How much free space does an SSD need ?

[Dr. McKay]

I have a 14 inch Macbook Pro with M1 Pro chip, 32Gb of RAM and a 1Tb SSD, with Mac OS Sequioa 15.7.

It's usually recommended (correct me if I'm wrong) to keep around 20% free space on your drive for it to perform optimally and to make it last longer.

I currently have around 282Gb free space, 51Gb of which is purgeable.
Just to have peace of mind : how much more space can I use before I start to have performance and/or other issues ?

Also, I understand that Time Machine snapshots can take up quite a lot of space on the internal SSD (I use a 2Tb Samsung T9 external SSD for Time Machine). Any way to optimise that ?

 

[Bigwaff]

It's usually recommended (correct me if I'm wrong) to keep around 20% free space on your drive for it to perform optimally and to make it last longer.

Probably not a matter of right or wrong, more preference. All SSD include overprovisioning, e.g. 1TB SSD actually has more space than 1TB (for lots of reasons .. you can web search). How much overprovisioning is dependent on manufacturer and whether it is a consumer or enterprise quality drive. You would have to look for comprehensive specs or benchmarks for your specific SSD or by manufacturer. Personally, I don't prescribe to the 20% free space rule. I've come close several times to filling up both internal and external SSD storage and have never experienced serious performance issues. Wasted space is wasted money.

Also, I understand that Time Machine snapshots can take up quite a lot of space on the internal SSD (I use a 2Tb Samsung T9 external SSD for Time Machine). Any way to optimise that ?

macOS will eventually delete TM snapshots so the only optimization possible is to manually review TM snapshots and manually delete them. You can do that using Disk Utility and choosing to view snapshots.

 

[Fishrrman]

I welcome correction from others regarding this post.

I recall reading previously that to do major OS updates now requires around 50gb of "free space" on the internal SSD. A LOT of stuff needs to be prepared, temporarily stored, etc.

Doesn't matter whether the drive is 128gb or 1tb or 4tb -- it's still going to need about that much space for the OS upgrade, or there could be problems.

So... the old rule of thumb about "xx% of free space" no longer applies, at least to modern Macs. These days it's a "hard number of gigabytes" of space.

Again, I welcome others with more knowledge than me to jump in and set things straight.

 

[Bigwaff]

So... the old rule of thumb about "xx% of free space" no longer applies, at least to modern Macs. These days it's a "hard number of gigabytes" of space.

Again, I welcome others with more knowledge than me to jump in and set things straight.

Leaving enough storage space for macOS updates/upgrades is a totally legit use case and recommendation.