bdb log file 预设长度的性能优化

postgres 同理的code：

backend/cdb/cdblogsync.c, createZeroFilledNewFile()

    /*
     * 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, , sizeof(zbuffer));

看代码随手记：log_put.c, __log_write()

    /*
     * 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 ==  && !__os_fs_notzero()) {
#else
    if (lp->w_off == ) {
#endif
        (void)__db_file_extend(env, dblp->lfhp, lp->log_size);
        if (F_ISSET(dblp, DBLOG_ZERO))
            (void)__db_zero_extend(env, dblp->lfhp,
                 , lp->log_size/lp->buffer_size, lp->buffer_size);

    }

我的理解：在flush log时使用fdatasync， 若log文件长度发生变化， 则仍需要写文件 metadata。

https://linux.die.net/man/2/fdatasync

fdatasync() is similar to fsync(), but does not flush modified metadata unless that metadata is needed in order to allow a subsequent data retrieval to be correctly handled. For example, changes to st_atime or st_mtime (respectively, time of last access and time of last modification; see stat(2)) do not require flushing because they are not necessary for a subsequent data read to be handled correctly. On the other hand, a change to the file size (st_size, as made by say ftruncate(2)), would require a metadata flush.