Backup folder on APFS - Does it take extra space?

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Level 1

40 points

Backup folder on APFS - Does it take extra space?

A while ago on another post someone seemed to say this, but I want to confirm:

If I have a file and I make an exact copy (while using a disk formatted as APFS), the space on disk doesn't double. It seems that the macOS knows that the second file is just a copy of the first.

So if I have a 1TB disk and my file is 100GB, I have 900GB free. If I make a copy of the file, it won't show me that I have 800GB, it will still have 900GB (give or take).

Is this true?

Context: I would like to create, along with my physical backup disks, a remote backup using iCloud/Dropbox/GoogleDrive, but for that those files have to be on my computer and I don't want to take more space, so I though that I can still create the backup on one of those folders and that won't take more space. Hope it's making sense?

Any help is greatly appreciated.

MacBook Pro 13″, macOS 10.15

Posted on Dec 11, 2023 9:11 AM

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Answer 1

John Galt

Level 10

157,936 points

Dec 11, 2023 11:55 AM in response to tiagorocha

tiagorocha wrote:

A while ago on another post someone seemed to say this, but I want to confirm:

If I have a file and I make an exact copy (while using a disk formatted as APFS), the space on disk doesn't double. It seems that the macOS knows that the second file is just a copy of the first.

So if I have a 1TB disk and my file is 100GB, I have 900GB free. If I make a copy of the file, it won't show me that I have 800GB, it will still have 900GB (give or take).

Is this true?

Yes. If you duplicate a file (files, folders, etc) the duplicate will consume exactly zero additional APFS disk space. You can duplicate it twice, ten times, a million times. No difference.

Context: I would like to create, along with my physical backup disks, a remote backup using iCloud/Dropbox/GoogleDrive, but for that those files have to be on my computer and I don't want to take more space, so I though that I can still create the backup on one of those folders and that won't take more space. Hope it's making sense?

Yes, although I confess I don't understand a perceived need to duplicate items before uploading them to some remote server. All I can tell you though is a remote / "cloud" based backup pales in comparison to a local backup. Different people have different needs, so have to develop and test your own disaster recovery plan.

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Answer 2

Dec 13, 2023 12:14 PM in response to tiagorocha

Yes, but will iCloud end up storing two separate copies since only APFS has this data deduplication feature? I would expect two copies to be stored in the cloud, but I don't use iCloud so am unable to test.

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Answer 3

Dec 13, 2023 11:08 AM in response to tiagorocha

tiagorocha wrote:

A while ago on another post someone seemed to say this, but I want to confirm:

If I have a file and I make an exact copy (while using a disk formatted as APFS), the space on disk doesn't double. It seems that the macOS knows that the second file is just a copy of the first.

So if I have a 1TB disk and my file is 100GB, I have 900GB free. If I make a copy of the file, it won't show me that I have 800GB, it will still have 900GB (give or take).

Is this true?

I'm not sure how macOS handles this situation. I think the Free space will only reflect a single copy being stored since the Free storage space is the most important value. However, you may see the "Used" space using more space, possibly so that if you want to transfer everything to another location you will know how much storage space will be needed. I've never had the time to test this out since you would need to test it with a significantly large enough file/folder to be sure what is occurring.

Keep in mind "Free" space is different from "Available" space. Available space is very misleading and is not what most people think it is. Free space is what you want to monitor and the only place in macOS where this Free space value is specifically listed is in Disk Utility when viewing the information of the "Data" volume.

Context: I would like to create, along with my physical backup disks, a remote backup using iCloud/Dropbox/GoogleDrive, but for that those files have to be on my computer and I don't want to take more space, so I though that I can still create the backup on one of those folders and that won't take more space. Hope it's making sense?

If you are transferring everything to another physical drive, then those items will become separate items which will end up taking up more space. I'm not sure how copying to another APFS volume would work, but I suspect it would require more space unless macOS is actually tracking the duplicated data.

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Answer 4

tiagorocha Author

Level 1

40 points

Dec 13, 2023 11:17 AM in response to HWTech

the disk and volume would be the same. In this case I will be using the iCloud folder (or the Google Drive folder, depending on which service I pick) so from what I was able to gather online it seems that APFS creates clones of files (like an alias), it doesn't duplicate them. And when the original file is changed, only the changes are added to the disk not the whole file, which is great

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Answer 5

tiagorocha Author

Level 1

40 points

Dec 13, 2023 12:47 PM in response to HWTech

I'm not 100% sure I understand your question, but let me explain what I'm trying to achieve:

I work locally on my disk, let's say ~/Documents/Projects

As I mentioned, I have physical disks with backups that I run everyday, multiple times a day, and I rotate them, meaning I run a backup now on 2 disks (DiskA and DiskB), then I go home I take those 2 disks and replace the ones I have there (DiskC and DiskD) with the ones that are updated (DiskA and DiskB). Then when I get to the studio I run backups on DiskC and DiskD. So I keep doing this 2-3 times a day. This is more than enough to create a safe backup system for me.

But, just in case, I would also like to send those files from ~/Documents/Projects to iCloud (~/Library/Mobile Documents/com~apple~CloudDocs/Projects) just to make sure that I have a copy of those files somewhere that's really remote.

So the disk and volume is the same for ~/Documents/Projects and the iCloud folder, meaning it will be creating clones and, apparently, not taking extra space (according to what I read about APFS), since the files are an exact copy of those in ~/Documents/Projects.

Hope it makes sense

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