Time Machine volumes. When you write a file to disk, the system will place it at the location that is easiest to access, and does not bother keeping it organized with others based on file type or task. The only organization the system does is maintain a hot zone where it keeps a relatively higher performing section of the drive free for placing frequently accessed documents.
Because of this, drive heads may need to move all over the disk when performing tasks, not because individual files are fragmented but because individual files required for the specific task at hand may be scattered. Optimizing a disk groups these files together so similar resources can be accessed as swiftly as possible. As with defragmentation, optimization benefits will depend on the drive's uses.
In my experiences the greatest benefit of optimizing a hard drive is that startup times can be noticeably reduced by keeping all the system files together on the drive.
Application launch times and file loading may also be decreased; however, because regular use will move files around the disk, the benefits of optimization will slowly degrade until you run it again. Whenever you access a file you increase the risk of some fault resulting in file corruption; defragmentation of a drive subjects all the files on disk not only to being read, but also to being copied, rearranged, and deleted. These tasks increase the risk of corruption, and iDefrag will run all of them on every file in the drive.
However, iDefrag is coded with numerous checks to prevent errors such as overwriting of files, or data loss in the event of a power outage or cancellation.
It also actively scans the drive for bad blocks and other integrity errors during its optimization. Regardless of the safety measures taken by any developer, your safest bet is always to have a full backup of your drives, so be sure you have Time Machine or other backup system fully updated before running iDefrag.
Keep in mind that running disk optimization and defragmentation uses the drive at full speed, which can cause some problems with heat if drives are not well ventilated though iDefrag monitors drive temperature during defragmentation.
In iDefrag's FAQ, there is specific warning against using the program with PowerMac G5 computers that have Western Digital hard drives, since intensive use of these drives can cause them to stop responding in the system.
Data loss will not occur, but the system may need to be restarted. If your computer is taking time to boot, open large files and applications, or your hard drive is making an inordinate amount of noise, I suggest doing the following steps:. Restart the computer -- many times slow systems are from programs and the system using up too much RAM and other resources.
If quitting the programs does not free up memory properly either from memory leaks or other errors , restarting the computer should do the trick. Check drive space -- full hard drives can result in fragmented virtual memory and data files, and low space for virtual memory. This can cause programs to run slowly and access the hard drive frequently while it manages the limited virtual memory resources.
Run Disk Utility -- use a disk utility program to check the drive's structure, integrity, and SMART status to ensure the hardware and formatting is stable. Run a defragmenter like iDefrag -- first defragment files, and then optimize them for an extra boost in drive efficiency.
I would compare it to shopping in a supermarket, which is really badly organized. You know, usually all the backing stuff is in one place. Now imagine you need backing power, flour, sugar, salt, etc. To make your life more interesting you must get them in precisely the order in which they are listed on your shopping list.
As files grow in size and large junks of empty space start to disappear, something else could become a potential problem. Go back to our first comparison of the hard drive as a rectangular space and data being the little building blocks.
It is quite easy to see how a large file e. I definitely recommend you read it. Basically the author argues many points that are quite true, here is a quick summary of the points I found most important. Remember the concept of the big video file being split up — this is really only a problem if your hard drive is nearly full and so Apple states:.
Another major cause, and Apple is quite right in pointing this out, was that data was frequently just appended to an existing file making it grow and ultimately become fragmented as space runs out and Apple has a good argument for that one as well:. Mac OS X The makers of iDefrag state on their website [3] that their tool is able to recognize these Hot Zones and deal with them appropriately.
With iDefrag you also get access to Coriolis CDMaker, which will create a bootable CD for you, in case you need to defragment your one and only hard drive as I did. After downloading and installing both products you are probably tempted as I was to take a look at your hard drive.
The iDefrag window has a little volumes panel, click your hard drive and let iDefrag analyze your disk. At first you will get a very generic window with a little banner underneath, just telling you about the free space fragmentation:.
Those colors of course have a lot of meaning, they tell you what type of file you are looking at and also the main window will have changed.
For the next part, you can follow what is going on by starting at the black spot in the right hand part of Figure 8 b and tracing the line as it goes clockwise around the disk. Assume we want to read File D from the disk, we rst have to go and nd it in the le list, which means reading block 0. The le list tells us where File D is on the disk, so the next sector we want to read is block 7, which on our disk is on track 1; were currently on track 0, so we move the head out out one track.
But we arent in the right place to read block 7, so we have to wait for the disk to revolve until the head is over block 7 so we can read it.
Having read block 7, we now need block 8; this is on track 2, and our disk head is over track 1, so we move the head outwards again. Yet again, we arent in the right place to read block 8 weve missed the beginning of it , so we need to wait for the disk to rotate again. If youre following along at this point, switch to the grey line. We can now read blocks 8 and 9 one after the other, without moving the disk head.
The next block, though, is block 12, and thats on the outer track, track 3, so we yet again need to move the head out one track, and since yet again we arent in the right place yet, we need to wait for the disk to rotate. Finally, we read blocks 12 and 13 one after the other, and were done.
During this process, the disk has revolved three and a half times, and weve had to move the head three times, not counting any initial movement to get to the starting point on track 0.
Lets say this disk is very slow indeed, and takes half a second to move the head, and rotates at 20rpm. Weve spent a total of 3 0. Now lets consider what might happen if we swapped the positions of the end of File D with File C.
Well, this one simple change will put the nal blocks of File D on the same track as the previous two blocks, so we could read 8, 9, 10 and 11 one after another, without moving the disk head and without waiting for the disk to rotate.
We save one head movement and one and a half rotations! In that case, we would only spend 2 0. When a le is split into more than one piece like File D was here, it is said that that le has become fragmented, and the pieces are typically referred to as fragments.
As you can hopefully see from this heavily simpli ed example, allowing the lesystem to fragment les is a trade-o. Without it, you might not be able to use all of the space on your disk, but on the other hand, a fragmented le might take a lot longer to read or write than it would if the le was in a single piece.
Clearly, therefore, what you dont want is for a le to be fragmented when it didnt have to be. What does my Mac do to avoid fragmentation?
The simplest of these is the way it chooses which blocks to allocate on the disk. In general, computer programs dont tell the computer the size of the le they intend to save before saving it. Even if the computer delays writing to the disk, it might not know the full size of the le within a reasonable amount of time. As a result, picking a space of exactly the right size is di cult. At rst sight, it might seem that this will be a huge disadvantage when our goal is to fragment les as infrequently as possible, but it turns out that computer scientists have shown that choosing a space of exactly the right size or best- t is no better in practice than other ways of choosing a space to allocate.
There are various other allocation strategies that have been investigated; for instance, we might simply pick the rst available space this is called rst- t.
Another rather surprising choice is to pick the largest space available, and always use that worst t. Again, it has been shown that in general neither of these is worse than best- t! Many lesystems use rst- t or variations on it , partly because they dont know the size of the le when they have to pick the rst block, and partly because it avoids searching through all of the free spaces to nd the best or worst option. Apples lesystem engineers, though, spotted something about the way people use disks.
Until very recently, the les you saved on your disk were very likely to be much smaller than the disk you were using.
As a result, it would take a long time for a typical user to use their disk completely. Imagine, rather than picking the rst space, or trying to nd the best space, just choosing the next space. If the user has yet to use every block on their disk, we know there is a good chance that, no matter how large the le we need to save, we wont need to fragment it, and the best part is that keeping track of the next space to use is easy we just keep a bookmark, or pointer to it.
This approach is known as a roving pointer algorithm. This clever choice worked well when most people used their computers to store word-processor documents, spreadsheets and other simple data les; it took a long time for a typical user to get through every block on a disk, and so it was a long time before the computer had to fragment a le.
However, modern media les, such as photos, iTunes or MP3 music les and particularly video les and movies are much larger and make it much easier for an ordinary user to defeat this simple approach. If you have journaling turned on, and the fragmented le is smaller than twenty megabytes, and it has more than eight fragments, Mac OS X will defragment it for you when you access it, assuming that there is su cient contiguous free space.
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Jump to Page. Search inside document. Figure 1 The computer is asking for your permission to access this disk. In this version of iDefrag, the available algorithms are: Full Defrag If you dont want to think about which algorithm to choose, choose the Full Defrag algorithm which is actually a combination of the Metadata and Optimize algorithms.
Figure 3 A heavily fragmented disk How can I tell if my disk needs defragmenting? Figure 6 Selecting a particular sector by cylinder, head and sector numbers. Correct at time of writing. Luis Vecina. Moe B. Vinod Rathore. Thanesh Kumar. Zaini Zulqarnain. Ethan Gabriel. Mehmood Abdul Razzaque. More From Kriyashakti. Willpower — By Roy F. Manny Resendiz. Popular in Cylinder Head Sector.
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