1024k 16384+0 records in 16384+0 records out real 1m47.220s user 0m0.009s sys 0m5.175s $ time dd if=f1 of=/dev/zero bs=128k 131072+0 records in 131072+0 records out real 2m52.157s user 0m0.056s sys 0m7.126s For grins I through in huge I/O sizes (yes this is acutally issuing 8MB blocking reads) $ $ time dd if=f1 of=/dev/zero bs=8192k 2048+0 records in 2048+0 records out real 1m32.710s user 0m0.002s sys 0m3.984s Large I/Os get chopped up in the scsi midlayer of Linux, but like what is happening if you get less tput with larger I/Os is you have few drives and a stripe with that is causing each disk to be hit more than once for every I/O (that is bad).