Add ReadAhead configuration support

[billziss-gh]

Enhancement Request

Windows includes the CcSetReadAheadGranularityEx DDI and the older CcSetReadAheadGranularity DDI, which can be used to influence the operating system read-ahead policy for a particular file. This functionality should be exposed to user mode file systems in some form.

• This may be implemented on a per-file basis (CcSetReadAheadGranularity* works on a FILE_OBJECT) or on a per-filesystem basis (e.g. via FSP_FSCTL_VOLUME_PARAMS).

  • Maximum flexibility might be provided by a mix of the two. Consider a ReadAhead field in FSP_FSCTL_VOLUME_PARAMS which defines a file system default (with 0 meaning the operating system default policy, i.e. the current behavior). In addition a file system may choose to provide a different read-ahead for a particular file (-- perhaps encoded as a power of 2 in the bitfields of FSP_FSCTL_TRANSACT_RSP::Rsp.Create.Opened, since read-ahead should be a power of 2 in general).

• FUSE read-ahead support should be added via a command-line option.

• .NET support should be added as well.

• See issue Support for streaming large files with lookahead sshfs-win#104, which requested such support for SSHFS-Win.

[tracymacding]

have not implement yet？