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.

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

        printf("File is %d%% fragmented\n",
            (int)(100 * (file_size - ckpt_size) / file_size));

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));
}