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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
+
+
+
+
+