A sector is a physical spot on a formatted disk that holds information. When a disk is formatted, tracks are defined (concentric rings from inside to the outside of the disk platter. Each track is divided into a slice, which is a sector. On hard drives and floppies, each sector can hold 512 bytes of data.
A block, on the other hand, is a group of sectors that the operating system can address (point to). A block might be one sector, or it might be several sectors (2,4,8, or even 16). The bigger the drive, the more sectors that a block will hold.
So why are there blocks. Why doesn't the operating system just point straight to the sectors? Because there are limits to the number of blocks, or drive addresses, that an operating system can address. By defining a block as several sectors, an OS can work with bigger hard drives without increasing the number of block addresses. For example, PC DOS (earlier versions at least) could only address 65,536 blocks (64K), and each block could could only be a single sector. Thus, the largest size a disk volume could be was 32mb (64K * 512K). (Earlier versions of the Mac OS had a 16mb volume limit for similar reasons). If you increase the size of a block to, say, 4K, that same version of DOS can now work with volumes as large as 256MB (64K addresses * 4K blocks).
With current versions of the OS's, the formatting software will look at the size of the drive, and figure out the smallest number of sectors that need to be in a block in order to be able to use the entire drive. So, when you format a floppy disk, the block size will be one sector. When you format a 230MB drive, for example, the block size is 8 sectors (4K). Why does this matter?
True or False: When a file is copied from a hard drive to a floppy, it will usually take up less space than it takes up on a hard drive.
TRUE. Although the size of the file will be the same, fewer sectors will be used to store the file. Conversely, when a file is copied from a floppy to a hard drive, it will usually take up more disk space. When files are stored on a disk, they always use up a whole number of blocks. Any unneeded space at the end of a block is unused and wasted. For example, say your hard drive has a block size of 4K, and you have a file that is 4.5K. This requires 8K to store on your hard drive (2 whole blocks), but only 4.5K on a floppy (9 floppy-size blocks).
Miscellaneous info...If you tend to store lots of small files on your hard drive (like running Windows and Windows apps), the blocks used to store all those little files can have a
lot of wasted space in them. Likewise, compressing lots of little files may not save as much space on a big hard drive with a big block size. If the block size is 4K, and you compress a 3K file, the file will be compressed, but it will still use 4K of disk space. If you do a Get Info on a file on a Mac, the Size info will say something like '12K on disk, 8320 bytes used'. The 12K is the amount of disk space used, based on the block size. Thus, if the block size of your drive is 4K, this number will always be in increments of 4K. 8320 bytes is the actual size of the file. Note that you have to go to Get Info to see the actual size of the file. This number does not show up in View by Name
-by Help at Filemaker
Most people that use a computer complain that it takes too long for it to boot up. Here is a quick tip to get your computer to boot faster in less time. When your computer is booting up the only thing stopping it from going straight to the desktop are the programs it has been told to startup when the computer is turned on or starts up. Many of these programs are important but there are also many unnecessary .exe files and programs that don’t need to be running. Many of these will not cause any problems even when they are disabled.
Easy as 1-2-3
Once your computer is booted up and you are on the desktop, press the Windows Key + R and type “msconfig” (without quotes).
First thing to do is go to the “Services” tab. Then hide all Microsoft services and click the “status” header at the top. Most all of the services that you see running are important but check and disable miscellaneous toolbars and other things that you dont need running. The services tab usually does not have anything you need to disable but checking to be sure is a good idea.
Next, click the “Startup” tab. You will see items like various updaters, messengers, media services, etc. These can be unchecked meaning disabled. Disabling the small unneeded programs from starting will get you to your desktop many times faster and will allow the computer to run faster. If you made a mistake and disabled one that you need then you just re-check the box and “enable” it again.
How Do Modern Hard Drive Store, Access Data? What is the inter- face? How is the data actually laid out and accessed?
Let’s start by understanding the interface to a modern disk drive. The basic interface for all modern drives is straightforward. The drive consists of a large number of sectors (512-byte blocks), each of which can be read or written. The sectors are numbered from 0 to n − 1 on a disk with n sectors. Thus, we can view the disk as an array of sectors; 0 to n − 1 is thus the address space of the drive.
Let’s start to understand some of the components of a modern disk. We start with a platter, a circular hard surface on which data is stored persistently by inducing magnetic changes to it. A disk may have one or more platters; each platter has 2 sides, each of which is called a surface. These platters are usually made of some hard material (such as aluminum), and then coated with a thin magnetic layer that enables the drive to persistently store bits even when the drive is powered off.
The platters are all bound together around the spindle, which is connected to a motor that spins the platters around and around (while the drive is powered on) at a constant fixed rate. The rate of rotation is often measured in rotations per minute (RPM), and typ- ical modern values are in the 7,200 RPM to 15,000 RPM range. Note that we will often be interested in the time of a single rotation, e.g., a drive that rotates at 10,000 RPM means that a single rotation takes 6 milliseconds (6 ms).
Data is encoded on each surface in concentric circles of sectors; we call one such concentric circle a track. A single surface contains many thousands and thousands of tracks, tightly packed together, with hundreds of tracks fitting into the width of a human hair.
To read and write from the surface, we need a mechanism that allows us to either sense (i.e., read) the magnetic patterns on the disk or to induce a change in (i.e., write) them. This process of reading and writing is accomplished by the disk head; there is one such head per surface of the drive. The disk head is attached to a single disk arm, which moves across the surface to position the head over the desired track.
Tracks and Seek Time
In the figure, the head is currently positioned over the innermost track (which contains sectors 24 through 35); the next track over con- tains the next set of sectors (12 through 23), and the outermost track contains the first sectors (0 through 11).
To understand how the drive might access a given sector, we now trace what would happen on a request to a distant sector, e.g., a read to sector 11. To accomplish this read, the drive has to first move the disk arm to the correct track (in this case, the outermost track), in a process known as a seek. Seeks, along with rotations, are one of the most costly disk operations.
The seek, it should be noted, has many phases: first an acceleration phase as the disk arm gets moving; then coasting as the arm is moving at full speed, then deceleration as the arm slows down; finally settling as the head is carefully positioned over the correct track. The settling time is often quite significant, e.g., 0.5 to 2 ms, as the drive must be certain to find the right track (imagine if it just got close instead!).
After the seek, the disk arm has positioned the head over the right track. Thus, the state of the disk might look like this (Figure 36.4).
As we can see in the diagram, during the seek, the arm has been moved to the desired track, and the platter of course has rotated, in this case about 3 sectors. Thus, sector 9 is just about to pass under the disk head, and we must only endure a short rotational delay to complete the transfer.
When sector 11 passes under the disk head, the final phase of I/O will take place, which is known as the transfer, where data is either read from or written to the surface. And thus, we have a complete picture of I/O time: first a seek, then waiting for the rotational delay, and finally the transfer.