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APFS is a copy on write filesystem if you use the right APIs, so it does what you describe but only for entire files. I believe as soon as you change a single

by MBCook 2y ago

APFS is a copy on write filesystem if you use the right APIs, so it does what you describe but only for entire files. I believe as soon as you change a single bite you get a complete copy that’s your own. And that’s how this program works. It finds perfect duplicates and then effectively deletes and replaces them with a copy of the existing file so in the background there’s only one copy of the bits on the disk.

mintplant 2y ago
I suppose this means that you could find yourself unexpectedly out of disk space in unintuitive ways, if you're only trying to change one byte in a cloned file but there isn't enough space to copy its entire contents?
- pansa777 2y ago
  It doesn't work like you think. If you change one byte of duplicated file - the only "byte" will be changed on disk (a "byte", because, technically is not a byte, but a block). As far as I understand, it works like a reflink feature in the modern linux FSs. If so, thats really cool, and thats also a bit better than the zfs's snapshots. Iam newbie on macos, but it looks amazing
  - jonhohle 2y ago
    That’s true as long as the writing application only writes to blocks that have changed. Is a VM tool going to write blocks, or write multi-MB segments of a sparse image that can be swapped atomically? Unfortunately, once a file changes there are no APIs to check which blocks are still shared (at least there weren’t as of macOS 13).
- MBCook 2y ago
  I’m not sure if it works on a file or block level for CoW, but yes. However APFS gives you a number of space related foot-guns if you want. You can overcommit partitions, for example. It also means if you have 30 GB of files on disk that could take up anywhere from a few hundred K to 30 GB of actual data depending on how many dupes you have. It’s a crazy world, but it provides some nice features.
tonyedgecombe 2y ago
> I believe as soon as you change a single bite you get a complete copy that’s your own. I think it stores a delta: https://en.m.wikipedia.org/wiki/Apple_File_System#Clones https://en.m.wikipedia.org/wiki/Apple_File_System#Clones
alwillis 2y ago
That’s not how this works. Nothing is deleted. It creates zero-space clones of existing files. https://en.wikipedia.org/wiki/Apple_File_System?wprov=sfti1#Clones https://en.wikipedia.org/wiki/Apple_File_System?wprov=sfti1#...

by MBCook 2y ago

mintplant 2y ago

I suppose this means that you could find yourself unexpectedly out of disk space in unintuitive ways, if you're only trying to change one byte in a cloned file but there isn't enough space to copy its entire contents?

pansa777 2y ago
It doesn't work like you think. If you change one byte of duplicated file - the only "byte" will be changed on disk (a "byte", because, technically is not a byte, but a block). As far as I understand, it works like a reflink feature in the modern linux FSs. If so, thats really cool, and thats also a bit better than the zfs's snapshots. Iam newbie on macos, but it looks amazing
- jonhohle 2y ago
  That’s true as long as the writing application only writes to blocks that have changed. Is a VM tool going to write blocks, or write multi-MB segments of a sparse image that can be swapped atomically? Unfortunately, once a file changes there are no APIs to check which blocks are still shared (at least there weren’t as of macOS 13).
MBCook 2y ago
I’m not sure if it works on a file or block level for CoW, but yes. However APFS gives you a number of space related foot-guns if you want. You can overcommit partitions, for example. It also means if you have 30 GB of files on disk that could take up anywhere from a few hundred K to 30 GB of actual data depending on how many dupes you have. It’s a crazy world, but it provides some nice features.

tonyedgecombe 2y ago

> I believe as soon as you change a single bite you get a complete copy that’s your own. I think it stores a delta: https://en.m.wikipedia.org/wiki/Apple_File_System#Clones https://en.m.wikipedia.org/wiki/Apple_File_System#Clones

alwillis 2y ago

That’s not how this works. Nothing is deleted. It creates zero-space clones of existing files. https://en.wikipedia.org/wiki/Apple_File_System?wprov=sfti1#Clones https://en.wikipedia.org/wiki/Apple_File_System?wprov=sfti1#...