APFS : new Apple iOS filesystem

Apple Quietly Upgrades the File System of Millions of iOS Devices

"With the release of iOS 10.3 to the general public, millions of devices will be upgraded to use Apple’s new file system, APFS,
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APFS does not normalize Unicode filenames, or the fact that APFS allows multiple files whose names differ only in case to exist in the same directory.
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the migration does not actually moves data, rather it writes APFS metadata into HFS+ free space. This approach has the advantage that if anything goes wrong with the conversion, the user is still left with a valid HFS+ partition."