Shuo-Jia commented on a change in pull request #695:
URL: https://github.com/apache/incubator-pegasus/pull/695#discussion_r594022492
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File path: rfcs/2021-02-22-disk-migrater.md
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+# Disk-Migrater
+
+## Design Goals
+Disk-Migrater is for migrating data among different local disks within one
node. This feature is different from node-rebalance that is for migrating data
among different nodes.
+
+## Flow Process
+Disk-Migrater operates by sending `RPC_REPLICA_DISK_MIGRATE` rpc to the
targeted node that triggers the node to migrate the specified replica from one
disk to another. The whole migration process is as follow:
+
+```
++---------------+ +---------------+ +--------------+
+| Client(shell) +------+ replicaServer +-------+ metaServer |
++------+--------+ +-------+-------+ +-------+------+
+ | | |
+ +------migrateRPC-----> +-----IDLE |
+ | | | (validate rpc)|
+ | | MOVING |
+ | | | (migrate data)|
+ | | MOVED |
+ | | | (rename dir) |
+ | | CLOSED |
+ | | | |
+ | +----- +<----LEARN<------------+
+ | | | |
+ | | | |
+ | LearnSuccess| |
+ | | | |
+ | | | |
+ | +----->+ |
+```
+
+1. The targeted node receives the migrateRPC and starts validating the request
arguments.
+2. If the RPC is valid, node starts migrating the specified replica.
+3. After replica migration finishes successfuly, the original replica will be
closed and ReplicaServer re-opens the new replica.
+4. If the new replica's data is inconsistent with its primary, MetaServer will
automatically start to trigger replica-learn to catch up with the latest data.
+5. After the learning process is completed, the entire disk-migration ends.
+
+## Replica States
+In the process of migration, the original replica and the new replica will
have different states as follow:
+| process |origin replica status[dir name] | new replica status[dir name] |
+|---|---|---|
+|IDEL |primary/secondary[gpid.pegasus] |--[--] |
+|START |secondary[gpid.pegasus] |--[--] |
+|MOVING |secondary[gpid.pegasus] |--[gpid.pegasus.disk.migrate.tmp] |
+|MOVED |secondary[gpid.pegasus] |--[gpid.pegasus.disk.migrate.tmp] |
+|CLOSED |error[gpid.pegasus.disk.migrate.ori] |--[gpid.pegasus] |
+|LEARNING |error[gpid.pegasus.disk.migrate.ori]
|potential_secondary[gpid.pegasus] |
+| COMPLETED |error[gpid.pegasus.disk.migrate.ori] |secondary[gpid.pegasus]
|
Review comment:
?, how format? now has been md fomat
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