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