导致 Oracle 10gR2 出现 ORA-39095 的另一种原因

Oracle 10g R2 的 Data Pump 是一个好工具，弥补了传统 export/import 工具的很多不足。相关信息可以参考一下我以前对 Data Pump 的介绍。
最近在 Linux 平台上经常遇到 ORA-39095 错误。这个错误。这个错误号，文档的解释：

ORA-39095:Dump file space has been exhausted: Unable to allocate string bytes
Cause: The Export job ran out of dump file space before the job was completed.
Action: Reattach to the job and add additional dump files to the job restarting the job.

这个解释只针对一般情况。我遇到的这个案例，目录的空间还有很多，可是也一样报错了。而且，一般在第 100 个 导出文件出错(更正：%U参数是2位,定长的，最大99)。开始以为是 Bug，可是遍查 Metalink，发现和 ORA-39095 错误有关只有两条信息，和我遇到的情况不符。
偶然在阅读 Oracle Database Utilities 手册的时候，发现了这一段话：

Because each active worker process or I/O server process writes exclusively to one file at a time, an insufficient number of files can have adverse effects. Some of the worker processes will be idle while waiting for files, thereby degrading the overall performance of the job. More importantly, if any member of a cooperating group of parallel I/O server processes cannot obtain a file for output, then the export operation will be stopped with an ORA-39095 error. Both situations can be corrected by attaching to the job using the Data Pump Export utility, adding more files using the ADD_FILE command while in interactive mode, and in the case of a stopped job, restarting the job.

真是孤陋寡闻了，我的并行度用的是 4 ，减小这个并行度，该错误不再出现。

You can supply multiple file_name specifications as a comma-delimited list or in separate DUMPFILE parameter specifications. If no extension is given for the filename, then Export uses the default file extension of .dmp. The filenames can contain a substitution variable (%U), which implies that multiple files may be generated. The substitution variable is expanded in the resulting filenames into a 2-digit, fixed-width, incrementing integer starting at 01 and ending at 99. If a file specification contains two substitution variables, both are incremented at the same time. For example, exp%Uaa%U.dmp would resolve to exp01aa01.dmp, exp02aa02.dmp, and so forth.

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