IBM General Parallel File System for AIX: Administration and Programming Reference

gpfsMultipleAccessRange_t Structure

Name

gpfsMultipleAccessRange_t - Defines prefetching and write-behind file access for an application.

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 application 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.)

• 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.
• Each range named in accRangeArray that is accepted for prefetching, should eventually be released via 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 will not produce an error.

• 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 will automatically recognize the pattern and perform prefetching and write-behind accordingly. In fact, using the multiple access range hint in programs having a sequential or strided access pattern may actually 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 write. A sample program gpfsperf, which demonstrates the use of the GPFS_MULTIPLE_ACCESS_RANGE hint, is included in the GPFS product and installed in the /usr/lpp/mmfs/samples/perf directory.

Members

structLen

Length of the gpfsMultipleAccessRange_t structure.

structType

The hint identifier GPFS_MULTIPLE_ACCESS_RANGE.

accRangeCnt

On input, the number of ranges in accRangeArray.

On output, the number of processed ranges, the first n, of the given ranges.

relRangeCnt

The number of ranges in relRangeArray.

accRangeArray

The ranges of blocks that the application will soon access.

relRangeArray

The ranges of blocks that the application has finished accessing.