Veritas File System 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Application Interface
Cache Advisories
Chapter 460
Cache Advisories
VxFS allows an application to set cache advisories for use when accessing files. These advisories are in
memory only and they do not persist across reboots. Some advisories are currently maintained on a per-file,
not a per-file-descriptor, basis. This means that only one set of advisories can be in effect for all accesses to
the file. If two conflicting applications set different advisories, both use the last advisories that were set.
All advisories are set using the VX_SETCACHE ioctl command. The current set of advisories can be
obtained with the VX_GETCACHE ioctl command. For details on the use of these ioctl commands, see
the vxfsio (7) manual page.
Direct I/O
Direct I/O is an unbuffered form of I/O. If the VX_DIRECT advisory is set, the user is requesting direct data
transfer between the disk and the user-supplied buffer for reads and writes. This bypasses the kernel
buffering of data, and reduces the CPU overhead associated with I/O by eliminating the data copy between
the kernel buffer and the user’s buffer. This also avoids taking up space in the buffer cache that might be
better used for something else. The direct I/O feature can provide significant performance gains for some
applications.
For an I/O operation to be performed as direct I/O, it must meet certain alignment criteria. The alignment
constraints are usually determined by the disk driver, the disk controller, and the system memory
management hardware and software. The requirements for direct I/O are as follows:
• The starting file offset must be aligned to a 512-byte boundary.
• The ending file offset must be aligned to a 512-byte boundary, or the length must be a multiple of 512
bytes.
• The memory buffer must start on an 8-byte boundary.
If the I/O is performed using the readv(2) and writev(2) system calls, these restrictions apply to each
element of the array of structiovec.
The requirements to perform direct I/O on a given platform and operating system release may be less
restrictive than above, but these requirements are met, then direct I/O will work on any platform. In
particular, Solaris and HP-UX do not require any alignment of the memory buffer.
Also note that on HP-UX, direct I/O will be the most efficient if the starting and ending file offsets are
aligned on file system block boundaries, as reported in the field f_frsize of statvfs(2).