sparsebundles

Sparsebundles are a type of disk image format used primarily on macOS. They store the contents of a virtual disk as a directory (a bundle) rather than as a single file. The bundle can grow in size as data is written, making sparsebundles efficient for backups and flexible storage.

Inside the .sparsebundle directory, metadata and a collection of data blocks are kept, organized into multiple fixed-size data files commonly referred to as bands. The disk image appears as a single mounted volume to the operating system, even though its data is distributed across many band files. As you add data to the image, new bands are created up to a defined maximum size, allowing the image to expand without needing to rewrite existing data.

Sparsebundles are supported by the macOS tools for creating disk images, such as Disk Utility and the hdiutil command-line tool. They are often used for network backups, such as Time Machine backups on network shares, because the bundle can be moved and stored as a single item.

Limitations include that sparsebundles are macOS-centric; while the files can be copied to other systems, other OSes may not natively read the format. They also rely on the integrity of the bundle's directory and band files; damage to a single band can affect the image. For resilience, it is common to back up the entire bundle and to ensure the hosting file system supports extended attributes and sparse files.

Creating one can be done with commands like: hdiutil create -size 10g -type SPARSEBUNDLE -fs 'APFS' -volname DiskName DiskName.sparsebundle