HDFS QUOTAS（hdfs配额）

1    overview：The Hadoop Distributed File System (HDFS) allows the administrator to set quotas for the number of names used and the amount of space used for individual directories. Name quotas and space quotas operate independently, but the administration and implementation of the two types of quotas are closely parallel.

     管理员可以设置名字和空间配额，名字配额和空间配额操作是独立的

2   name quotas：The name quota is a hard limit on the number of file and directory names in the tree rooted at that directory. File and directory creations fail if the quota would be exceeded. Quotas stick with renamed directories; the rename operation fails if operation would result in a quota violation. The attempt to set a quota will still succeed even if the directory would be in violation of the new quota. A newly created directory has no associated quota. The largest quota is Long.Max_Value. A quota of one forces a directory to remain empty. (Yes, a directory counts against its own quota!)

Quotas are persistent with the fsimage. When starting, if the fsimage is immediately in violation of a quota (perhaps the fsimage was surreptitiously modified), a warning is printed for each of such violations. Setting or removing a quota creates a journal entry.

      配额的最大值是Long.Max_Value，配额持久化在fsImage中。如果fsImage违反了配额，当启动时违反一个配额打印一条警告信息。

3   space quotas：The space quota is a hard limit on the number of bytes used by files in the tree rooted at that directory. Block allocations fail if the quota would not allow a full block to be written. Each replica of a block counts against the quota. Quotas stick with renamed directories; the rename operation fails if the operation would result in a quota violation. A newly created directory has no associated quota. The largest quota is Long.Max_Value. A quota of zero still permits files to be created, but no blocks can be added to the files. Directories don't use host file system space and don't count against the space quota. The host file system space used to save the file meta data is not counted against the quota. Quotas are charged at the intended replication factor for the file; changing the replication factor for a file will credit or debit quotas.

Quotas are persistent with the fsimage. When starting, if the fsimage is immediately in violation of a quota (perhaps the fsimage was surreptitiously modified), a warning is printed for each of such violations. Setting or removing a quota creates a journal entry.