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PowerScale OneFS supports storing Apple resource fork data transparently in an alternate data stream, also called a named stream. The named stream prevents creation of AppleDouble files, increases macOS performance when writing to OneFS, and helps prevent file corruption.
To better understand this feature, see the following background information.
When macOS stores a file, that file is composed of two forks. There is a resource fork which contains extended attributes about the file itself, such as the file’s icon. There is also a data fork which contains the file data. When macOS writes data to local file systems such as HFS+ or APFS-formatted volumes, the resource fork data is stored in an alternate data stream. The resource fork is not visible to users or systems accessing those files; only the file itself is shown with all its associated metadata.
When macOS writes to a storage system that does not support alternate data streams, the resource fork data still needs to be stored somewhere. In this case, macOS stores the resource fork data in AppleDouble files. These files begin with ._ followed by the file name. For example, saving a file called file.dat results in two files being created on the file system: file.dat and ._file.dat. These “extra” files degrade performance because it increases the file operations on the target storage system. File corruption can also become an issue if a non-macOS system overwrites or deletes the AppleDouble files.
As mentioned previously in this section, OneFS supports storing resource fork data in an alternate data stream, which prevents the creation of AppleDouble files. It is the default behavior of macOS 10.5+ to use an alternate data stream on an SMB server if the feature is available (as it is on OneFS). When writing through NFS, macOS will revert to storing resource fork data as AppleDouble files (another good reason always to use SMB with macOS).
OneFS intelligently manages resource fork data so that even non-Apple systems connected by NFS can interact with files created or used by SMB-connected macOS systems.
It is not necessary to force macOS to use alternate data streams (the default since macOS version 10.5). Nonetheless, some administrators feel more comfortable explicitly enabling this feature in /etc/nsmb.conf by adding the following line:
streams=yes
The following archived support document outlines this functionality: Mac OS X v10.5, v10.6: About named streams on SMB-mounted NAS, Mac OS X, and Windows servers; "-36" or "-50" alerts may appear.