How Full is Too Full?



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SSD's or Solid State Drives don't apply here... but with classic hard drives here's the skinny...

All platter based hard drives have the highest level of performance in their first 10% of drive space, as a hard drive becomes filled with data, data is written further away from this high performance area and performance drops.   You can see this effect by running any of the many hard drive benchmarks available. 

Using my 1TB Western Digital Caviar Black SATAII hard drive & HD Tach as a benchmark, my drive averages 110MB/sec read speed for the first 100GB of capacity, but by the time HD Tach reaches the last 100GB of capacity, that average read speed drops to just 58MB/sec. 

Together with file fragmentation, this can have a seriously negative impact on overall system performance.  With older & smaller IDE drives, this negative impact is even more dramatic as read speeds in the last 20% of capacity can be down as low as 7-15MB/sec.  



As a hard drive fills up, it has more bits of data on it.  The more bits of data, the heavier the platters become.  After all, a hard drive is also a mechanical device.  The heavier a spinning object, the more angular momentum it has.  The more angular momentum an object has, the more resistance it has to changes in rotational velocity.  Thus, a full hard drive will spin slower.






just kidding



I didn't see that just kidding at the bottom and almost asked how you even came up with that lol.  I needed that laugh today thank you.  :)



the previous comment is correct. however, there is more, im just throwing this up for anyone who wants to know the finer details.  as the previous commenter said more data spins under the read/write head on the outside of the disk than the inside. this is because of how hard drive storage area is arranged on each disk(platter) surface. the space is arranged in X number of concentric circles. each concentric circle is divided into Y number of slices. Each sub-area(sector) is a 512 byte block, also each circle has the same number of sectors as the circle inside and outside of itself. So the sectors on the outside of the platter are long and thin, the sectors on the inside of the platter are comparatively short and tall

This is why hard drives "slow down" as they fill up.   it takes fewer circles to store a file of X size on the outside of a platter than inside. therefore less search time. add to this that the outside of a platter spins faster than the center, because a point on the outside of a platter has to cover more distance than a point on the inside of a platter in the same amount of time.



You missed one of the main causes of hard drive slow downs.  Even without fragmentation a hard drive will slow down as it becomes full.  This is related to how hard drives write data to the disk.  On a basic level, hard drive speeds are related to the amount of data passing under the read/write heads.  The farther out on the platter, the faster reads and writes are.  This is because the circumfrence of the disk is larger at this point, so as the hard drive spins at a given speed, the data is moving faster on the outside of the disk and hence the read/write speeds are better.  A new HDD will begin writing to the outside of the disk first, moving in as more data is writen.  This is the other source of slow down, when a drive is near full, it is writing on the inside of the disk, and hence will be slower because less data is moving under the heads.  This effect is clearly visible if you run a program like hd tach or hd tune.  These test locations all across the disk and then complile them into a graph.  You will clearly see the speeds steadly decrease as the program writes closer to the middle of the disk.


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