What is "purgeable" data, and can I safely delete it?

Having also been confused about what space is actually available on my iMac for storage, this macOS Sequoia 15 support article helped me understand "purgeable space" better: Get detailed information about a disk in Disk Utility on Mac - Apple Support

[“purgeable space”—or space that macOS can free up when needed by removing files from your computer (you can’t manually remove the files that are designated purgeable, but macOS removes them as space is required)]

This was also another helpful article on managing storage space: Free up storage space on Mac - Apple Support

Otterysteve wrote:

No, I deleted these files about two or three weeks ago. The space has not been "released" at all, it is still designated "purgeable" by Disk Utility.

Then those files must be preserved in a local snapshot. Normally, those are automatically deleted after a day or so. But if you've done any hacking on Time Machine (or some 3rd party app has done that for you), or if you've installed any 3rd party software that creates its own local snapshots, then this data will sit in "purgeable" forever. And there may be other explanations - see below.

In Disk Utility, select one of the real volumes like "Macintosh HD" or "Macintosh HD - Data", go to the View menu and choose "Show APFS snapshots".

On my computer, I haven't done a backup since I updated to Sequoia. Long story. But I still have a Time Machine snapshot sitting there since March 10. It says it's holding onto 193 GB. So let's delete that to recover my storage.

And presto! I went from 123 GB free to 304 GB free instantly.

Purgeable Space is controlled by the operating system. When the system needs more empty space, it moves some purgeable space to empty space.

There’s no user action to hasten this transition. It can take days or longer.

From contributor @etresoft regarding Free Space and Available Space 

Free vs available disk space huge differe… - Apple Community

Quote >>  “ The "available" storage is the amount of used storage that the operating system could automatically delete if it felt that it was really necessary. The "free" storage is the amount that you can actually use for something.

There are system processes that run in the background and automatically delete some of the "available" storage and convert it to "free". If you completely run out of storage, then those system processes will try a little harder. When you "delete" files you are just hinting to the operating system that you don't need those files anymore. The operating system will eventually remove them, but on its own schedule.

Certain tools will allow you to force the issue and manually clean up some of this storage and manually delete local snapshots. But that is only temporary. "  << End Quote 

Otterysteve wrote:

What is this "purgeable" data please?

The same as unicorns, mermaids, and other fantasy creatures.

Can I safely delete it?

In theory.

And if I can safely delete it, how do I do so please?

I could really use the extra storage this would free-up, so any help will be much appreciated please.

You don't need to. You have 81 GB of free space. That's not too bad for a 1 TB drive.

The problem with this "purgeable" data is that it consists of various system caches, temp files, etc. There's nothing wrong with any of that. It's designed to improve your computer's performance. You can delete it, but you really, really don't want to.

The reason is that you don't control the creation of these files. They will be recreated as needed. So the problem is that if you free up this 250 GB and then use it for your own data, then it's no longer available for all those system files. Your performance will crash hard. You'll start deleting files like crazy. As you delete files, your free space will only go down. It's an endless cycle that can only lead to a system crash.

But right now, you have 81 GB of free space. As I said, that's not too bad. 181 GB would be better. So find 100 GB of your own data that you don't need and delete it or move it to an archive disk. And then copy it to another archive disk. That's the solution. There is no other solution.

Otterysteve wrote:

I wanted to download the War Thunder game for example, Steam recommended specifications require 95GB of available space (Minimum is 40GB). So clearly in that case 81GB would not be enough.

That's fine. Just follow my instructions and you'll be able to install the game. Find find 100 GB of your own data that you don't need and delete it or move it to an archive disk. Easy-peasy.

The thing that I don't understand is on one hand I am told I have about 250GB "available" to me, and then on another hand I am told that I only have about 80GB "free" for me to use. On the face of it they can't both be right, although there is too much I don't know for me to make such a statement.

Yes. I explained that too. The "available" is a fantasy. It's like those old fairy tales that are now wholesome children's stories but were originally very dark and scary. You are out of free storage space, but don't worry, because in this story, all of that storage is really "available". Just pretend you are playing that War Thunder game and think of how entertaining it will be. A healthy imagination is really better for you than computer games anyway.

Purgeable suggests the 170GB can be "purged", which according to my dictionary means it is or may be considered undesirable and can be got rid of. Your comments suggest otherwise (in which case I wonder why it is called "purgeable" data?) and that these are important data necessary for the smooth-running on my MBA. So I wonder, if I were to download War Thunder (which I'm not actually going to do for other reasons), would the OS purge enough of the purgeable data to make room for the new downloaded data?

No and yes. You don't have enough free space for that came, so you can't download it - end of story.

But if you tried to do it anyway, it would fail. That would trigger various system tasks that will try to free up that "purgeable" data. In a day or so, you might be able to download the game.

But there's a catch-22. In the process of all this, you run out of free storage space. This runs the risk of corrupting various system databases like e-mail, or other important system tasks. And then, even if you are able to download then game, you will have virtually no free storage space left. Your computer will then struggle to keep enough free storage to run the operating system. All that "purgeable" space will have been used, but now it can't be used for system caches. Everything will run much, much more slowly.

I have no idea whether or not this is so. I certainly can't delete any of the purgeable data because I don't know how to identify it - and I wouldn't want to risk deleting any of it as I don't know if your obsevations are correct (sorry, I don't mean to imply you are wrong, just that I simply don't know enough about all this to really know one way or the other).

By all means, you are welcome to try the procedure above. You can work through these things rationally. I might be wrong and you won't be able to recover the purgeable data at all. Or I'm right and you will be able to eventually run that game with really poor performance. But my guess is that you are still holding out hope for some magical 3rd option. Everyone loves fairy tales, eh?

Would the OS purge purgeable data to make space for new data I wonder?

Absolutely, unequivocally - YES!

So that, in reality, I HAVE got about 250GB that I can use? Can anyone tell me this please?

Can anyone add to this please?

Tell you what? From your screenshots, you have a 1 TB hard drive. Maybe you should investigate what's going on with that other 750 GB first. That's where you'll be able to recover more useable space.

As I said before, there's no magic "fix it" button here. These are your options:

1) Delete files you don't need.

2) Live with a full hard drive and accept the limitations.

3) You can push it to the bitter end, maybe risk some corruption, and a performance hit

4) Erase the hard drive and start fresh

5) Buy a new computer with a 2 GB hard drive

Otterysteve wrote:

As I know what the contect of the deleted files was, I know that they are NOT system files or anything like that. Removing them (as I did) would have absolutely no detrimental effect on the running of my MBA. Yet, according to Disk Utility there are 434GB of "purgeable" data. This is clearly nonsense. That data is, as I said, data I created or downloaded, such a photographs taken with my cameras and phones, music I copied onto the MBA, music that I created and recorded, and other such self-generated data. NONE of it is system files or suchlike.

That available data will be released in a couple of days. When you delete files, they aren't actually deleted right away. The operating system will hold on to them for about 24 hours. During this time, they will be considered "purgeable".

But ironically enough, if you actually did run out of free space during this time, it won't actually purge any of this data. You still have to wait 1-2 days. So in other words, don't run out of free storage.