Sparse image

Sparse Disk Image

Filename extension	.sparseimage
Internet media type	application/octet-stream
Uniform Type Identifier (UTI)	com.apple.disk-image-sparse
UTI conformation	com.apple.disk-image
Developed by	Apple Inc.
Type of format	disk image
Container for	file system objects

A sparse image is a type of disk image file used on Mac OS X that grows in size as the user adds data to the image, taking up only as much disk space as stored in it. Encrypted sparse image files are used to secure a user's home directory by the FileVault feature in Mac OS X Snow Leopard and earlier. Sparse images can be created using Disk Utility.

Unlike a full image file (.dmg), which takes up as much actual space as the real disk it represents (regardless of the amount of unused space), a sparse image file (.sparseimage) takes up only as much actual disk space as the data contained within, up to a maximum of the capacity assigned during creating.

Limitations

Two limitations are therefore worth noting regarding the use of this image file format:

1. A customized ".sparseimage" image file can be assigned a larger total capacity than the physical volume (or HD partition) on which it originally resides. While the virtual volume will seem to make that capacity available, attempting to exceed the physical capacity of the underlying volume will result in a disk error: "ran out of space". The .sparseimage file must first be moved to a larger physical disk or partition.
2. As noted above, while mounted .sparseimage image files automatically expand to their preassigned limit when data is added, they cannot be arbitrarily resized without the use of Disk Utility, hdiutil, or other such software.

Sparse bundle disk images

Mac OS X v10.5 (Leopard) introduced the concept of the sparse bundle. Instead of a monolithic file, a sparse bundle is a bundle (directory) containing a number of files called bands, each in the order of 8 MB in size. This means even though to the end user the sparse bundle appears as a single file, it is composed of smaller files. As of Mac OS X 10.8, the bands are 8.4 MB each. When the content of the image changes, one or more band files is changed, created, or deleted. This allows backup software (such as Time Machine) to operate more efficiently. You may use a tool such as rsync to keep your disk image(s) consistent across various systems.