XFS inodes are dynamically allocated on demand, rather than being
allocated at mkfs time. Chunks of 64 inodes are allocated at once, but
they are never freed. Over time, this can lead to filesystem
fragmentation, clusters of inodes and the btrees which point at them can
be scattered around the system.

By freeing clusters as they are emptied, we will reduce fragmentation of
the free space after removing files. This in turn will allow us to make
better placement decisions when repopulating a filesystem. The
XFSMNT_IDELETE mount option enables freeing clusters when they get empty.

Unfortunately a side effect of freeing inode clusters is that the inode
generation numbers of such inodes would be reset to zero when the cluster
is reclaimed. This is a problem in particular for a DMAPI enabled
filesystem as the the DMAPI handles need to be unique and persistent in
time. An unique DMAPI handle is built with the help of the inode
generation number. When the last one is prematurely reset by an inode
cluster reclaim, there is a high probability of different generation
inodes to end up having identical DMAPI handles.

To avoid the problem with identical DMAPI handles, the XFSMNT_IDELETE
mount option should be set as default, only if the filesystem is not
mounted with XFSMNT_DMAPI.

SGI-PV: 969192
SGI-Modid: xfs-linux-melb:xfs-kern:29486a
Signed-off-by: Vlad Apostolov <vapo@sgi.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Mark Goodwin <markgw@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>