Streamline mount/unmount for dehydrating folders #1894
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In microsoft#1890 I demonstrated that it's possible to dehydrate folders without unmounting at all. Unfortunately that requires deleting all the placeholders and hydrated files which is too slow for dehydrating anything substantial. The current implementation avoids this by moving instead of deleting (which has the additional benefit of providing a backup) but ProjFS doesn't support moving or renaming folders, so we can't do that while mounted. This pull request takes a different approach to reducing the overhead of unmounting for dehydration. Instead of unmounting, moving, and remounting from the dehydrate verb, those steps are moved into the mount process under its dehydrate message handler. The mount process only disposes and recreates the components required for virtualization, avoiding several costly steps (eg authentication with Azure DevOps, verification of the cache server, verification of ProjFS installation). For the repo I work in, dehydrating a top-level directory is reduced from 33 seconds to 11 seconds with this change. Specific changes: * Backup of non-src folders (.git, .gvfs) is added to dehydrate folders. Previously it was only done for full dehydrate. * Unmount, move/backup of folders, and mount are moved from DehydrateVerb to InProcessMount. To support this, the DehydrateFolders message has the backup folder added to its fields. * The core methods of Mount and Unmount have a parameter added to skip disposing (on unmount) and initialization (on mount) of certain components which are ok to leave alive during the temporary unmount. * Ownership of GVFSContext disposal fixed - FileSystemCallbacks was disposing it despite not owning it. * Missing disposal of a file stream in BackgroundFileSystemTaskRunner is fixed. * WindowsFileSystemVirtualizer.DehydrateFolder will now delete a tombstone file for the directory if present. This allows us to support fixing a directory that the user manually deleted while mounted (perhaps in a misguided attempt to dehydrate it), though that would typically require running 'gvfs dehydrate --no-status' to skip verifying that the working directory matches the index. * '--no-status' is now supported with '--folders'
ShiningMassXAcc
approved these changes
Jan 7, 2026
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ShiningMassXAcc
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Not an expert on this codebase, but LGTM
YingyHuang
approved these changes
Jan 8, 2026
wilbaker
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Jan 9, 2026
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| private bool TryBackupNonSrcFiles(ITracer tracer, GVFSEnlistment enlistment, string backupRoot) |
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For my own understanding, what was the reason for backing up the non-src folders as well?
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The .gvfs/* and .git/index files are modified during this operation - my thought was, why aren't they backed up like they are for full (rather than folder) dehydrate?
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Ahh, I missed that they're backed up for full. The dehydrate procedure is not something I'm super familiar with :)
Thanks!
wilbaker
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Jan 9, 2026
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In #1890 I demonstrated that it's possible to dehydrate folders without unmounting at all. Unfortunately that requires deleting all the placeholders and hydrated files which is too slow for dehydrating anything substantial. The current implementation avoids this by moving instead of deleting (which has the additional benefit of providing a backup) but ProjFS doesn't support moving or renaming folders, so we can't do that while mounted.
This pull request takes a different approach to reducing the overhead of unmounting for dehydration. Instead of unmounting, moving, and remounting from the dehydrate verb, those steps are moved into the mount process under its dehydrate message handler. The mount process only disposes and recreates the components required for virtualization, avoiding several costly steps (eg authentication with Azure DevOps, verification of the cache server, verification of ProjFS installation). For the repo I work in, dehydrating a top-level directory is reduced from 33 seconds to 11 seconds with this change.
Specific changes: