A System or Data volume that isn’t correctly firmlinked to its partner simply can’t function.Īs these firmlinks are created during the creation of the paired volumes, adding them later may not be possible. In Big Sur and later, System and Data volumes are joined together by special bi-directional firmlinks, which are crucial to their function. This can be used in a two-step process on a disk image made from the original volume, or directly.Ĭloning a general-purpose APFS volume should be relatively straightforward using asr, but when it comes to the volumes most users want to clone, the System or Data volume from a macOS boot volume group, this becomes more complex. Because APFS volumes can have snapshots and inaccessible directories, and in special circumstances aren’t free-standing from other volumes, there’s now only one tool capable of cloning volumes, Apple Software Restore, asr. Because it’s usually performed between two different storage units, it isn’t normally expected that contents will be stored at exactly the same locations, and in the past it has been used to defragment data and free space, for instance by cloning a volume to another disk, then cloning it back to the original.Ĭloning HFS+ is fairly straightforward, and has been longstanding practice which isn’t reliant on specific tools. Most faithful copying occurs when the source and destination storage use the same file system, and no network transfer is involved, but methods of copying should always preserve the data in the file.Ĭloning a volume or a whole disk is a special form of copying in which the aim is to create a perfect replica of the original, including a file system identical in every respect, with identical data and metadata. Some xattrs are never copied, others always, provided the destination supports some means of storing them, which varies between different file systems. Non-standard file metadata, particularly extended attributes (xattrs), are even less predictable in their behaviour. That could mean that a 20 MB original is copied as a 50 GB file full of empty space. Do the same from an APFS volume to HFS+, which doesn’t support the format, and the sparse file explodes to its full size. As that copy changes from its original, APFS will normally store changed data, until eventually the data of those two files are completely separate.Ĭopy a sparse file within the same APFS volume, or to another APFS volume, and it should remain in this special space-saving format. Much of this stems from the capabilities of different file systems, and it’s useful to consider some examples from APFS.Ĭopy a file within an APFS volume and you’ll normally not duplicate the data at all, but end up with a clone file, which temporarily shares the same data in storage. Although copying should always be faithful to the data in files, it’s less reliable when it comes to its metadata and the precise representation of the data. You can copy individual files from one folder to another, to a different storage medium, and between remote systems. This is the generic term, and applies widely to many different processes. This article explains how they differ, and their consequences. There’s a tendency to use the terms copy, clone and backup as if they mean essentially the same thing. When making copies or backups of our files, it’s vital to make clear distinctions between different methods.
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