Policies - usage errors

Certain misunderstandings that may be encountered while using policies and the suggestions to overcome such mistakes.

The following are common mistakes and misunderstandings encountered when dealing with policies:
  • You are advised to test your policy rules using the mmapplypolicy command with the -I test option.

    Also consider specifying a test-subdirectory within your file system. Do not apply a policy to an entire file system of vital files until you are confident that the rules correctly express your intentions. Even then, you are advised to do a sample run with the mmapplypolicy -I test command using the option -L 3 or higher, to better understand which files are selected as candidates, and which candidates are chosen. The -L flag of the mmapplypolicy command can be used to check a policy before it is applied. For examples and more information on this flag, see The mmapplypolicy -L command.

  • There is a 1 MB limit on the total size of the policy file installed in GPFS.
  • Ensure that all clocks on all nodes of the GPFS cluster are synchronized. Depending on the policies in effect, variations in the clock times can cause unexpected behavior.

    The mmapplypolicy command uses the time on the node on which it is run as the current time. Policy rules may refer to a file's last access time or modification time, which is set by the node which last accessed or modified the file. If the clocks are not synchronized, files may be treated as older or younger than their actual age, and this could cause files to be migrated or deleted prematurely, or not at all. A suggested solution is to use NTP to keep the clocks synchronized on all nodes in the cluster.

  • The rules of a policy file are evaluated in order. A new file is assigned to the storage pool of the first rule that it matches. If the file fails to match any rule, the file creation fails with an EINVAL error code. A suggested solution is to put a DEFAULT clause as the last entry of the policy file.
  • When a policy file is installed, GPFS verifies that the named storage pools exist. However, GPFS allows an administrator to delete pools that are mentioned in the policy file. This allows more freedom for recovery from hardware errors. Consequently, the administrator must be careful when deleting storage pools referenced in the policy.