"Time Machine is a backup utility, not an archival utility, it is not intended as offline storage. Time Machine captures the most recent state of your data on your disk. As snapshots age, they are prioritized progressively lower compared to your more recent ones."[3]

For backups to a network drive, Time Machine allows the user to back up Apple Mac computers through Apple's AirPort networking, and supports backing up to certain network attached storage devices or servers, depending on the version of Time Machine. Earlier versions worked with a wide variety of NAS servers, but later versions require the server to support a recent version of Apple's Apple Filing Protocol (AFP), and Time Machine no longer works with servers using the Server Message Block (SMB) protocol typical for Windows servers. Some of the legacy support can be re-enabled by using hand-tuned configuration options, accessed through the Terminal. Apple's Time Capsule acts as anetwork storage device specifically for Time Machine backups, allowing both wired and wireless backups to the Time Capsule's internal hard drive. Time Machine may alternatively be used with any external or internal volume.

Time Machine saves hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for everything older than a month until the volume runs out of space. At that point, Time Machine deletes the oldest weekly backup.

User interface[edit]

Time Machine's user interface when retrieving a file uses Apple's Core Animation API. Upon its launch, Time Machine "floats" the active Finder or application window from the user's desktop to a backdrop depicting the user's blurred desktop wallpaper. Behind the current active window are stacked windows, with each window representing a snapshot of how that folder or application looked on the given date and time in the past. When toggling through the previous snapshots, the stacked windows extend backwards, giving the impression of flying through a "time tunnel." While paging through these "windows from the past," a previous version of the data (or presently deleted data) may be retrieved.

Storage[edit]

Time Machine works with:

local storage (volumes directly connected to the local machine, and formatted with HFS+)

remote storage (network resources shared from other Macs, from Time Capsule or compatible device)

The significant difference is that when using remote storage, Time Machine uses sparse bundles; this acts as an isolation layer, which makes the storage neutral to the actual file system used by the network server, and also allows to replicate the backup from one storage to another.

Sparse bundles are mounted by OS X like any devices, presenting their content as a HFS+ formatted volume, functionally similar to a local storage.

Operation[edit]

Time Machine creates a folder on the designated Time Machine volume (local or inside a remote sparsebundle) into which it copies the directory tree of all locally attached disk drives, except for files and directories that the user has specified to omit, including the Time Machine volume itself. Every hour thereafter, it creates a new subordinate folder and copies only files that have changed since the last backup and creates hard links to files that already exist on the backup drive. A user can browse the directory hierarchy of these copies as if browsing the primary disk.[4]

Some other backup utilities save deltas for file changes, much like version control systems. Such an approach permits more frequent backups of minor changes, but can often complicate the interaction with the backup volume. By contrast, it is possible to manually browse a Time Machine backup volume without using the Time Machine interface; the use of hard links presents each backup to the user as a complete disk copy.

Time Machine appears to create multiple hard links to unmodified directories. Multiple linking of directories is different from conventional UNIX operating systems. As a result, tools like rsync cannot be used to replicate a Time Machine volume; replication can only reliably be done by imaging the entire filesystem.

Apple system events record when each directory is modified on the hard drive. This means that instead of examining every file's modification date when it is activated, Time Machine only needs to scan the directories that changed for files to copy (the remainder being hard-linked). This differs from the approach taken by similar backup utilities rsyncand FlyBack, which examine modification dates of all files during backup.