Sparse image

A Sparse image is a type of disk image file that can be created under Mac OS X using Disk Utility. Encrypted sparse image files are used to secure a user's home directory by Mac OS X's FileVault feature.

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.

This can be advantageous when backing up data. For example, assume a user would like to make a backup of a hard disk volume named "INFO" that is 5GB in size, yet only contains 2.5GB of actual data. A single DVD-R (with around 4GB capacity) would be too small to store an INFO.img file of the 5GB hard disk. However, though a full-sized "INFO.dmg" image file will not fit on one (single-layer) DVD, an "INFO.sparseimage" image file will fit. This is because the unused 2.5GB from the original volume is not actually allocated within the sparse image file. The "INFO.sparseimage" file will occupy only 2.5GB.

Note that if the 2.5GB "INFO.sparseimage" backup on the DVD is subsequently opened and mounted as a "clone" of the source volume, the Finder will report the clone as having the original 5GB capacity, with 2.5GB available, but the virtual disk will be "read-only", because the source image file resides on read-only media. However, the first-generation 2.5GB "INFO.sparseimage" on a hard drive, when mounted, may have files added to it until the originally designated 5GB capacity is reached. The ".sparseimage" file will expand in size to accommodate the additional data. Deleting files from the mounted virtual volume will not result in a reduction in the size of the corresponding ".sparseimage" file. However, the size of the image can be reduced when the volume is not mounted.

Two limitations are therefore worth noting regarding the use of this image file format:
# 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.
# As noted above, while mounted ".sparseimage" image files automatically expand to their preassigned limit if and when data is added, they cannot be arbitrarily resized without using additional third-party software tools. A new ".sparseimage" file of the desired size must be created, and the source volume properly cloned onto it.

However, when the sparseimage file is "not" mounted, the .sparseimage file may be resized from the Terminal. The following example resizes a .sparseimage so that it can expand to a maximum size of 50 gigabytes:

hdiutil resize -size 50g MyFile.sparseimage

Similarly, a .sparseimage file that has expanded in size but then had files deleted, can be "compacted" to a smaller size with the following command:

hdiutil compact MyFile.sparseimage

Additional information about the hdiutil command may be seen by typing the following in the Terminal:

man hdiutil

Sparse Bundles

Mac OS X 10.5, Leopard, Introduced the concept of Sparse Bundles [http://macosx.com/article/live-filevaultsparse-bundle-backups-in-leopard.html 1] . In comparison to a Sparse Image, a Sparse Bundle is a directories full of banded data in 8M chunks. When you add, modify and remove files to the image, one or more of the band files will change, depending on where your data is stored. This allows for more efficient backups using Leopard's Time Machine feature.

References

* [http://docs.info.apple.com/article.html?artnum=152051 Mac OS X 10.3 Help] , apple.com