[svn-r5550] Purpose:

    Code Improvement

Description:
    Split the metadata accumulator code into two parts: one for allowing writes
    of the accumulator buffer during reads (when the buffer is dirty and needs
    to be flushed to disk in order to hold the new metadata being read in) and
    another for only allowing writes of the buffer during writes.

    This allows the MPI-I/O VFL driver to use the metadata accumulator (but
    only during writes) and benefit from the reduced number of metadata I/O
    operations that it brings.

Platforms tested:
    IRIX64 6.5 (modi4) w/parallel

1 files changed, 9 insertions, 0 deletions

diff --git a/src/H5FDmpio.c b/src/H5FDmpio.c
index 0ec8db3..2d81469 100644
--- a/src/H5FDmpio.c
+++ b/src/H5FDmpio.c
@@ -944,6 +944,15 @@ H5FD_mpio_query(const H5FD_t *_file, unsigned long *flags /* out */)
     if(flags) {
         *flags=0;
         *flags|=H5FD_FEAT_AGGREGATE_METADATA; /* OK to aggregate metadata allocations */
+
+        /* Distinguish between updating the metadata accumulator on writes and
+         * reads.  This is particularly (perhaps only, even) important for MPI-I/O
+         * where we guarantee that writes are collective, but reads may not be.
+         * If we were to allow the metadata accumulator to be written during a
+         * read operation, the application would hang.
+         */
+        *flags|=H5FD_FEAT_ACCUMULATE_METADATA_WRITE; /* OK to accumulate metadata for faster writes */
+
         *flags|=H5FD_FEAT_AGGREGATE_SMALLDATA; /* OK to aggregate "small" raw data allocations */
     } /* end if */