GPFS: Administration and Programming Reference - IRA Home

gpfsMultipleAccessRange_t Structure
Name
gpfsMultipleAccessRange_t – Defines prefetching and write-behind file access for an application.
Library
GPFS Library (libgpfs.a for AIX, libgpfs.so for Linux)
Structure
typedef struct
{
offset_t blockNumber; /* data block number to access */
int start; /*start of range (from beginning of block)*/
int length; /* number of bytes in range */
int isWrite; /* 0 - READ access 1 - WRITE access */
char padding[4];
} gpfsRangeArray_t;
typedef struct
{
int structLen;
int structType;
int accRangeCnt;
int relRangeCnt;
gpfsRangeArray_t accRangeArray[GPFS_MAX_RANGE_COUNT];
gpfsRangeArray_t relRangeArray[GPFS_MAX_RANGE_COUNT];
} gpfsMultipleAccessRange_t;
Description
The gpfsMultipleAccessRange_t structure defines prefetching and write-behind access where the
application will soon access the portions of the blocks specified in accRangeArray and has finished
accessing the ranges listed in relRangeArray. The size of a block is returned in the st_blksize field of the
stat command, so the offset, OFF, of a file is in the block, OFF/st_blksize.
v Up to GPFS_MAX_RANGE_COUNT, as defined in the header file gpfs_fcntl.h, blocks may be given in
one multiple access range hint. The current value of GPFS_MAX_RANGE_COUNT is eight. Depending
on the current load, GPFS may initiate prefetching of some or all of the blocks.
v Each range named in accRangeArray that is accepted for prefetching, should eventually be released
with an identical entry in relRangeArray, or else GPFS will stop prefetching blocks for this file.
Note: Naming a subrange of a block in relRangeArray that does not exactly match a past entry in
accRangeArray has no effect, and does not produce an error.
v Applications that make random accesses or regular patterns not recognized by GPFS may benefit from
using this hint.
GPFS already recognizes sequential and strided file access patterns. Applications that use such
patterns should not need to use this hint, as GPFS automatically recognizes the pattern and performs
prefetching and write-behind accordingly. In fact, using the multiple access range hint in programs
having a sequential or strided access pattern may degrade performance due to the extra overhead to
process the hints.
Notice that the units of prefetch and release are file blocks, not file offsets. If the application intends to
make several accesses to the same block, it will generally get better performance by including the entire
range to be accessed in the GPFS_MULTIPLE_ACCESS_RANGE hint before actually doing a read or
360 GPFS: Administration and Programming Reference