I have never heard of the myth that "HDDs provide random access". As far back as I can remember, HDDs were always the archetypal example for non-random access storage media.
That's funny, but I realize you probably didn't mean it to be.
the old "SAM" (Sequential Access Method) libraries talked to tape, and "DASD" (Direct Access Storage Devices) were what you and I would call a hard drive.
Think 'relative seek time' on a tape seeking to the correct block can take minutes, on disks it generally takes milliseconds. Both of them are accessed sequentially once you seek to the correct block, and that was the point of the article. Relative to seek time, disk reads are taking longer (a consequence of one physical head serving more potential bits).
I tried to get Seagate to put two independent R/W heads in a disk once. They wouldn't go for it.
Back to your comment though "RAM" as in Random Access Memory is often distinguished from Hard Drives, especially in early computer science classes to point out the differences. And again what this article is saying is that even dynamic RAM today, looks more like an I/O device than memory.
In the early 2000's at NetApp, one of the performance engineers managed to track down the performance limiter on Pentium 4's to the frontside bus memory controller. The FSB could only issue so many memory transactions per second, and the P4 had doubled the length of a cache line (that improved streaming performance at the expense of everything else). The tricky bit was that the chip reported CPU 100% busy, but in reality it was the memory controller that was busy, not the CPU.
I'd say "random access" is basically just a relative term. Tape, yes, is about the least random-access-friendly medium I can think of. Compared to that, HDDs are quite efficient at random access. DRAM then makes HDDs look like tape in comparison, but even that in turn has its own slight differences in behaviour between sequential and random access (row buffers, prefetching...).
but even that in turn has its own slight differences in behaviour between sequential and random access
It's not slight. Modern RAM has about 20x more bandwidth doing sequential reads vs. random reads. It's almost entirely due to the addressing setup times needed to access different parts of RAM (something I suspect most programmers don't even know exists).
Programmers should actually be treating RAM the same way they treat disks, if they want the best possible performance the hardware can deliver. You can treat the L3 cache the way you used to treat RAM—they're certainly big enough these days!
One of the problem I work on is completely random access memory bound, so to scale, we now use ridiculously cheap CPUs (AMD Kaveri chips if anyone cares) paired with DDR3 2400 DRAM, $100 motherboards, and an Infiniband fabric and switch to keep them talking to each other quickly. (We use Kaveri's because we also are doing GPU computation at the same time, and thanks to the slow RAM, the on-chip R7 GPU is more than fast enough to keep up.)
On the plus side, I get to spend way less time optimizing the code since the CPU just sits there idling most of the time, which is a nice change. :)
It's less a matter of age than environment -- there are still plenty of places using lots of tape. (It's not like we're talking about vacuum tubes or core memory here...)
Clarification: I didn't mean to imply that tape is dead, but that during the 15 years that I've been programming, tape drives have only been used in enterprise-level backup systems, and are not used as "archetypal example" in articles intended for a more general audience.
Tape is still in use today. Widely. I bet there are more companies archiving to tape than archiving to AWS. For most of the past 27 years it would be the dominant archival method.
Random Access simply means that you can hit any part of the device 'at will' without skip all the data in between where you are and where you want to be.
Sequential access indicates that you have to pass all the data between where you are and where you want to go underneath the read head. So a denser medium would increase your access time unless you also changed some other fundamental (such as the speed with which you can move your medium around).
For a disk drive rotational speeds and head seek times have been more or less constant for a very long time, so the time to access a certain block can be computed without the underlying storage density becoming a factor.
So historically RA indicates disks/drums and so on and SA indicates tapes and derivatives. Bubble memory and its more recent incarnations are somewhere in the middle.
>'I have never heard of the myth that "HDDs provide random access".'
I haven't heard it stated directly, but I've seen it implied plenty. The typical scenario is simply someone who doesn't grok the gulf between random and sequential disk performance.
Indeed. IOPS has been the achilles head of magnetic disks for time eternal.
But ultimately calling these "myths" is somewhat absurd. Everything is relative. Relative to a magnetic hard drive, main memory offers impossibly fast random access. Sure, it's slow compared to cache memory, but that is a different discussion, and a different level of optimization. Similarly, when your SSD offers you 10s to 100s of thousands of random seeks per second, it is a universe better than the 10s to low 100 of a magnetic disk.
