Raid常用代码片段备忘

DistributedRaidFileSystem其实就是装饰者设计模式中的装饰者，对DistributedFileSystem进行了封装，关键是DistributedRaidFileSystem.ExtFSDataInputStream.ExtFsInputStream对FSInputStream.read(byte[] b, int offset, int len)封装，捕获BlockMissingException，ChecksumException用unraid恢复。

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Configuration使用了DistributedRaidFileSystem，要得到DistributedFileSystem：

??? FileSystem fs = (new Path(path)).getFileSystem(conf);
??? // if we got a raid fs, get the underlying fs
??? if (fs instanceof DistributedRaidFileSystem) {
????? fs = ((DistributedRaidFileSystem) fs).getFileSystem();
??? }

??? // check that we have a distributed fs
??? if (!(fs instanceof DistributedFileSystem)) {
????? throw new IOException("expected DistributedFileSystem but got " +
??????????????? fs.getClass().getName());
??? }

??? final DistributedFileSystem dfs = (DistributedFileSystem) fs;