branch-2.0.4-alpha/hadoop-hdfs-project/hadoop-hdfs/src/site/apt/HdfsQuotaAdminGuide.apt.vm

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  HDFS Quotas Guide
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HDFS Quotas Guide

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* 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.

* 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.

* 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.

* Administrative Commands

   Quotas are managed by a set of commands available only to the
   administrator.

     * <<<dfsadmin -setQuota <N> <directory>...<directory> >>>

       Set the name quota to be N for each directory. Best effort for each
       directory, with faults reported if N is not a positive long
       integer, the directory does not exist or it is a file, or the
       directory would immediately exceed the new quota.

     * <<<dfsadmin -clrQuota <directory>...<directory> >>>

       Remove any name quota for each directory. Best effort for each
       directory, with faults reported if the directory does not exist or
       it is a file. It is not a fault if the directory has no quota.

     * <<<dfsadmin -setSpaceQuota <N> <directory>...<directory> >>>

       Set the space quota to be N bytes for each directory. This is a
       hard limit on total size of all the files under the directory tree.
       The space quota takes replication also into account, i.e. one GB of
       data with replication of 3 consumes 3GB of quota. N can also be
       specified with a binary prefix for convenience, for e.g. 50g for 50
       gigabytes and 2t for 2 terabytes etc. Best effort for each
       directory, with faults reported if N is neither zero nor a positive
       integer, the directory does not exist or it is a file, or the
       directory would immediately exceed the new quota.

     * <<<dfsadmin -clrSpaceQuota <directory>...<director> >>>

       Remove any space quota for each directory. Best effort for each
       directory, with faults reported if the directory does not exist or
       it is a file. It is not a fault if the directory has no quota.

* Reporting Command

   An an extension to the count command of the HDFS shell reports quota
   values and the current count of names and bytes in use.

     * <<<fs -count -q <directory>...<directory> >>>

       With the -q option, also report the name quota value set for each
       directory, the available name quota remaining, the space quota
       value set, and the available space quota remaining. If the
       directory does not have a quota set, the reported values are <<<none>>>
       and <<<inf>>>.