Hard disk drives (HDDs) are nonvolatile magnetic storage devices that retain data without power, available in 3.5-inch and 2.5-inch form factors and commonly connected via SATA interfaces. Rotational speed, measured in RPM, determines read performance, while onboard cache helps manage write speed limitations.
Hard Disk Drive (HDD)
A hard disk drive can also be called an HDD. A hard disk drive would be nonvolatile memory. That is, we can store data on this for a long period of time, the machine can be shut down and that data is still stored.
Although I will mention, there are times if we run out of volatile — that is, like RAM — if we run out of RAM, sometimes what it'll do is it will create some storage space on the nonvolatile, in this case the HDD, so that way it can still operate, your computer can still operate. It's not an ideal situation, because this type of memory that we have on our computer is much slower.
With the media, HDD is going to be our magnetic media. It also has a couple different form factors, and there are also some different protocols and interfaces to it.
From a form factor perspective, there's really just two form factors we should be aware of. There are more than just these two, but these are the ones that are going to be most prevalent that you're going to see out there: one of them is the three and a half inch, and the other is two and a half inch.
We haven't really continued to develop this or try to get it into any kind of smaller container. One reason is because solid state drives have become much more prevalent, but the other reason is that there are physical components in here, moving parts, and so it doesn't make a lot of sense to continue to invest into this older technology, trying to shrink the size of this smaller and smaller. Especially since we've got a lot of moving parts in here and things that need to actually be spinning, you can only get it really so small.
Over the years there's been a lot of different interfaces as well, but the two main ones that you need to be concerned with are really SAS — which you're probably not going to see a lot of SAS drives in your early career, in fact a lot of them are being replaced — or SATA. This would be an example of a SATA. A lot of our drives, when it comes to this magnetic storage or hard disk drives, we use these SATA connections.
When it comes to hard disk drives, the other thing that we need to be aware of is that this has a certain speed to it. That is, I can spin this. There's a motor that spins this around, and then this arm comes out and reads this, and so the faster that this spins, the faster we can actually read that data.
Now what I'm going to do is actually put a little dot on here as a reference point, so we can talk about rotations per minute. There's something called rotations per minute, and I'm going to put this on here, and when I spin that around one full rotation right there, that would be one full rotation. If I were to just do that once every minute, that would be one RPM, or rotations per minute. But let's say I want to go faster, so now maybe I'm spinning it many times here, maybe I'm spinning it around every second it goes around, and so now I'm spinning it at 60 rotations per minute, so that'd be 60 RPMs.
Well, these drives have certain ratings to them, and so maybe I want a faster drive. Faster is better, so they go up to 15,000 RPMs. That means that this spins around 15,000 times every single minute, and then what happens is this arm can then read that data faster, because it's spinning around faster and it can read more data in a shorter period of time.
There's a few different options we have when it comes to rotations per minute: we have 5400, 7200, 10,000 and 15,000. One of the things we know is that 15,000 would mean that the drive could actually read data faster, so why would we always go with 15,000? The answer to that is just cost. This 5400 is quite a bit cheaper, and that would work if we're just browsing the internet or doing some simple things. However, if we're doing things like video editing, then we might want something a little more powerful. If we're looking at servers, then we would want something definitely more powerful, because there's a lot of reads and writes that generally happen on servers — not always the case. So we would choose based off of the cost and what application we're using this for, the need that we have for speed.
Now there is a problem, because this has to spin and write the data. These drives can be really slow, even in the faster 15,000, when you're working with servers. It can be slow to write data to this. Writing data takes a little bit longer than reading data, and so what we need to do is — there might be more data that's coming to this device than it has time to write to. So what happens is we have some caching that happens on these devices.
We can see that there's a little board on the back here, and the same thing with this bigger unit right here, there's a little board. It's a control unit, and there's some chips on there, and one of the things that we incorporate into this is some caching, some short-term memory. Think of something like RAM, that it's storing some short-term memory waiting to be written to the computer. So when you have the higher end drives, what happens is those caches on these drives become bigger, so that way you can store more data temporarily before it gets written to the drive. So that's what this looks like: we've got some sort of temporary cache, and it goes into those caches, and then that's when it's waiting to be written to the actual drive.
Something beyond the scope of this course, but I'm going to mention it just so you have a heads up about this, is there is something called a paging file. When we have RAM on the computer, it creates that fast connection between the CPU before stuff gets processed, and then things that come out will go to the RAM. There are times when we don't have enough RAM on a system. In that case, what will happen is the data will start being stored on your hard disk drive or your SSD or whatever, and we call that a paging file. It gets stored in a paging file.
Well, as soon as that happens, that information that's being stored on the paging file becomes really slow in comparison to the rest of the data that's on your computer. It used to be a big problem, because RAM was very expensive and so we'd try to get away with less RAM. Nowadays it's not as expensive as it used to be, so we don't have this issue as much anymore. But just be aware that there's this paging file in case your RAM actually gets filled up, then we can use that paging file.
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