Copy-on-write on APFS

APFS (like many modern file systems but unlike its predecessor HFS+) supports copy-on-write. This means you can logically copy a file – it looks and behaves like a distinct file – but it doesn’t immediately copy the file’s contents on disk – it merely shares them with the original. Only if and as you modify… Read more

Creating a Fusion Drive from an existing disk, without erasing it

Photo of Phil Schiller on stage presenting Apple's Fusion Drive technology, at Apple's iPad Mini event on October 23rd, 2012.

Note: this guide was written circa macOS 10.13 High Sierra, in 2018. Its accuracy has not been verified for newer macOS releases. Curiously there’s very little information out on the web (at time of writing) on how to create (or expand) a Fusion Drive without erasing its contents first.  It’s entirely possible to do so… Read more

Silent data corruption

Alternate title:  Apple’s file system engineers are sadly naive. I was quite disappointed to see that APFS isn’t even trying to provide data integrity.  Data integrity is kind of step 0 of any file system, and checksums or use of ECC is pretty much standard in modern & leading-edge file systems.  APFS doesn’t want to… Read more