bdblogfile预设长度的性能优化

    /*
     * Zero-fill the file.    We have to do this the hard way to ensure that all
     * the file space has really been allocated --- on platforms that allow
     * "holes" in files, just seeking to the end doesn't allocate intermediate
     * space.  This way, we know that we have all the space and (after the
     * fsync below) that all the indirect blocks are down on disk.    Therefore,
     * fdatasync(2) or O_DSYNC will be sufficient to sync future writes to the
     * log file.
     */
    MemSet(zbuffer, 0, sizeof(zbuffer));

    /*
     * If we're writing the first block in a log file on a filesystem that
     * guarantees unwritten blocks are zero-filled, we set the size of the
     * file in advance.  This increases sync performance on some systems,
     * because they don't need to update metadata on every sync.
     *
     * Ignore any error -- we may have run out of disk space, but that's no
     * reason to quit.
     */
#ifdef HAVE_FILESYSTEM_NOTZERO
    if (lp->w_off == 0 && !__os_fs_notzero()) {
#else
    if (lp->w_off == 0) {
#endif
        (void)__db_file_extend(env, dblp->lfhp, lp->log_size);
        if (F_ISSET(dblp, DBLOG_ZERO))
            (void)__db_zero_extend(env, dblp->lfhp,
                 0, lp->log_size/lp->buffer_size, lp->buffer_size);

    }