Mounts

Mounts are a way to specify filesystem scopes and rules for those scopes. They can be referred to by name in build specs as a way to abstract specification of common locations. Mounts are specified in the DScript configuration file for the build.

Example mount definition

config({
    mounts: [
        {
            // Unique name for the mount
            name: a`pictures`,

            // Path of the directory to mount (mount point)
            path: p`data/pictures`,

            // Mount is readable
            isReadable: true,

            // Mount is a system folder
            isSystem: false,

            // Mount is scrubbable. You can have "input" files that are not
            // registered in the current build graph, or "output" files that
            // BuildXL can delete.
            isScrubbable: true,

            // Mount is not writable
            isWriteable: false,

            // File changes are tracked for this mount by hashing file contents
            trackSourceFileChanges: true
        }
    ]
});

Mount permissions

The isReadable and isWritable mount settings do not automatically give all pips access to read/write files without specifying those accesses. They are more like rule for what pips are allowed to specify. For example the pictures mount above has the isReadable property set to true. That means you may define a pip that reads from a path under this mount. However if a pip does not specify the read it makes under pictures, it will still get a Disallowed File Access at runtime.

System Mounts

BuildXL has a few behavioral changes for system mounts. The primary one is that it may tokenize those paths. So if a system mount is at different locations on disk across different machines, they still may get cache hits if using the same shared cache.

Default mounts

BuildXL ships with the following built-in mounts. See MountsTable.cs for a full accounting of default system mounts based on your platform.

Name	Description	Writable?	Scrubbable?
"BuildEnginePath"	layout.BuildXLBinDirectory	no	no
"SourceRoot"	layout.SourceDirectory	no	no
"ObjectRoot"	layout.ObjectDirectory	yes	yes
"TempRoot"	layout.TempDirectory	yes	yes
"LogsDirectory"	layout.ObjectDirectory	yes	no