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new 17c0843 doc: add disk-migrater rfc (#695)
17c0843 is described below
commit 17c0843c5018b3e32961fbe7bbb5a92d0c7b12d7
Author: Jiashuo <[email protected]>
AuthorDate: Thu Mar 18 17:08:55 2021 +0800
doc: add disk-migrater rfc (#695)
---
rfcs/2021-02-22-disk-migrator.md | 66 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 66 insertions(+)
diff --git a/rfcs/2021-02-22-disk-migrator.md b/rfcs/2021-02-22-disk-migrator.md
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+# Disk-Migrator
+
+## Design Goals
+Disk-Migrator is used 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-Migrator 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]
|
+
+**Note:**
+* If replica status is `primary`, you need assign it as `secondary` manually
via [propose](http://pegasus.apache.org/administration/rebalance).
+* Once any process is failed, the operation will be failed and reverted the
`IDEL` status.
+
+## Client Command
+The client sending migrateRPC is
[admin-cli](https://github.com/pegasus-kv/admin-cli) which supports
`disk-capacity`, `disk-replica` and `disk-migrate` commands.
+
+Use `help` to see the command manuals. For example:
+```
+# query replica capacity
+disk-capacity -n node -d disk
+# query replica count
+disk-replica -n node -d disk
+# migrate data
+disk-migrate -n node -g gpid -f disk1 -t disk2
+```
+
+It should be noticed that disk migration is currently a manual operation. It's
in our future plan to design a disk-rebalance planner. It can generate a series
of steps, which automatically migrate data and eventually make all disks
balanced.
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