39722: Acronis Files Connect: When I save a file using SMB protocol, what information is saved in the "dot-underscore" (._) files?

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Last update: 13-07-2017


When I save a file using SMB protocol, what information is saved in the "dot-underscore" (._) files? How is this information stored on an NTFS file system?



Answer: Macintosh files have 2 parts;

The data fork: This is where the data is saved by an application. It is the equivalent of a "file" on the PC.

The resource fork: This hidden file fork contains additional information about the file depending upon what application created it. For example, BBedit, a text editing application, stores the text of a document in the data fork like any other application, but also saves the location of the cursor in the resource fork so that the next time you open the document the cursor will be right where you left off. Although different applications use the resource fork for different purposes, in general, it is used to store additional information about a file beyond the generic data.

In addition to the two forks that make up a Macintosh file, the Mac maintains information within the file system itself about each file. This information is called "metadata" (data about data). The metadata, maintained by Mac OS X, is required to provide the full Macintosh user experience. It includes information such as owner, type, creator, date modified, date created, name, label, and visibility. Some of this metadata (such as creation date and owner) is common to both Windows and the Mac file systems. Other metadata (such as type, creator and label) are unique to just the Macintosh.

Example file types are "text" or "JPEG". Applications can use this file "type" information to determine how to handle the file when it is opened. The "creator" metadata tells a Mac what application created the file and should be used to open the file when it is double-clicked by the user.

How is this Mac specific metadata and resource fork stored on a Windows NTFS file system?

When connecting to a volume that does not support Macintosh metadata or resource forks such as an SMB/CIFS share, the Mac client will use a special format called AppleDouble to encode the metadata and resource fork. As one would expect from the name, the AppleDouble format uses two files, the data fork and another file that contains the combined metadata and resource fork information.

This second file is the so called "dot-underscore" (._) file. Because the dot-underscore file contains the metadata for the file in addition to the resource fork, if this information is lost, you will lose the linkage between the file & it's creating application. In cross platform environments this is a frequent occurrence because when Windows users move the file to a new folder, the dot-underscore will not be moved at the same time. Also it is a fairly common occurrence for a Windows user to delete the "._" extra file with a similar name, because they do not know what it is!

How does Acronis Files Connect Improve Macintosh File Compatibility?

Acronis Files Connect saves all of the Macintosh file information as a single file on the Windows NTFS file system.

Acronis Files Connect supports Macintosh resource forks and metadata using Windows NTFS alternate data streams. Acronis Files Connect use of alternate data streams is compatible with Microsoft's "Services for Macintosh," meaning that files saved using SFM can be shared by Acronis Files Connect without the needs to migrate or convert the data. However, Acronis Files Connect offers also supports the 3.x versions of the Apple Filing Protocol and that Mac OS X introduced.

Unlike the SMB format that uses two files to keep track of the data fork, resource fork, and metadata, Acronis Files Connect uses one file that has a main data stream, and two alternate data streams. These streams are the main stream for the data fork, the AFP_Info stream that contains metadata about the file (finder info), and the AFP_Resource stream that contains the resource fork if one is present. All files created by the Mac will have an AFP_Info stream to maintain the Finder information, but not all files will have a resource fork. Because the NTFS file system shows all of these data streams as one file, when a Windows user moves this file to a new folder, the entire file including all three streams will be moved to the new folder and all of the Macintosh metadata such as type and creator will be maintained.