Comment 75 for bug 1208993

This fix my problem. Add deadline i/o sheduler for non rotating disks (in my case usb pen)

sudo nano /etc/udev/rules.d/60-schedulers.rules

and paste these

# set cfq scheduler for rotating disks
ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/rotational}=="1", ATTR{queue/scheduler}="cfq"

# set deadline scheduler for non-rotating disks
ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/rotational}=="0", ATTR{queue/scheduler}="deadline"

before change i has

ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/rotational}=="1", ATTR{queue/scheduler}="cfq"

i don't know why. By default form ubuntu 14.4 io sheduler miust be deadline by default
https://wiki.ubuntu.com/TrustyTahr/ReleaseNotes/14.04

but may be is not :)