4 ms·
> 15 km/h The tape stores 580TB for a length of 1255m, does that mean the read speed is 580TB/1255m*15km/h = 1.9TB/s ? Seems too high.
by ajnin 6y ago
> 15 km/h
The tape stores 580TB for a length of 1255m, does that mean the read speed is 580TB/1255m*15km/h = 1.9TB/s ? Seems too high.
- jeremyjh 6y agoI'm guessing that is the fastest it can traverse the tape, it didn't say it could read at that speed.
- LeifCarrotson 6y ago> When tape is being read it is streamed over the head at a speed of about 15 km/h... It did say that. I wonder if there's some multi-track/double-layer/double-sided stuff going on that needs multiple passes to read, that does sound awful fast!
- texse 6y agoMost recent tape standards have multiple "bands" and "wraps" placed in parallel. The head reads only one wrap at a time, so it takes many passes to read the whole tape. For example an LTO-8 tape has 52 wraps each within 4 bands, requiring 208 passes to read completely.
- TedDoesntTalk 6y agoWhy not just have multiple heads then?
- nine_k 6y agoPositioning the tape should become harder, I suppose. Also, I suppose they limit the read bandwidth with something like what a single Infiniband connection would support; few disk arrays support much higher speeds.
- paulmd 6y agoIt would be like downloading a file using GetRight back in the day, where you'd have one thread downloading the 0-25% chunk, one downloading the 25-50% chunk, and so on. You could hypothetically do that, for sure, but the software basically is derived from the tape era where you'd just have one logical stream coming out of the tape.
- Dylan16807 6y agoThere are. The tradeoff of speed vs. complexity is currently sitting at 32 heads.
- bastawhiz 6y agoTapes are often structured in bands, and those bands are divided into wraps, and those wraps are divided into tracks. There are many tracks on a tape, and they snake back and forth from end to end on the wrap (so you don't need to "rewind" when you get to the end of the tape—you just start reading back in the other direction). In newer tape drives, you physically can't read all of the data at once because the tape head is only a fraction of the width of a single band: it physically moves (laterally) to position itself over the right data.
- Dylan16807 6y agoSince I had to look up more info to understand this explanation, I'll try to give my own, using the numbers for LTO-8. The drive has 32 heads, and reads/writes 32 tracks at a time. It goes from the start of the tape to the end, then aligns with the next 32 tracks and reverses direction. Each group of 32 tracks is called a wrap. There are 208 wraps, so 6656 total data tracks. Even wraps go one direction, and odd wraps go the other direction. That's the important part. But also the tape is divided into 4 "bands", each one holding a quarter of the wraps/tracks. Between the bands, and at the edges of the tape, are special servo tracks that are used for alignment. So when a source talks about "wraps per band", it's pointless abstraction. Unless you're really in the weeds, the only thing you want to know is the total number of wraps.
- jeffbee 6y agoThat is too high but tape really does have surprisingly high transfer rates. You need a real computer to keep them streaming.
- nine_k 6y agoWow, it's like a segment of a floppy disk of infinite radius!
- paulmd 6y agoYeah I've been wondering about the numbers too. The numbers they cite for "previous generations" don't match up to the progression of the LTO tape series' capacity. Maybe they are citing "research numbers" that they can do in a lab but aren't production-ready yet. I would certainly assume they are citing "compressed" data figured there. Also bear in mind that tapes typically store data striped across the tape in multiple tracks and multiple bands. There are four bands per tape and 12-52 wraps per band, so reading the whole tape requires up to 208 passes across the tape. https://en.wikipedia.org/wiki/Linear_Tape-Open#Physical_structure https://en.wikipedia.org/wiki/Linear_Tape-Open#Physical_stru... But yes, to agree with another parallel comment, tape data rates are quite high sequentially (abysmal in random of course, but that's not how they're used). LTO-8 does 750 megabytes per second compressed / 360 megabytes per second raw.