Course 3, Module 2. In this module, we're going to talk about sectors and clusters. Sectors, like we talked about in our last section, are formatted during low-level formatting. They are written to the disk during low-level formatting which is performed by the manufacturer. In order for our operating system to be able to read from these sectors, they must have an address for the operating system to locate and read and write data to and from the sectors. Sectors are also relative to the physical disk, which means they can be relative to the physical disk, meaning that the disk itself from the absolute sector zero, absolute meeting prior to the operating system, sectors will have addresses that is relative to the physical disk itself and not the volume within the drive itself, that physical drive. But we'll talk more about that as we go along. Again, this is performed by the manufacturer. They are usually 512 bytes in size. That can vary with advanced format disks where then they'll be 4,096 bytes in size. But those disks are marked AAF and you will be able to recognize them. Sectors are the smallest readable unit on a disk and the smallest writeable unit on a disk, and that's going to become important as we move along. Sector numbering starts at zero, and that's very important to remember because when we're locating items on our disk like our partitions or a specific file, and we have to calculate that numbering, we have to remember that it starts at zero, so we must always account from zero. Sector numbering can be physical or logical. If it's physical, it's usually relating prior to the operating system, but it can also be addressed in a physical address or a logical address. The physical address would be from the start of the disk and the logical address would be from the start of the volume or the partition. How are these sectors addressed? Well, we use something called logical block address or an LBA, is what is used on all modern hard drives today. When you look at your label on your hard drive, as you can see in the diagram on the slide, you can see our disk, we have an LBA, we look to the right, highlighted in pink there. We have a number. Now that is the logical block address, with status keeping us the total number of sectors on the disk. That is the total number of sectors. If we wanted to find the total number of bytes, we would multiply the LBA times our sector size, which is usually going to be 512, and that would give us our total number of bytes on the disk. Also on this disk label, we can see a capacity of one terabyte, highlighted in blue, we have the serial number down here in green, and a model number down here highlighted in yellow. These is all important information you want to note on your hard drives because this serial number is going to be different than the serial number on the outside of the computer case and you always want to make note of that. Like I said, sectors are numbered sequentially, so we'll have sector 0, then sector 1, 2, 3, 4, all the way continuing to the end of the drive. Legacy addressing or cylinder, head, sector, CHS addressing is no longer used. This was relative to the structure of the physical drive itself. It took into account the number of cylinders, heads, and sectors. We looked at cylinders and heads when we looked at the physical structure, that is layout, in our previous module. This was relative to the physical structure of the drive, but it was limited to 8.2 gigabytes, and most modern drives are going to be much larger than 8.2 gigabytes. This also has to do with the number of bytes available to address the sectors, only allowed us to address up to 8.2 gigabytes. We're going to talk more about sector addressing as we go through this course. Clusters. Clusters, like we learned, were a group of sectors, and these are written during high-level of formatting. When we format a drive, we have the option to set the cluster size or it can be set automatically by the operating system. Cluster size will vary with the size of the disk. The larger the disk, usually the larger the cluster size. The most common cluster size you're going to see is eight sectors, but it does vary with the size of the disk. Cluster numbering starts at one, not zero, like sectors. Cluster numerating starts at one. Clusters are going to be relative to the partition of the volume, to that drive letter. Cluster of one would start at the beginning, say of our C drive, for example, not at the beginning of the physical disk itself. They are relative to the volume. Clusters are the smallest allocable unit. What that means is when you write a file to disk, the operating system saves a certain amount of space for that file and it's going to save it in clusters. It's going to save, say, two clusters for that file. It can't save 1.5 or one and a quarter, it has to save by full clusters. So there will be leftover space usually because most files aren't going to be exactly the same size as the clusters. They won't usually end on a cluster boundary. We'll talk more about that space that's left over as we go through this path. Unpartitioned space. This is space that is going to be prior to the start of the logical volume. Before my C drive actually starts, there is space on my physical disk prior to that logical volume, then that is considered unpartitioned space. That's going to be addressed in sectors and it's going to be relative to the physical disk itself. What we're going to find here is usually like our master boot record, our boot code, our partition tables and we're not going to have cluster addressing in this area. In our next module, we are going to go ahead and learn how to use active disk editor so we can take a look at these data structures on disk and get a better look into what we're talking about.
