Metastore Management

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Alluxio stores most of its metadata on the master node. The metadata includes the filesystem tree, file permissions, and block locations. Alluxio provides two ways to store the metadata:

  • ROCKS: an on-disk, RocksDB-based metastore
  • HEAP: an on-heap metastore

The default metastore is the ROCKS metastore.

RocksDB Metastore

The ROCKS metastore uses a mixture of memory and disk. Metadata eventually gets written to disk, but a large, in-memory cache stores recently-accessed metadata. The cache buffers writes, asynchronously flushing them to RocksDB as the cache fills up. The default cache size is dynamically set to be (JVM Heap size / 4KB). In the default configuration of 8GB heap size, Alluxio master will cache 2 million inodes. The cache performs LRU-style eviction, with an asynchronous evictor evicting from a high water mark (default 85%) down to a low water mark (default 80%).

You can explicitly configure Alluxio to use the ROCKS metastore by adding this setting in conf/alluxio-site.properties for the master nodes.

alluxio.master.metastore=ROCKS

Configuration Properties

  • alluxio.master.metastore.dir: A local directory for writing RocksDB data. Default: {alluxio.work.dir}/metastore, e.g. /opt/alluxio/metastore
  • alluxio.master.metastore.inode.cache.max.size: A hard limit on the number of entries in the on-heap inode cache. Increase this to improve performance if you have spare master memory. The default value for this configuration is dynamically set to be (JVM Heap size / 4KB). For example, when xmx setting is set to the default 8GB, Alluxio master will cache 2 million inodes.

Advanced Tuning Properties

These tuning parameters primarily affect the behavior of the cache.

  • alluxio.master.metastore.inode.cache.evict.batch.size: Batch size for flushing cache modifications to RocksDB. Default: 1000
  • alluxio.master.metastore.inode.cache.high.water.mark.ratio: Ratio of the maximum cache size where the cache begins evicting. Default: 0.85
  • alluxio.master.metastore.inode.cache.low.water.mark.ratio: Ratio of the maximum cache size that eviction will evict down to. Default: 0.8

Metrics and memory usage

Alluxio exposes all RocksDB metrics found here. Alluxio uses one RocksDB database for blocks and one for inodes. Both databases have their own set of metrics. The naming of such metrics follows the pattern of rocksdb.name-of-metric > Master.Rocks<Block | Inode>NameOfMetric. All metrics are aggregated across all columns present in RocksDB.

The metrics that concern memory usage are explored in the RocksDB wiki. These are important as RocksDB is written in C++ and used through the JNI in Java. This means that RocksDB’s memory usage is not governed by the JVM arguments -Xmx and -Xms. Here’s how they are exposed in Alluxio:

  • Master.RocksBlockBlockCacheUsage and Master.RocksInodeBlockCacheUsage (derived from rocksdb.block-cache-usage).
  • Master.RocksBlockEstimateTableReadersMem and Master.RocksInodeEstimateTableReadersMem (derived from rocksdb.estimate-table-readers-mem).
  • Master.RocksBlockCurSizeAllMemTables and Master.RocksInodeCurSizeAllMemTables (derived from rocksdb.cur-size-all-mem-tables).
  • Master.RocksBlockBlockCachePinnedUsage and Master.RocksInodeBlockCachePinnedUsage (derived from rocksdb.block-cache-pinned-usage).

These four metrics are aggregated on a blocks and inodes basis to estimate total memory usage. Master.RocksBlockEstimatedMemUsage and Master.RocksInodeEstimatedMemUsage estimate the total memory usage for the blocks table and the inodes table, respectively. These two metrics are further combined in Master.RocksTotalEstimatedMemUsage to estimate the total memory usage of RocksDB across all of Alluxio.

Heap Metastore

The heap metastore is simple: it stores all metadata on the heap. This gives consistent, fast performance, but the required memory scales with the number of files in the filesystem. With 64GB of Alluxio master memory, Alluxio can support around 30 million files.

To configure Alluxio to use the on-heap metastore, set the following in conf/alluxio-site.properties for the master nodes:

alluxio.master.metastore=HEAP

Switching between RocksDB MetaStore and Heap MetaStore

Alluxio master stores different journal information for RocksDB metastore and heap metastore. Switching the metastore type of an existing Alluxio requires formatting Alluxio journal which will wipe all Alluxio metadata. If you would like to keep the existing data in Alluxio cluster after the switch, you will need to perform a journal backup and restore:

First, run the journal backup command while Alluxio master is running.

$ ./bin/alluxio fsadmin backup

This will print something like

Backup Host        : ${HOST_NAME}
Backup URI         : ${BACKUP_PATH}
Backup Entry Count : ${ENTRY_COUNT}

By default, this will write a backup named alluxio-backup-YYYY-MM-DD-timestamp.gz to the /alluxio_backups directory of the root under storage system, e.g. hdfs://cluster/alluxio_backups. This default backup directory can be configured by setting alluxio.master.backup.directory

Alternatively, you may use the --local <DIRECTORY> flag to specify a path to write the backup to on the local disk of the primary master. Note that backup directory paths must be absolute paths. For example:

$ ./bin/alluxio fsadmin backup --local /tmp/alluxio_backup

Then stop Alluxio masters:

$ ./bin/alluxio-stop.sh masters

Update the metastore type in conf/alluxio-site.properties for the master nodes:

alluxio.master.metastore=<new value>

Format the masters with the following command:

$ ./bin/alluxio formatMasters

Then restart the masters with the -i ${BACKUP_PATH} argument, replacing ${BACKUP_PATH} with your specific backup path.

$ ./bin/alluxio-start.sh -i ${BACKUP_PATH} masters

See here for more information regarding journal backup.