Storage Foundation 7.4.1 Administrator's Guide - Linux

Multiple placement classes in <ON> clauses of CREATE statements and in <TO> clauses of RELOCATE statements

Both the <ON> clause of the CREATE statement and the <TO> clause of the RELOCATE statement can specify priority ordered lists of placement classes using multiple <DESTINATION> XML elements. VxFS uses a volume in the first placement class in a list for the designated purpose of file creation or relocation, if possible. If no volume in the first listed class has sufficient free space or if the file system's volume set does not contain any volumes with that placement class, VxFS uses a volume in the second listed class if possible. If no volume in the second listed class can be used, a volume in the third listed class is used if possible, and so forth.

The following example illustrates of three placement classes specified in the <ON> clause of a CREATE statement:

 <CREATE>
   <ON>
     <DESTINATION>
       <CLASS>tier1</CLASS>
     </DESTINATION>
     <DESTINATION>
       <CLASS>tier2</CLASS>
     </DESTINATION>
     <DESTINATION>
       <CLASS>tier3</CLASS>
     </DESTINATION>
   </ON>
 </CREATE>

In this statement, VxFS would allocate space for newly created files designated by the rule's SELECT statement on tier1 volumes if space was available. If no tier1 volume had sufficient free space, VxFS would attempt to allocate space on a tier2 volume. If no tier2 volume had sufficient free space, VxFS would attempt allocation on a tier3 volume. If sufficient space could not be allocated on a volume in any of the three specified placement classes, allocation would fail with an ENOSPC error, even if the file system's volume set included volumes in other placement classes that did have sufficient space.

The <TO> clause in the RELOCATE statement behaves similarly. VxFS relocates qualifying files to volumes in the first placement class specified if possible, to volumes in the second specified class if not, and so forth. If none of the destination criteria can be met, such as if all specified classes are fully occupied, qualifying files are not relocated, but no error is signaled in this case.