Thanks for the help! Vikram On Mon, Jun 1, 2009 at 5:17 AM, Alan D. Brunelle <Alan.Brunelle@xxxxxxxxx> wrote: > Vikram Oberoi wrote: >> >> Hey folks, >> >> I have three specific questions about btt's output. I've searched the >> list, Google, and read the user guide, but I'm still not completely >> sure what the answers are. I'm finally posting my questions here, and >> I hope I'm not in the wrong place or going against etiquette by doing >> so! Please let me know if I am. Here are my questions: >> >> Under "Device Merge Information", are BLKmin/BLKavg/BLKmax the >> min/avg/max I/O size in *number of filesystem blocks* (in my case, 4 >> KB each) being *issued to the device*? > > > "Blocks" are all 512 bytes. > > >> >> Under "Device Seek Information", is the mean seek distance *the >> average number of 512 byte disk sectors* over which the disk head had >> to move before beginning its next IO? Finally, are the median and mode >> also *distances*? If so, I find it hard to believe that my mean seek >> distance is regularly in the tens or hundreds of thousands of disk >> sectors when the mode -- always 0 -- constitutes over 95% of my seek >> distances. Or is there a gap in my understanding here? > > The problem with that field - and it's always bugged me - is that it truly > represents distances from the previous I/O to the next I/O. (It is actually > the closest distance - meaning: if where the previous I/O ends is closer to > where the next one begins we use that, else if where the previous I/O begins > is closer to where the next I/O ends (backwards seek) we use that distance.) > > But remember: with disks that are very large - as most every disk is today - > some seeks can be tremendously large. So a (very) few (very) large seeks can > dwarf lots (and lots) of small seeks. > > The mode field provides a better idea as to what is going on - it will show > that sequential (or nearly sequential) I/Os predominate for a lot of typical > I/O patterns (highly sequential). In my typical FS runs I usually see on the > order of 80-90+ percent of the I/Os being 0 or 8 blocks off (8 being > equivalent to the 4,096 byte FS block). > >> >> Thanks, >> Vikram > -- To unsubscribe from this list: send the line "unsubscribe linux-btrace" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
