Saturday, January 03, 2009
Making Resource-Fork-Aware Backups with rsync on Mac OS X
The Mac Resource Fork Problem:from http://www.quesera.com/reynhout/misc/rsync+hfsmode/:
Mac OS X uses the HFS+ filesystem, by default. HFS+ files are often composed of a data fork, a resource fork, and Finder metadata. The data and resource forks contain what you normally think of when you think of file information: data, program code, etc. Finder metadata includes information like file type and creator, comments, modification dates, locked and invisible status, and Finder colors.
Traditional UNIX filesystems only store a single stream of file data (the HFS+ equivalent of the data fork). Mac OS X (or Darwin, more precisely) is a genuine BSD UNIX, but with a nontraditional filesystem. Because of this, standard UNIX tools can only see certain portions of OS X files.
The difficulties caused by HFS/HFS+ aren't new. Early Mac users of BBSes and the Internet had the same problems when uploading files from a Mac for storage by other operating systems. Since the foreign OS had no way to store the additional HFS data, the uploads would be incomplete/useless. To solve this problem, Apple and others invented conversion formats that collected the full set of file information into a single data stream. Examples (some with varying and/or additional design goals) include AppleSingle, AppleDouble, MacBinary, BinHex, and Stuffit.
The relatively new development is OS X. Now that Mac users can run decades of software written for traditional UNIX machines, some of that software needs to be updated to work properly with HFS+ files.
One such standard UNIX tool is rsync, an excellent file synchronization utility which is also great for use as filesystem backup software. Rsync builds and runs without errors on OS X, but because it is unaware of resource forks and other metadata, it creates incomplete (and therefore corrupt) backups.
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This patch will make rsync HFS+ metadata-aware. Resource forks and Finder metadata are assembled on the sender into an ephemeral file in standard AppleDouble format, before being sent to the destination.
This method preserves disk space on both sides, with zero redundant data and only a small amount of overhead per file (~100 bytes of AppleDouble headers for each file that has a resource fork and/or HFS+ metadata). It works with any destination filesystem and operating system (tested with Solaris and Linux), and even with older or unpatched versions of rsync.
The Solution in Detail : Making Resource-Fork-Aware Backups with rsync on Mac OS X
Article Link posted by Edward at 9:38 AM
