APFS incompatibilities and how to live with them
APFS has many of the features of modern file systems that make most efficient use of space, and others that aren’t found in more traditional file systems like HFS+. While those should all work well when used locally with other APFS volumes, they can prove incompatible with other file systems, and may have untoward side effects. This article looks at those features that you could encounter problems with, and how best to work around them.
Sparse files
Most modern file systems store files that contain significant amounts of blank or absent data in a special sparse file format. In APFS, this is implemented by only allocating file extents to those blocks that do contain data. In many cases, this can save significant disk space. Initially, sparse files were unusual in APFS, but over time they have become increasingly common, and can now be found in some types of disk image, virtual machine storage, and in some databases. Note that disk image types whose name includes the word sparse, sparse bundles and sparse disk images, don’t use sparse file format at all, but are the victims of an unfortunate name collision.
Neither macOS nor APFS can simply convert regular files to sparse format; for a file to be written in sparse format, the code writing it must explicitly skip the empty space. As a result, sparse files are prone to explode to full size when they’re copied or moved unless that’s between two local volumes both using APFS. Examples of where you should expect them to explode include:
copy or move to HFS+, as that has no sparse file format
copy between Macs using AirDrop or file sharing
back up to network storage, although local Time Machine backups to APFS should preserve them
copy or move to any other local file system other than APFS, even if that file system has its own support for sparse files.
In each of those cases, sparse files explode in size as they’re copied from the source volume. If you have a 100 GB sparse file that only takes 20 GB of local disk space, when it’s copied over the network, or to a local HFS+ volume, the full 100 GB has to be transferred.
One potential workaround is to compress the sparse file before transferring it. All good compression algorithms will work efficiently on the blank space in the file, so when compressed its size could be as small as the original sparse file. However, when it’s decompressed, even on another APFS volume, it will explode to full size. For disk images, that can be corrected by mounting them, as APFS will then trim their contents, and the disk image should be saved back into sparse format.
Clone files
These are two distinct files that share common data, normally the result of duplicating the original within the same APFS volume. Those two cloned files then only require the storage of the whole file, plus those data blocks that differ between them. This only works within the same volume, and the moment that either of the clones is moved or copied to any other volume, it assumes full size, as it can no longer share data with the other clone.
However, most other file systems don’t support file cloning in this way. When you duplicate a file in an HFS+ volume, there’s no shared data between them, and the two require twice the amount of space as one does.
Snapshots
Snapshots consist of a complete copy of the volume at an instant in time, so require a copy of the file system metadata for that volume, and retain copies of storage blocks for each file as they are changed subsequent to the snapshot being made, so you could roll back that volume to its state at the moment of the snapshot.
Although Time Machine backups contain snapshots of the volume they back up, snapshots can’t normally be copied to another disk or volume. Some have been able to make a complete copy of a disk including its snapshots using the dd command tool, but that should be considered experimental. In all other circumstances, snapshots stay where they were made, but you can always copy from an existing snapshot to reconstitute a volume.
Directory hard links
These aren’t available in APFS, but are supported in HFS+, where they’re used extensively in Time Machine backups. They work like regular hard links, but act on directories rather than individual files. They can’t be copied in any way to an APFS volume, but can be used to reconstitute a volume.
Extended attributes
These are additional metadata associated with files and folders, and are fully supported in both APFS and HFS+. However, many are treated as being ephemeral, and may not be preserved during copying or other actions. The system of flags used to determine which are preserved is detailed in this article.
Several other file systems also support extended attributes, but copying between them is unlikely to transfer them between file systems. In some cases, extended attributes are preserved using AppleDouble format, in which each file can have a hidden shadow with its name prefixed with ._ (dot-underscore) characters. These are most often seen in FAT and ExFAT volumes, but are prone to confuse users of other computers.
Key points
APFS sparse files are only preserved when copying or moving files between local APFS volumes. In other circumstances they explode to full size.
Good compression methods can keep a sparse file to a similar size, but decompression explodes them to full size. Disk images may then be restored as sparse files after they have been mounted again from APFS.
Clone files aren’t preserved when copied or moved to any other volume.
Snapshots can’t be copied at all, although they can be reconstituted as volumes.
APFS doesn’t support the directory hard links used in Time Machine backups to HFS+, but a backup can be reconstituted as a volume.
Extended attributes are preserved between APFS and HFS+, but not with other file systems, except in the shadow files of AppleDouble format, seen in FAT and ExFAT volumes.
