On 19 Apr 2020, at 18:40, Matt Neuburg wrote:

I’m not sure I believe that. Safe save is in fact one of the boasts of APFS. You ask for safe save, which causes the file system to clone the file and now you get copy-on-write automatically, which is far more efficient because only the changed blocks actually need to be written. When you’re done saving, the original is atomically swapped with the clone. m.

Before APFS we had exchangedata which atomically swaps the data part of two files. Majority of applications derive no advantage from first cloning the file to be updated, they simply write out a new file and then swap it with the old one. In-place/partial update of a file during save is for applications where the file may be too big to hold in memory, like a disk image or a database backend, but require a specially crafted file format to support this.

After APFS we have to use rename for atomically overwriting the old file, but that has the disadvantages I listed in my email.

What you may have seen regarding atomic save and APFS is that Apple extended rename to also allow atomic renaming of directories even when the destination exist. This allows atomic save of bundles, which previously was not possible. But it is unrelated to my grief about APFS dropping support for exchangedata.