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
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