On Sun, May 13, 2012 at 4:28 PM, walt <w41...@gmail.com> wrote: > I have a usb3 docking station which is showing some behavior I don't > understand: > > #hdparm -t /dev/sdc > > /dev/sdc: > Timing buffered disk reads: 266 MB in 3.00 seconds = 88.59 MB/sec > Timing buffered disk reads: 268 MB in 3.01 seconds = 89.05 MB/sec > Timing buffered disk reads: 266 MB in 3.01 seconds = 88.43 MB/sec > Timing buffered disk reads: 266 MB in 3.02 seconds = 88.10 MB/sec > Timing buffered disk reads: 306 MB in 3.01 seconds = 101.72 MB/sec > Timing buffered disk reads: 266 MB in 3.00 seconds = 88.59 MB/sec > Timing buffered disk reads: 306 MB in 3.00 seconds = 101.84 MB/sec > Timing buffered disk reads: 306 MB in 3.00 seconds = 101.86 MB/sec > > That's all the same disk, repeating hdparm as fast as I could. The > disk was not even mounted at the time, and no other disks were active. > > Two very different but reproducible numbers, changing values at random > times. The only thing I can think of is that the disk may be doing > a SMART self-test, but for some reason the USB connection prevents > me from accessing the data so I can't test my theory. > > Any other ideas?
bonnie++? My first guess is that something on the same USB bus might be periodically active, changing how the kernel manages talking to USB devices. Try ensuring no other USB devices are connected (or active), and running something like "hdparm -t /dev/sdc; hdparm -t /dev/sdc; hdparm -t /dev/sdc; hdparm -t /dev/sdc; hdparm -t /dev/sdc; hdparm -t /dev/sdc" ...letting the shell automate it for you. My second guess would be something relating to the kernel's page cache...but I'm unsure if a filesystem needs to be mounted first. My third guess would be some internal buffering behavior inside the USB disk, and hdparm's behavior happens to straddle an edge condition. -- :wq
