How often does an LTO tape brake and return for a full write?

0

A return could sound similar to "shoe shining" but it slews down very quick before the turn. How many returns are required to write a full tape for the LTO generations 1 to 6 respectively?
This is the image I have from the LTO system:

BOT |-------------->-------| EOT first write till end, 
brake, move servo of the head to the next track, write in reverse direction
BOT |-----<----------------| EOT write till start is reached and so on...

Jonas Stein

Posted 2017-07-30T17:11:29.330

Reputation: 773

Answers

2

The way I interpret the word "rewind" the answer is "none" - well, maybe one at the end of the whole operation. "Rewind" to me means shuttling the tape from takeup to supply reel without reading or writing anything.

LTO doesn't write to the end of the tape, rewind, and then start again from the beginning of the tape. All LTO versions use "serpentine" recording, meaning that alternate passes record from beginning to end, then end to beginning, etc.

The Wikipedia article on LTO describes this quite well. Look at the table of LTO parameters. The next-to-last row shows the "End-to-end passes required to fill tape". This is simply the number of "data bands per tape" (so far this is four in all versions) multiplied by the number of "wraps per band". For example, LTO-4 needs 4 x 14 = 56 wraps (end-to-end passes) to write the tape's full capacity.

But never does it "rewind" without actually doing any writing... except at the end of the run, when you're at the physical EOT and you want to remove the cartridge.

Jamie Hanrahan

Posted 2017-07-30T17:11:29.330

Reputation: 19 777

Doesn't that mean it writes 4 tracks at once and moves 7 times from start to end and back 2*7=14? – Jonas Stein – 2017-07-30T19:34:52.637

No. Read the article in detail. – Jamie Hanrahan – 2017-07-30T20:34:19.017

For LTO-6 the time to write a full tape at max uncompressed speed is 5.5h for 136 passes end to end, so we should hear every 2.4 minutes a turn over at full speed. – Jonas Stein – 2017-07-30T23:53:34.393

1I have no personal experience with LTO-6, but about two minutes per pass is about right for my ancient LTO-3 drive, assuming it's being written from a system and disks that are otherwise very un-busy. – Jamie Hanrahan – 2017-07-31T01:08:38.253

AFAIK the tape do not rewind even at EOT. The number of passes are even, so the EOT is actually at the start of the tape. – Hontvári Levente – 2017-08-09T19:37:40.267

When you're done writing whatever you want to write, what if you haven't written an even number of complete passes? It most certainly doesn't have to "write nothing" to the end of that pass and then maybe another "dummy" pass to get all the tape back into the cartridge. – Jamie Hanrahan – 2017-08-09T22:02:41.353