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Subject [GitHub] [drill] paul-rogers commented on a change in pull request #1953: Add docs for Drill Metastore
Date Tue, 04 Feb 2020 03:12:29 GMT
paul-rogers commented on a change in pull request #1953: Add docs for Drill Metastore
URL: https://github.com/apache/drill/pull/1953#discussion_r374455079
 
 

 ##########
 File path: _docs/performance-tuning/drill-metastore/030-drill-iceberg-metastore.md
 ##########
 @@ -0,0 +1,69 @@
+---
+title: "Drill Iceberg Metastore"
+parent: "Drill Metastore"
+date: 2020-01-31
+---
+
+Drill uses Iceberg Metastore implementation based on [Iceberg tables](http://iceberg.incubator.apache.org).
For Drill 1.17,
+ this is default Drill Metastore implementation. For details on how to configure Iceberg
Metastore implementation and
+ its option descriptions, please refer to [Iceberg Metastore docs](https://github.com/apache/drill/blob/master/metastore/iceberg-metastore/README.md).
+
+{% include startnote.html %}
+Iceberg table supports concurrent writes and transactions but they are only effective on
file systems that support
+ atomic rename.
+If the file system does not support atomic rename, it could lead to inconsistencies during
concurrent writes.
+{% include endnote.html %}
+
+### Iceberg Tables Location
+
+Iceberg tables will reside on the file system in the location based on
+Iceberg Metastore base location `drill.metastore.iceberg.location.base_path` and component
specific location.
+If Iceberg Metastore base location is `/drill/metastore/iceberg`
+and tables component location is `tables`. Iceberg table for tables component
+will be located in `/drill/metastore/iceberg/tables` folder.
+
+Metastore metadata will be stored inside Iceberg table location provided
+in the configuration file. Drill table metadata location will be constructed
+based on specific component storage keys. For example, for `tables` component,
+storage keys are storage plugin, workspace and table name: unique table identifier in Drill.
+
+Assume Iceberg table location is `/drill/metastore/iceberg/tables`, metadata for the table
+`dfs.tmp.nation` will be stored in the `/drill/metastore/iceberg/tables/dfs/tmp/nation` folder.
+
+Example of base Metastore configuration file `drill-metastore-override.conf`, where Iceberg
tables will be stored in
+ hdfs:
+
+```
+drill.metastore.iceberg: {
+  config.properties: {
+    fs.defaultFS: "hdfs:///"
+  }
+
+  location: {
+    base_path: "/drill/metastore",
+    relative_path: "iceberg"
+  }
+}
+```
+
+### Metadata Storage Format
+
+Iceberg tables support data storage in three formats: Parquet, Avro, ORC. Drill metadata
will be stored in Parquet files.
+This format was chosen over others since it is column oriented and efficient in terms of
disk I/O when specific
+columns need to be queried.
+
+Each Parquet file will hold information for one partition. Partition keys will depend on
Metastore
+component characteristics. For example, for tables component, partitions keys are storage
plugin, workspace,
+table name and metadata key.
+
+Parquet files name will be based on UUID to ensure uniqueness. If somehow collision occurs,
modify operation
+in Metastore will fail.
+
+### Iceberg metadata expiration
+
+Iceberg table generates metadata for each modification operation:
+snapshot, manifest file, table metadata file. Also when performing delete operation,
+previously stored data files are not deleted. These files with the time can occupy lots of
space.
+Two table properties `write.metadata.delete-after-commit.enabled` and `write.metadata.previous-versions-max`
+control expiration process. Metadata files will be expired automatically if
+`write.metadata.delete-after-commit.enabled` is enabled.
 
 Review comment:
   Maybe we should take a set back. Sounds like Iceberg maintains multiple versions of each
file stored in Iceberg. This allows Drill to read a prior version of the file while ANALYZE
TABLE creates a new version (true? Do we have such synchronization? If not, it is a bug.)
   
   Similarly, if a file is delete, prior versions remain.
   
   Therefore Iceberg requires that Drill periodically remove old file versions. Drill does
this on a time interval defined by ....
   
   Note that none of the above ensure atomic operation: we can always come up with a scenario
in which updates happen faster than readers. This is a bug that needs fixing.

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