Long test taking ages

Hi to all
I have a server with scheduled SMART background long test every
saturday at 03 AM
This server has only 4 300GB SAS 15K disks.

The smart long test is taking ages and making very high load on server
(iowait more than 30)

In example, currently I have:

Self-test execution status: 0% of test remaining
SMART Self-test log
Num  Test              Status                 segment  LifeTime
LBA_first_err [SK ASC ASQ]
     Description                              number   (hours)
# 1  Background long   Self test in progress ...  80     NOW
      - [-   -    -]


background test is still running from 03 AM. 6 hours and is still running.

I had to cancel the other 3 background test to keep load under
reasonable values.

Some questions:
1) is this normal?
2) AFAIK, smart test should not interfere with server load and should
be "transparent"
3) why 0% remaining and "self test in progress" at the same time?

This doesn't happens on all server but only on a couple of them (same
operating system, same kind of disks)

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