Sunday, February 12, 2017

Testing Out Snapshots in APFS

Adam H. Leventhal:

It’s 2017, and Apple already appears to be making good on its promise with the revelation that the forthcoming iOS 10.3 will use APFS. The number of APFS tinkerers using it for their personal data has instantly gone from a few hundred to a few million. Beta users of iOS 10.3 have already made the switch apparently without incident. They have even ascribed unscientifically-significant performance improvements to APFS.


We figured out the proper use of the fs_snapshot system call and reconstructed the WWDC snapUtil. But all this time an equivalent utility has been lurking on macOS Sierra. If you look in /System/Library/Filesystems/apfs.fs/Contents/Resources/, Apple has included a number of APFS-related utilities, including apfs_snapshot (and, tantalizingly, a tool called hfs_convert).


After the volume is mounted again, not only are the contents reverted (to an empty directory in this case), but any snapshots taken after the snapshot used for the revert operation are deleted as well. One might expect APFS snapshot revert to immediately take effect and restore the contents of the volume to the previous state. Some technical issues likely make that challenging, such as what to do about programs that have files within in that volume open. So seeing if and how Apple decides to expose this functionality will be interesting.

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