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BypassIO in storage drivers

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About BypassIO

BypassIO is an optimized I/O path for reading from files. The goal of this path is to reduce the CPU overhead of doing reads, which helps to meet the I/O demands of loading and running next-generation games on Windows. BypassIO is a part of the infrastructure to support DirectStorage on Windows, and is available starting in Windows 11.

It's important that storage drivers implement support for BypassIO, and that you keep BypassIO enabled as much as possible. Without storage stack support, game performance is degraded, resulting in a poor gaming experience for end users.

There will be broader application uses beyond gaming in future Windows releases.

IOCTL_STORAGE_MANAGE_BYPASS_IO and an equivalent FSCTL_MANAGE_BYPASS_IO were added as a part of this infrastructure. IOCTL_STORAGE_MANAGE_BYPASS_IO is sent by file systems to the volume/storage stacks, while minifilters process FSCTL_MANAGE_BYPASS_IO. These control codes are designed to be diagnosable: they both return the identity of the driver that failed the BypassIO request, and the reason for vetoing it.

See BypassIO in filter drivers for BypassIO system architecture details across the file system filter and storage stacks.

Scope of BypassIO support

Starting in Windows 11, BypassIO is supported as follows:

  • Only on Windows client systems. Server system support will be added in a future release.

  • Only on NVMe storage devices. Support for other storage technologies will be added in a future release.

  • Only on the NTFS file system. Support for other file systems will be added in a future release.

  • Only noncached reads are supported. Support for noncached writes will be added in a future release.

  • Only supported on files (not supported on directory or volume handles).

DDIs changes and additions for BypassIO

The following DDIs relevant to storage drivers were added to provide BypassIO support:

What storage stack drivers need to do to support BypassIO

Starting in Windows 11, storage driver developers need to update their driver's INF or MANIFEST files to add the StorageSupportedFeatures infrastructure with STORAGE_SUPPORTED_FEATURES_BYPASS_IO for BypassIO as follows:

  • Define a "Parameters" key under your Service definition
  • Add a DWORD value named "StorageSupportedFeatures", and set this value to 0x1 to indicate BypassIO support.

This registry key indicates to the system that the driver understands bypass I/O. The driver must also call StorPortSetUnitAttributes with BypassIOSupported set to 1 to indicate which logical unit (disk) supports bypass I/O.

The driver then processes IOCTL_STORAGE_MANAGE_BYPASS_IO as needed. The FS_BPIO_OP_QUERY operation fails if there's a storage stack driver that didn't opt in.

Note

A driver that can never support BypassIO should still add the StorageSupportedFeatures state to the INF and then veto appropriately inside the driver, specifying the reason.

If a storage driver doesn't update its INF or MANIFEST file to indicate BypassIO support, all BypassIO operations on that volume or storage driver are immediately blocked. The system falls back to the traditional I/O path, which results in degraded game performance.

IOCTL_STORAGE_MANAGE_BYPASS_IO implementation details

The file system (currently NTFS) generates an IOCTL_STORAGE_MANAGE_BYPASS_IO control code in response to a generated FSCTL_MANAGE_BYPASS_IO as needed.

Input to IOCTL_STORAGE_MANAGE_BYPASS_IO is similar to its FSCTL_MANAGE_BYPASS_IO counterpart, but only supports BypassIO enabling, disabling, and querying.

Output from IOCTL_STORAGE_MANAGE_BYPASS_IO is similar to its FSCTL_MANAGE_BYPASS_IO counterpart, identifying the failing driver's name and reason, and operation status as to why the driver vetoed BypassIO. The file system propagates the IOCTL_STORAGE_MANAGE_BYPASS_IO output from the volume and storage stacks up to FSCTL_MANAGE_BYPASS_IO.