What are the symbols in VDT's output?

Question

I've been messing with a curses-based disk check tool in Linux called VDT (visual disk test). It puts out a nice little chart on the screen as it's going, but I have no idea what each unit of output on the chart means. I've read the man page several times, but there doesn't seem to be any mention of the visual output.

For instance, right now I have a chart that looks like this during a read test. What do the o's and the Z mean?

┌──vdt5.4.67───────────────────────────────────────────────────────────────────┐
│┌────────────────────────────────────────────────────────────────┐┌──────────┐│
││ooooooZooooooooooooooooooooooooooooooooooooooooooooooooooooooooo││100 % 232G││
││oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo││ 90 % 211G││
││oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo││ 80 % 186G││
││oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo││ 70 % 163G││
││oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo││ 60 % 140G││
││oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo││ 50 % 116G││
││oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo││ 40 %  93G││
││oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo││ 30 %  70G││
││oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo││ 20 %  46G││
││oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo││ 10 %  23G││
││oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo│└──────────┘│
││ooooooooooooooooooooooooooooooooooooooooooooooo.................│┌──────────┐│
││................................................................││avg.  time││
││................................................................││  2.905203││
││....................................                            ││avg. speed││
││                                                                ││     90M/s││
│└────────────────────────────────────────────────────────────────┘└──────────┘│
│┌────────────────────────────────────────────────────────────────┐┌──────────┐│
││</dev/sda                                                       ││read  incr││
││232G{931x256M+139M}                                             ││          ││
│└────────────────────────────────────────────────────────────────┘└──────────┘│
└──────────────────────────────────────────────────────────────────────────────┘

Answer 1

I was able to contact the author of the VDT (alexandre@botao.org) and he explained the output to me.

The o's are regions of the drive which pass, but the Z is a region where VDT wasn't able to read the number of bytes expected (aka, it contains a bad sector). He also said that on write tests, Z could indicate a write-size error instead.