05-15-2020, 01:19 PM #1Requirement to keep free space on SSD?
With a traditional spinner hard drive, it is suggested that one keeps a certain amount of free space available - maybe 5 or 10% on a large 500GB drive.
Suggestion of the percentage varies depending where one looks.
Is that suggestion to keep free space also valid for an SSD with APFS considering the file structure is totally different?
- 05-15-2020, 03:49 PM #2
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05-15-2020, 06:07 PM #3
The amount of free space required has nothing to do with the type of media involved. It has more to do with the space needed for the swap file and other dynamic things. If you run out of space, you will not be able to swap memory to disk and thus the OS will start misbehaving VERY badly until you free up space.
So keep at least 20% of your drive free.--
05-15-2020, 06:29 PM #4
05-15-2020, 06:59 PM #5
The 20%+ number has been around for a while.
I thought it was that high because
a. Typical hard drives used to be much smaller, so 20% of an 80GB drive was only 16GB whereas 20% of the 500GB drive I use now is 100GB. 16GB of disk space for swap seems reasonable, 100GB for that same purpose now, does not
b. There was a lot less RAM in a typical system, 4MB was very common, I now have 16MB, so with much more RAM, memory swap should be less - activity moitor for me shows swap usedas 3.3 MB
c. Why I thought an SSD requires less free space for memory swap is because read/write is much faster than a spinner drive. Thus 20% is recommended on a spinner drive to try to use free disk space that physically is quickly accessible for read/writes whereas on an SSD all free space anywhere would be quickly accessible.
Maybe I'm overthinking the last part.
However,I think 20%+ on a 500GB or 1TB drive seems rather excessive.
PS: Did a bit of googling.
Opinions are all over the map - from a low of 5% to a high of 30%
Few seem to actually attach a hard number. ie something like 15% or 20GB.
The point many make about SSD's is that they need additional free space because of the way data is stored compared to a traditional magnetic media.
The SSD itself includes extra storage not accessible (or reported) to the user, but how much varies from unit to unit.
Last edited by krs; 05-15-2020 at 07:30 PM.
05-15-2020, 10:54 PM #6
Here's a good recent read that might give you a bit more information and some reasoning...
WHY SOLID-STATE DRIVE (SSD) PERFORMANCE SLOWS DOWN AS IT BECOMES FULL
Why solid-state drive (SSD) performance slows down as it becomes full • Pureinfotech
But their comment about the amount of free storage space to leave that I missed on my first glance seems rather excessive to me:
The rule of thumb to keep SSDs at top speeds is to never completely fill them up. To avoid performance issues, you should never use more than 70% of its total capacity.
Last edited by pm-r; 05-15-2020 at 11:54 PM.
05-16-2020, 07:00 AM #7
The 20% rule was always a rough rule of thumb, never a hard and fast rule. It's just that empirically, it seemed that was about the level where most folks who were hitting their head with rotating disk hard drives (RDHD's) fell. But not all of them. When good utilities came available that showed RDHD fragmentation, it became clear that if a RDHD was highly fragmented that it could become full as early as 60% full, but if a RDHD was meticulously kept defragmented, it might be able to go beyond 90% full with no adverse effects.
As hard drives became bigger and bigger, one might have expected the 20% rough rule of thumb to have gotten smaller. It didn't. One can only guess that the metadata that fills up a hard drive also becomes larger as the amount of ordinary data also becomes larger. And fragmentation also probably plays a bigger role on bigger RDHD's too.
SSD's are different. They don't suffer from fragmentation. But they still fill up with virtual memory, databases, caches, scratch files, etc. They don't follow the 20% rule though. It turns out that, as you would expect with RDHD's, SSD's do better with higher total capacities. They also do better based on how good the TRIM they have running works. (Not all third party SSD's work with Apple's implementation of TRIM.) I've already started hearing from folks with tiny 256GB SSD's starting to hit their head, and it happens well before their hard drive is full. However, I haven't heard from a single person who has hit their head with a 1TB SSD yet. It's still too soon to estimate a rough rule for how full it's safe to let an SSD get. But I guaranty you that it will be well short of 100%.
05-16-2020, 11:27 AM #8
I read a fair amount of articles on this subject yesterday - some mentioned that the 20% number came about years back specifically for Windows to allow for defragmentation. The Mac still had only a niche market share then.
So perhaps for the Mac the 20% number was always an overkill.
I remember years ago I downloaded some software that generated tons and tons of log files in the background to the point where I got a warning message that my drive was getting full. When I received the warning there were only a few MB of free space left.
I panicked for a short while because I thought my Mac would shut down and I would not be able to boot up again, but I managed to delete the log files, got rid of that problem app and everything was good.
Right now, with a 500GB drive, I try to keep 100GB free, but right now I'm down to 63GB free and so far I have not noticed any problems. But I want to switch to a 500GB SSD to try Mojave and was trying to get some idea if that amount of free space is still OK....or even less is acceptable.
05-16-2020, 09:11 PM #9
05-16-2020, 10:08 PM #10
Like I said, I was down into the MB range of free space when I got a pop up with this warning that I was running out of free space on my drive.
I tried to find out at what amount of free space Apple brings up this warning - hopefully before there is an issue.
Came across this thread where the warning seems to come up with about 3GB free space left on a 128GB drive.
That is a lot less than 20%
Stop receiving "Your disk is almost f… - Apple Community
PS: I'm planning to run Mojave on an SSD I ordered today.
This will be on a test basis only - I'll keep my main drive with El Capitan, so if Mojave crashes it won't be a problem.
05-16-2020, 10:31 PM #11
I go into this entire topic in a lot of detail here:
OS X Maintenance And Troubleshooting
Item #5 and Note #1
I know that it's hard to believe that Apple's hard drive management is so bad that you can have a huge RDHD and that it could be for all intents and purposes "full" when it has many gigabytes of free space still reported as available. But this happens routinely to users. It's really old news at this point, and I'm not going to debate it. I present my findings for folks who feel that "forewarned is forearmed."
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