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发表于 29-12-2011 07:19 PM
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Time Machine creates a folder on the designated Time Machine volume which is named the current date and time. It then copies all locally attached drives (except for files and directories that it has specifically been told not to copy, including the Time Machine volume itself) to the folder. Every hour thereafter, it creates a new folder on the remote drive using the same naming scheme, but instead of making another complete copy of the primary hard drive, Time Machine instead only backs up files that have changed and creates hard links to files that already exist on the remote drive. A user can browse these "versions" of the primary drive and see each file as if it were right where it was left.
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 software's use of hard links makes each backup session appear to the user like a full backup, rather than an incremental or "delta" backup.
Time Machine appears to create multiple hard links to unmodified directories. Multiple linking of directories is quite different from conventional UNIX.
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 rsync and FlyBack, which examine modification dates of all files during backup.
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