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I'm using this Windows Gadget for information about my hard disks. Besides available/used space, it has read and write speed. Those are pretty much self-explanatory. Then there is this "disk activity" measurement, and is a percentage indicator.

I have noticed that when it is really high, my computer gets slow, specially right after boot, when lots of programs are being started. However, many times the read/write speed indicators are quite low, and the disk activity percentage is high.

I don't think I understand well enough the concept behing this disk activity indicator, why is it in percentages, what does it mean?

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I've found this official page addgadgets.com/drives_meter/info.html with a screenshot of the gadget, it explains many of the fields, but unfortunately not the "disk acticity". –  That Brazilian Guy Dec 2 '12 at 13:42
    
I have sent (again) a message to the original developer, and I'm waiting for his/her reply. –  That Brazilian Guy Dec 2 '12 at 13:53

3 Answers 3

What exactly it means you probably have to ask the developers or look at the code. But the concept itself seems to be similar to what Resource Monitor displays as Queue length in its graphs:

enter image description here

which manifests itself as a percentage in the main view as well:

enter image description here

Basically this represents how many hard disk commands are waiting to be fulfilled. If there is little I/O, then the disk can read/write fast enough to satisfy them all quickly, but if many things are to be done at various places on the hard disk there will be lots of waiting for the arm to reposition, etc. (Side note: above screenshots are from an SSD which suffers from the problem a little later).

Once the queue length rises above 1 you'll usually start feeling the system slow down. Especially so if it's the disk where your pagefile is on. Since this is a very low-level thing it affects everything and even the commands to read a faulted page from the pagefile will have to wait. If you thought performance went down when paging starts due to full RAM ... this is a lot worse (and it happened to me every time I made an SVN update in a large repository).

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"If there is little I/O" -- does it mean as "many bytes / throughput speed" or "many read/write commands, doesn't matter how small they are"? Because if the latter, then it does begin to make sense to me. And overlap with the concept of disk fragmentation. –  That Brazilian Guy Oct 28 '12 at 0:09
    
Well, for this it comes down to the commands but also to the behaviour. E.g. large, sequential reads (e.g. when copying a file) are easier to satisfy as the OS will read ahead. Smaller reads across the disk will be slower. Since it's more about the usage pattern you could see many commands or high throughput while having little activity and vice-versa. –  Joey Oct 28 '12 at 0:13

A hard disk is made from one of more circular platters with magnetic material on one them. A read/write head floats on a thin air cushion over the platter. It can senses magnetic charge below it (reading). Or it can use a small electromagnet to change it (writing).

In the picture below the platter would be the gray disk. The green line would be the arm with the head at the end.

painted harddisk

As you can see the head only covers a small part of the gray disk. Since the disk is rotating it will eventually cover all the spots in the light green ring.

If I move the head a bit to the side it will cover another circular track.

However both moving the head and rotating the disk takes time.

If I want to read some information which just happens to be below the head I can do quite quickly. If I want to read some information which just moved away from the head I will have to wait until that part comes around again. On average this takes half a rotation.

On a 7200RPM drive the platter rotates 7200 times per minute. That is 120 times per second. Or once per 0.008sec. That means when you want to read something new you will have to wait a 0.004 sec on average for the disk to rotate to the right place. (4ms).

It gets worse. Because the disk is not set up to read from just once small circle in the platter. it can move inward and outward to read many concentric circles. Moving a head also takes time.

If I summarize this it comes down to three small points:

  • A disk can be busy reading data from disk
  • A disk can be busy writing data to disk
  • Or a disk can be busy doing other things, such as waiting for the head to arrive at the right position.

Using the pre-edit book analogy.

Say you are reading a story to a child. You have a book and you read a sentence or two. You then pronounce these so your child knows what the story is. You read more. You speak more.

But, eventually you reach the end of the page.

Your child will have to wait while you turn the page over before you can continue with the story.

That might seem to be something trivial. However with hard disk speeds your read speed is very high and turning a page (moving the RW head to another circle) takes a significant chunk of time.

If I let a disk just read from a single track it might read about 200 megabytes per second. This is about the same as a book reader who can rapidly read a page.

Now if I ask you to read the first word of every page things will slow down. But you will be very busy. Just not with reading but with turning pages.

Returning you your question, you wrote many times the read/write speed indicators are quite low, and the disk activity is high.

The read/write indicator is indicating how fast you are reading data from the disk. In MB/sec (or, bookish, in words/second).

The activity indicator is telling you how much time the disk is busy by either reading or writing data, or by waiting for the RW heads to get to the right place. If it indicates 100% then the disk is working as fast as it can. If you add additional tasks then you will have to wait longer until they complete because the disk simply has no time left to do more work in.

As why this is displayed as a percentage: I think it is because it is easy to read. 0% from being totally idle, 100% for working as hard as the disk can, and anything in between to indicate just how hard the drive is working.


Fragmentation (as asked in the comment).

Say I have a notebook with twenty numbered pages. I write a short story which takes up 5 pages. I then write three more stories of the same length. My notebook now looks like this.

page:     1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20
Content  S1 S1 S1 S1 S1 S2 S2 S2 S2 S2 S3 S3 S3 S3 S3 

I decide that story 2 is bad and I delete it with an erasor. The notebook now looks like this:

page:     1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20
Content  S1 S1 S1 S1 S1                S3 S3 S3 S3 S3 

I now write fourth and firth story. Both nice short ones, 3 pages long. I could either start writing at page 13 (after story 3). But that would waste free space and I would quickly have to buy a new notebook.

Or I could write my first story starting from the now empty page 3. If I do that my notebook now looks like this:

page:     1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20
Content  S1 S1 S1 S1 S1 S4 S4 S4       S3 S3 S3 S3 S3 

I then start to pencil story number 5. Starting with the first empty page. After writing two pages I run out of free space. At the bottom of page 10 I scribble 'continued at page 16' and write the last page of my story on that page.

page:     1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20
Content  S1 S1 S1 S1 S1 S4 S4 S4 s5 s5 S3 S3 S3 S3 S3 s5

Now whenever someone reads my stories they can quickly read most of them. However story number 5 requires them to turn a few extra pages and thus costs a bit more time to read then similar 3 page stories. In other words, reading that is slower.

A similar thing happens with filesystems. The disk is the notebook.

If I open a book I have two facing pages. I can quickly read both of them. Similarly on a disk I can quickly read all the sectors on the same cylinder (e.g. the green ring in the picture). The sectors on the disk never get moved, they are like the page numbers in my notebook. However the story on it (files in a filesystem) can get scattered around the disk. This is fragmentation.

(And yes, this is a bit short as an explanation, but it was a side question)

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Correct me if I'm wrong, but I think you just answered the question in the title, not the question posted. –  Mehrdad Oct 27 '12 at 23:55
    
Well, "what does it mean?" It means how busy the disk is. Either reading, writing or doing other things such as waiting until it has moved its heads to the next place to read or write. "why in percentages" is a choice of the program maker. But a 0% busy (fully idle), 50% busy (working at half max speed) etc is easy to understand for humans. –  Hennes Oct 27 '12 at 23:59
    
Did you read this in the question? "I have noticed that when it is really high, my computer gets slow, specially right after boot, when lots of programs are being started. However, many times the read/write speed indicators are quite low, and the disk activity is high." I'm not sure I see how your answer responds to this problem. –  Mehrdad Oct 28 '12 at 0:01
    
@Hennes You're describing fragmentation, right? I understand this concept (Used disk defraggers since the DOS era). However, this "busyness" can be measured in terms of what? Read and write speeds are measured in bytes per second. I tend to think the more bites per second being written/read, the more busy the disk is? Or am I wrong? –  That Brazilian Guy Oct 28 '12 at 0:08
    
No, not fragmentation. And not what I can answer in just a comment. Let me expand the post and I add what fragment and make the post clearer. –  Hennes Oct 28 '12 at 0:24

As others stated you should ask the developer what the disk activity percentage exactly is. But nevertheless let's give a short explanation what it might mean.

I have noticed that when it is really high, my computer gets slow, specially right after boot, when lots of programs are being started.

That is not surprising. When disk activity is high, then the disk is busy reading or writing. Processes reading or writing have to wait until their data is read or written. The more disk activity, the longer they'll have to wait and the lower will be the system's responsiveness.

However, many times the read/write speed indicators are quite low, and the disk activity percentage is high.

If the disc wants to read a sector, it has to position the head first, then wait for the sector to be under the head and then read the data from the sector. Positioning and waiting for the sector is the slowest part and takes some milliseconds. If you are reading/writing from a contigious file you might end up sending a big read to the disc and the disc will have to position the head once and then read a lot of data. This is a case where you will have a high data rate. At other times you might need to read sectors that are spread over the complete disc. For each sector the disc will have to position the head first. The disc will be busy, but the data rate will be low because most of the time the disc is waiting for the head to position.

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