Performance monitoring with statistics

WiredTiger optionally maintains a variety of statistics, when the statistics configuration string is specified to wiredtiger_open; see Statistics for general information about statistics, and Statistics Data for information about accessing the statistics.

The resulting statistics can be displayed using the wtstats visualization tool. For more information, see Visualizing performance with wtstats.

Note that maintaining run-time statistics involves updating shared-memory data structures and may decrease application performance.

The statistics gathered by WiredTiger can be combined to derive information about the system's behavior. For example, a cursor can be opened on the statistics for a table:

        if ((ret = session->open_cursor(session,
            "statistics:table:access", NULL, NULL, &cursor)) != 0)
                return (ret);

Then this code calculates the "fragmentation" of a table, defined here as the percentage of the table that is not part of the current checkpoint:

        uint64_t ckpt_size, file_size, percent;
        ret = get_stat(cursor, WT_STAT_DSRC_BLOCK_CHECKPOINT_SIZE, &ckpt_size);
        ret = get_stat(cursor, WT_STAT_DSRC_BLOCK_SIZE, &file_size);

        percent = 0;
        if (file_size != 0)
                percent = 100 * ((file_size - ckpt_size) / file_size);
        printf("Table is %" PRIu64 "%% fragmented\n", percent);

The following example calculates the "write amplification", defined here as the ratio of bytes written to the filesystem versus the total bytes inserted, updated and removed by the application.

        uint64_t app_insert, app_remove, app_update, fs_writes;

        ret = get_stat(cursor, WT_STAT_DSRC_CURSOR_INSERT_BYTES, &app_insert);
        ret = get_stat(cursor, WT_STAT_DSRC_CURSOR_REMOVE_BYTES, &app_remove);
        ret = get_stat(cursor, WT_STAT_DSRC_CURSOR_UPDATE_BYTES, &app_update);

        ret = get_stat(cursor, WT_STAT_DSRC_CACHE_BYTES_WRITE, &fs_writes);

        if (app_insert + app_remove + app_update != 0)
                printf("Write amplification is %.2lf\n",
                    (double)fs_writes / (app_insert + app_remove + app_update));

Both examples use this helper function to retrieve statistics values from a cursor:

int
get_stat(WT_CURSOR *cursor, int stat_field, uint64_t *valuep)
{
        const char *desc, *pvalue;
        int ret;

        cursor->set_key(cursor, stat_field);
        if ((ret = cursor->search(cursor)) != 0)
                return (ret);

        return (cursor->get_value(cursor, &desc, &pvalue, valuep));
}