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https://issues.apache.org/jira/browse/HDDS-638?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16657038#comment-16657038
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Mukul Kumar Singh commented on HDDS-638:
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Thanks for taking a look at the patch [~jnp].
With this patch, the snapshot file generated for TermIndex (t:1, i:4) is of the
name sm/snapshot.1_4.
So the snapshot file is unique each time, Also as part of Raft code,
ServerState#initStatemachine, the last applied index is read from the term
index, which is inturn formed from the file name.
The following log snippet explain the file names and recovery on restart.
{code}
2018-10-17 21:39:19,532 INFO ratis.ContainerStateMachine
(ContainerStateMachine.java:takeSnapshot(195)) - Taking snapshot at
termIndex:(t:1, i:4)
2018-10-17 21:39:19,534 INFO ratis.ContainerStateMachine
(ContainerStateMachine.java:takeSnapshot(198)) - Taking a snapshot to file
/Users/msingh/code/apache/ozone/ozone_review/hadoop-ozone/tools/target/test-dir/MiniOzoneClusterImpl-65253a3a-c5ca-4e5e-b7b5-4ed4bcc92c99/datanode-0/data/ratis/a85420d0-71d2-497b-8890-b0689bf55a6e/sm/snapshot.1_4
2018-10-17 21:39:19,534 INFO impl.RaftServerImpl (ServerState.java:close(381))
- 84aca8dc-41f8-4a21-a957-8d2cdd51d75e closes. The last applied log index is 4
{code}
After restart, the 4 is the loaded index.
{code}
2018-10-17 21:39:26,535 INFO ratis.ContainerStateMachine
(ContainerStateMachine.java:addRequest(452)) - Setting the last applied index
to 4
{code}
> enable ratis snapshots for HDDS datanodes
> -----------------------------------------
>
> Key: HDDS-638
> URL: https://issues.apache.org/jira/browse/HDDS-638
> Project: Hadoop Distributed Data Store
> Issue Type: Bug
> Components: Ozone Datanode
> Affects Versions: 0.3.0
> Reporter: Mukul Kumar Singh
> Assignee: Mukul Kumar Singh
> Priority: Blocker
> Attachments: HDDS-638.001.patch
>
>
> Currently on a restart, a hdds datanode, starts applying log entries from the
> start of the log.
> This should can be avoided by taking a ratis snapshot to persist the last
> stable state and on restart the datanodes start applying log from that index.
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