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).      

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