[Info-vax] TK50 - this is annoying...

[Johnny Billquist]

On 2012-10-19 21:27, glen herrmannsfeldt wrote:
> Johnny Billquist <bqt at softjar.se> wrote:
>
> (snip)
>
> I should probably remember, but I don't. Are DECtapes factory formatted
> and can't be low-level formatted by the user?

You can reformat them, but you do not want to have any other format than 
consecutive block numbers on tape. It's important so that seeks will work...

> Or maybe only the timing track?

No, you rewrite everything when you format, including the timing track. 
Actually, the timing track is the most important thing, since all other 
data on the tape are read based on the timing track.
That's why you can also reformat the tape to hold more data by just 
breaking the tape a little while formatting, so that the clock track 
becomes more compressed.

> Many tapes now have a servo track that is factory written
> (LTO, for one) and so can't be bulk erased and reused.

Good point, but no, DECtapes can be reformatted by anyone.

>> Right. In which case, if we want to read blocks 0,1,2, we actually need
>> to read physical block 0,2,4, which means you can't set up the
>> controller to read multiple blocks, but instead have to issue an
>> individual read for each block. It then also becomes a seek between each
>> read, and then you need to make sure that there isn't a stop-and-go
>> effect between the read of one block, and the read of the next block.
>> Not to mention that you still need to know in which direction to read
>> the blocks, so that you don't get that wrong. :-)
>
> If it can be reformatted, then you can write the new block numbers
> into the block headers. Otherwise, yes, you need to map to the
> physical block numbers.

You need to have a mapping.

	Johnny