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-
-===============
-Subtree exports
-===============
-
-Normal Migration
-----------------
-
-The exporter begins by doing some checks in export_dir() to verify
-that it is permissible to export the subtree at this time. In
-particular, the cluster must not be degraded, the subtree root may not
-be freezing or frozen (\ie already exporting, or nested beneath
-something that is exporting), and the path must be pinned (\ie not
-conflicted with a rename). If these conditions are met, the subtree
-freeze is initiated, and the exporter is committed to the subtree
-migration, barring an intervening failure of the importer or itself.
-
-The MExportDirDiscover serves simply to ensure that the base directory
-being exported is open on the destination node. It is pinned by the
-importer to prevent it from being trimmed. This occurs before the
-exporter completes the freeze of the subtree to ensure that the
-importer is able to replicate the necessary metadata. When the
-exporter receives the MExportDirDiscoverAck, it allows the freeze to proceed.
-
-The MExportDirPrep message then follows to populate a spanning tree that
-includes all dirs, inodes, and dentries necessary to reach any nested
-exports within the exported region. This replicates metadata as well,
-but it is pushed out by the exporter, avoiding deadlock with the
-regular discover and replication process. The importer is responsible
-for opening the bounding directories from any third parties before
-acknowledging. This ensures that the importer has correct dir_auth
-information about where authority is delegated for all points nested
-within the subtree being migrated. While processing the MExportDirPrep,
-the importer freezes the entire subtree region to prevent any new
-replication or cache expiration.
-
-The warning stage occurs only if the base subtree directory is open by
-nodes other than the importer and exporter. If so, then a
-MExportDirNotify message informs any bystanders that the authority for
-the region is temporarily ambiguous. In particular, bystanders who
-are trimming items from their cache must send MCacheExpire messages to
-both the old and new authorities. This is necessary to ensure that
-the surviving authority reliably receives all expirations even if the
-importer or exporter fails. While the subtree is frozen (on both the
-importer and exporter), expirations will not be immediately processed;
-instead, they will be queued until the region is unfrozen and it can
-be determined that the node is or is not authoritative for the region.
-
-The MExportDir message sends the actual subtree metadata to the importer.
-Upon receipt, the importer inserts the data into its cache, logs a
-copy in the EImportStart, and replies with an MExportDirAck. The exporter
-can now log an EExport, which ultimately specifies that
-the export was a success. In the presence of failures, it is the
-existence of the EExport that disambiguates authority during recovery.
-
-Once logged, the exporter will send an MExportDirNotify to any
-bystanders, informing them that the authority is no longer ambiguous
-and cache expirations should be sent only to the new authority (the
-importer). Once these are acknowledged, implicitly flushing the
-bystander to exporter message streams of any stray expiration notices,
-the exporter unfreezes the subtree, cleans up its state, and sends a
-final MExportDirFinish to the importer. Upon receipt, the importer logs
-an EImportFinish(true), unfreezes its subtree, and cleans up its
-state.
-
-
-PARTIAL FAILURE RECOVERY
-
-
-
-RECOVERY FROM JOURNAL
-
-
-
-
-