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Chapter 7. Communicating file access patterns to GPFS
GPFS attempts to recognize the pattern of accesses that an application makes to an open file, and
optimizes its behavior accordingly. For example, GPFS can recognize sequential reads and therefore
prefetch blocks in advance of when they are required by the application. However, in many cases GPFS
does not recognize the access pattern of the application, or cannot optimize its data transfers. In these
situations, performance may improve if the application explicitly discloses aspects of its access pattern to
GPFS using the gpfs_fcntl() subroutine. These subroutines are exploited by MPI Version 2.0.
The gpfs_fcntl() subroutine allows application programs to pass two classes of file access information
giving GPFS an opportunity to improve throughput and latency of file system requests:
1. Hints
2. Directives
Hints allow an application to disclose its future accesses to GPFS. Hints are always optional. Adding or
removing hints from a program, even incorrectly specified hints, will never alter the meaning of a program.
Hints can only affect the performance of an application. The hints that may be passed to GPFS are:
1. gpfsAccessRange_t
2. gpfsClearFileCache_t
3. gpfsFreeRange_t
4. gpfsMultipleAccessRange_t
Note: GPFS is free to silently ignore a hint if system resources do not permit the hint to be processed.
In contrast, directives are stronger than hints. They may affect program semantics and must be either
carried out by GPFS or return an error. The directives which may be passed to GPFS are:
1. gpfsCancelHints_t
2. gpfsDataShipMap_t
3. gpfsDataShipStart_t
4. gpfsDataShipStop_t
To communicate hints and directives to GPFS, an application program builds a data structure in memory,
then passes it to GPFS. This data structure consists of:
v A fixed length header, mapped by gpfsFcntlHeader_t.
v Followed by a variable number of, and any combination of, hints and directives.
Hints and directives may be mixed within a single gpfs_fcntl() subroutine, and are performed in the
order that they appear. A subsequent hint or directive may cancel out a preceding one.
The header and hints and directives that follow it are defined as C structures.
The gpfs_fcntl() subroutine takes the handle of the opened file as its first parameter and the address of
the data structure as its second parameter. For complete definitions of the gpfs_fcntl() subroutine, the
header, hints, directives, and other operations, see “gpfs_fcntl() Subroutine” on page 284.
© Copyright IBM Corp. 1998, 2006 57