Migrate Metadata within a Filesystem

Lustre 2.8 includes a feature to migrate metadata (directories and inodes therein) between MDTs, but it doesn't support striped directory. Lustre 2.12 adds this support. The migration command can only be performed on directory, though it will migrate both directory and its sub entries recursively. for example, to migrate contents of /testfs/testdir from its current MDTs (MDT0 and MDT1) to MDT1 and MDT3, the command is as follows:

$ lfs migrate -m 1,3 /testfs/testdir

Metadata migration will migrate file dirent and inode to other MDTs, but won't touch file data. After migration file will have a new FID, so if a Lustre system is reexport to NFS, the files to migrate will become inaccessible during and after migration, you'll have to stop NFS service before migration, and start again after migration. During migration, directory and its sub files can be accessed like normal ones. Migration may fail for various reasons, e.g., MDT restart, or disk full, in this case some of the sub files may have been migrated, while others are still in the old place, still they can be accessed normally, and the same command can be executed again when issues are fixed to finish this migration, but you can't abort a failed migration, or migrate to other MDTs different from previous migration command,