Does running a full hard drive really slow system performance?

Discussion in 'MacBook Air' started by james*b, Feb 12, 2015.

  1. james*b macrumors regular

    Jan 2, 2011
    Does running a full-ish hard drive really slow system performance? I am currently thinking about which model MBA and the size of SSD to upgrade to, so any advice would be welcome.
    When my MBP hard drive was 75%+ full, an Apple Genius told me this was slowing the machine.
    Thanks in advance,
  2. motrek macrumors 68020

    Sep 14, 2012
    If your MBP had a hard drive instead of SSD then that's nonsense. But if it had an SSD, there might be something to it.

    The concern is write amplification:

    Basically, the less space you have available on the drive, the more data the drive has to shuffle around every time you write to it.

    That will wear the drive out faster and cause it to be slower. By exactly how much I don't know. I would guess it's not something you should lose sleep over.
  3. Weaselboy Moderator


    Staff Member

    Jan 23, 2005
    The issue motrek mentions is legit, but with either a hard drive or flash storage the drive would have to very close to full for you to start feeling it was slowing the machine. Short of that you might see some difference is you measured speeds with a benchmark app.

    But no... if you are noticing your machine is real slow all of a sudden, there is no way it is because the drive is 75% full.
  4. gnasher729 macrumors P6


    Nov 25, 2005
    What you say about SSD is correct. What you say about hard drives isn't.

    A hard drive is just a round disc (or several) rotating at constant high speed. The tracks at the outside of the disc are significantly longer than the ones on the inside, and therefore can contain more data. So reading data on the outermost tracks of a drive can read more than twice as many megabytes per second than tracks on the inside.

    Every operating system first uses the "fast" tracks on the outside, but as the drive fills, it has to use the slower tracks on the inside as well, and gets slower. Because of this, buying a drive that is significantly bigger than you need, can make your computer run faster.
  5. joshlalonde macrumors 6502


    Jul 12, 2014
    Interestingly enough, I had about 50GB of free space out of 256GB and my computer was much slower. I got rid of bootcamp and suddenly it was good again.
  6. motrek macrumors 68020

    Sep 14, 2012
    Good point, thanks.
  7. AFEPPL macrumors 68030


    Sep 30, 2014
    Yes hard drives slow down as the used capacity increases..

    HDD use mechanical moving heads. The further the head has to move the longer the transaction will take to complete, thus the longer it will take before the next transaction can take place. This travel time is described as latency an this is whats resulting in poor performance. The disk platters are round and always rotate at the same speed, so you have to factor in something called angular velocity. So the outer parts of the disk covers or travels over more tracks in the same about of time - or put another way is able to transfer more data in the same amount of time.

    So basically as you consume the drive you do so from the outside in, and that's why you get the so called slow down. Drive fragmentation can also play a part too.

    Im sure lots of so called experts will start jumping in, so heres a real simple test you can do and try it out for yourself...

    Take an unused disk.. don't do it on a used disk..!!
    Partition up the disk into 3 or 4 partitions and using a benchmark tool perform a test on each of the partitions. You need to understand how files work in terms of random read/write and sequential read/write and the block size in relation to your data, but if you run the tests with various sizes large say 512k, small being 8k just for giggles - you will see a performance reduction from the first to the last partition.

    Just look at the thought-put for each of the partitions... it tells you all you need to know.
  8. Bomb Bloke macrumors regular

    Bomb Bloke

    Feb 12, 2015
    Tasmania (AU)
    Of course, the data written to the outer rim of the drive (eg your OS!) won't take any longer to access just because other bits of data have been written closer to the center - and clearing data from the outer rim won't automatically move other bits of data to take its place.

    If your system is slowing to the point where you notice it in day-to-day use, odds are the amount of data on the drive has nothing to do with it - it'll more likely be related to the apps your system is rigged to load on boot.
  9. joshlalonde macrumors 6502


    Jul 12, 2014
    You'll want to get the larger SSD if possible. It's nice not to have to worry about free space. I have like 4-5 games installed and if I had the 128GB model, I'd already be out of disk space. I still have another 100GB free. I also have some larger software packages such as Unity installed.

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8 February 12, 2015