Reading and writing

When reading and writing to files, the VFS server is responsible for maintaining file position and for having access checks performed.

Each call to v_rdwr() must specify:

The file offset from which the operation is to start. This differs from the POSIX API, where the LFS maintains a file cursor.
Whether security access checks are to be performed. If the VFS server maintains sufficient state information to associate a sequence of reads and writes from the same end user, it can limit these access checks to that end user's first reference, thus improving performance.

Additionally, the VFS server may request a "sync on write", which forces the current write, and all previously written data, to be saved to disk before v_rdwr() returns.