In Accumulo each tablet has a list of files associated with it. As data is written to Accumulo it is buffered in memory. The data buffered in memory is eventually written to files in DFS on a per-tablet basis. Files can also be added to tablets directly by bulk import. In the background tablet servers run major compactions to merge multiple files into one. The tablet server has to decide which tablets to compact and which files within a tablet to compact.

Within each tablet server there are one or more user configurable Compaction Services that compact tablets. Compaction Services can be configured with one or more named queues, which may use internal executors or external executors. An internal executor will be configured with some sized threshold and number of threads in which to compact files. An external executor is configured only with the name of an external queue.

Each Accumulo table has a user configurable Compaction Dispatcher that decides which compaction services that table will use. Accumulo generates metrics for each compaction service which enable users to adjust compaction service settings based on actual activity.

Each compaction service has a compaction planner that decides which files to compact. The default compaction planner uses the table property table.compaction.major.ratio to decide which files to compact. The compaction ratio is real number >= 1.0. Assume LFS is the size of the largest file in a set, CR is the compaction ratio, and FSS is the sum of file sizes in a set. The default planner looks for file sets where LFS*CR <= FSS. By only compacting sets of files that meet this requirement the amount of work done by compactions is O(N * log_CR(N)). Increasing the ratio will result in less compaction work and more files per tablet. More files per tablet means higher query latency. So adjusting this ratio is a trade-off between ingest and query performance.

When CR=1.0 this will result in a goal of a single per file tablet, but the amount of work is O(N²) so 1.0 should be used with caution. For example if a tablet has a 1G file and 1M file is added, then a compaction of the 1G and 1M file would be queued.

Compaction services and dispatchers were introduced in Accumulo 2.1, so much of this documentation only applies to Accumulo 2.1 and later.

Configuration

Below are some Accumulo shell commands that do the following :

Create a compaction service named cs1 that has three executors. The first executor named small has 8 threads and runs compactions less than 16M. The second executor medium runs compactions less than 128M with 4 threads. The last executor large runs all other compactions.
Create a compaction service named cs2 that has three executors. It has similar config to cs1, but its executors have fewer threads. Limits total I/O of all compactions within the service to 40MB/s.
Configure table ci to use compaction service cs1 for system compactions and service cs2 for user compactions.

config -s tserver.compaction.major.service.cs1.planner=org.apache.accumulo.core.spi.compaction.DefaultCompactionPlanner
config -s 'tserver.compaction.major.service.cs1.planner.opts.executors=[{"name":"small","type":"internal","maxSize":"16M","numThreads":8},{"name":"medium","type":"internal","maxSize":"128M","numThreads":4},{"name":"large","type":"internal","numThreads":2}]'
config -s tserver.compaction.major.service.cs2.planner=org.apache.accumulo.core.spi.compaction.DefaultCompactionPlanner
config -s 'tserver.compaction.major.service.cs2.planner.opts.executors=[{"name":"small","type":"internal","maxSize":"16M","numThreads":4},{"name":"medium","type":"internal","maxSize":"128M","numThreads":2},{"name":"large","type":"internal","numThreads":1}]'
config -s tserver.compaction.major.service.cs2.rate.limit=40M
config -t ci -s table.compaction.dispatcher=org.apache.accumulo.core.spi.compaction.SimpleCompactionDispatcher
config -t ci -s table.compaction.dispatcher.opts.service=cs1
config -t ci -s table.compaction.dispatcher.opts.service.user=cs2

For more information see the javadoc for compaction, DefaultCompactionPlanner and SimpleCompactionDispatcher

The names of the compaction services and executors are used for logging and metrics.

External Compactions

In Accumulo 2.1 we introduced a new optional feature that allows compactions to run outside of the Tablet Server. External compactions introduces two new server processes in an Accumulo deployment:

Compactor: Accumulo process that runs external compactions and is started with the name of a queue for which it will perform compactions. In a typical deployment there will be many of these processes running, some for queue A, queue B, etc. This process will only run a single compaction at a time and will communicate with the Compaction Coordinator to get a compaction job and report its status.
Compaction Coordinator: a process that manages the compaction queues for all external compactions in the system and assigns compaction tasks to Compactors. In a typical deployment there will be one instance of this process in use at a time with a backup process waiting to become primary (much like the primary and secondary manager processes). This process communicates with the TabletServers to get external compaction job information and report back their status.

Starting the Components

The CompactionCoordinator and Compactor components are started in the same manner as the other Accumulo services.

To start a CompactionCoordinator:

accumulo compaction-coordinator &

To start a Compactor:

accumulo compactor -q <queueName>

Configuration

Configuration for external compactions is very similar to the internal compaction example above. In the example below we create a Compaction Service cs1 and configure it with a queue named DCQ1. We then define the Compaction Dispatcher on table testTable and configure the table to use the cs1 Compaction Service for planning and executing all compactions.

config -s tserver.compaction.major.service.cs1.planner=org.apache.accumulo.core.spi.compaction.DefaultCompactionPlanner
config -s 'tserver.compaction.major.service.cs1.planner.opts.executors=[{"name":"all","type":"external","queue":"DCQ1"}]'
config -t testTable -s table.compaction.dispatcher=org.apache.accumulo.core.spi.compaction.SimpleCompactionDispatcher
config -t testTable -s table.compaction.dispatcher.opts.service=cs1

Note that you can mix internal and external options, for example:

config -s 'tserver.compaction.major.service.cs1.planner.opts.executors=[{"name":"small","type":"internal","maxSize":"16M","numThreads":8},{"name":"medium","type":"internal","maxSize":"128M","numThreads":4},{"name":"large","type":"external","queue":"LargeQ"}]'

Overview

The CompactionCoordinator is responsible for managing the global external compaction work queue. For each external compaction queue, the tablet server will maintain an in memory priority queue of the tablets loaded on it that require external compactions. The coordinator polls all tservers to get summary information about their external compaction queues to combine the summary information to determine which tablet server to contact next to get work. The coordinator does not maintain per tablet information, it only maintains enough information to allow it to know which tablet server to contact next for a given queue. The tablet server will then know what specific tablet in that queue needs to compact.

When a Compactor is free to perform work, it asks the CompactionCoordinator for the next compaction job. The CompactionCoordinator contacts the next TabletServer that has the highest priority for the Compactor’s queue. The TabletServer returns the information necessary for the compaction to occur to the CompactionCoordinator, which is passed on to the Compactor. The Compaction Coordinator maintains an in-memory list of running compactions and also inserts an entry into the metadata table for the tablet to denote that an external compaction is running. When the Compactor has finished the compaction, it notifies the CompactionCoordinator which inserts an entry into the metadata table to denote that the external compaction completed and it attempts to notify the TabletServer. If successful, the TabletServer commits the major compaction. If the TabletServer is down, or the Tablet has become hosted on a different TabletServer, then the CompactionCoordinator will fail to notify the TabletServer, but the metadata table entries will remain. The major compaction will be committed in the future by the TabletServer hosting the Tablet.

External compactions handle faults and major system events in Accumulo. When a compactor process dies this will be detected and any files it had reserved in a tablet will be unreserved. When a tserver dies, this will not impact any external compactions running on behalf of tablets that tserver was hosting. The case of tablets not being hosted on a tserver when an external compaction tries to commit is also handled. Tablets being deleted (by split, merge, or table deletion) will cause any associated running external compactions to be canceled. When a user initiated compaction is canceled, any external compactions running as part of that will be canceled.

External Compaction in Action

Below are some examples of log entries and metadata table entries for external compactions. First, here are some metadata entries for table 2 . You can see that there are three files of different sizes (file size and number of entries are stored in the value portion of the metadata table rows with the “file” column qualifier).

2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/A0000047.rf []   12330,99000
2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/F0000048.rf []   1196,1000
2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/F000004j.rf []   1302,1000
2< last:10000bf4e0a0004 []  localhost:9997
2< loc:10000bf4e0a0004 []   localhost:9997
2< srv:compact []   111
2< srv:dir []   default_tablet
2< srv:flush [] 113
2< srv:lock []  tservers/localhost:9997/zlock#1950397a-b2ca-4685-b70b-67ae3cd578b9#0000000000$10000bf4e0a0004
2< srv:time []  M1618325648093
2< ~tab:~pr []  \x00

Below are excerpts from the TabletServer, CompactionCoordinator, Compactor logs and metadata table. I have merged the logs in time order to make it easier to see what is happening.

In the logs below the Compactor requested a compaction job from the Coordinator with an ExternalCompactionId of de6afc1d-64ae-4abf-8bce-02ec0a79aa6c. The Coordinator knew that TabletServer localhost:9997 had a Tablet that needed compacting and contacted it to get the details. The CompactionManager, a component running in the TabletServer, returned the information to the Coordinator. The Coordinator then updates the metadata table (below the logs) for the external compaction and returns the information to the Compactor:

2021-04-13T14:54:10,580 [compactor.Compactor] INFO : Attempting to get next job, eci = ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c
2021-04-13T14:54:10,580 [coordinator.CompactionCoordinator] DEBUG: getCompactionJob called for queue DCQ1 by compactor localhost:9101
2021-04-13T14:54:10,580 [coordinator.CompactionCoordinator] DEBUG: Found tserver localhost:9997 with priority 288230376151711747 compaction for queue DCQ1
2021-04-13T14:54:10,580 [coordinator.CompactionCoordinator] DEBUG: Getting compaction for queue DCQ1 from tserver localhost:9997
2021-04-13T14:54:10,581 [compactions.CompactionManager] DEBUG: Attempting to reserve external compaction, queue:DCQ1 priority:288230376151711747 compactor:localhost:9101
2021-04-13T14:54:10,596 [compactions.CompactionManager] DEBUG: Reserved external compaction ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c
2021-04-13T14:54:10,596 [coordinator.CompactionCoordinator] DEBUG: Returning external job ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c to localhost:9101

2< ecomp:ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c []   {"inputs":["hdfs://localhost:8020/accumulo/tables/2/default_tablet/F0000048.rf","hdfs://localhost:8020/accumulo/tables/2/default_tablet/A0000047.rf"],"tmp":"hdfs://localhost:8020/accumulo/tables/2/default_tablet/A000004k.rf_tmp","dest":"hdfs://localhost:8020/accumulo/tables/2/default_tablet/A000004k.rf","compactor":"localhost:9101","kind":"USER","executorId":"DCQ1","priority":288230376151711747}
2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/A0000047.rf []   12330,99000
2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/F0000048.rf []   1196,1000
2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/F000004j.rf []   1302,1000
2< last:10000bf4e0a0004 []  localhost:9997
2< loc:10000bf4e0a0004 []   localhost:9997
2< srv:compact []   111
2< srv:dir []   default_tablet
2< srv:flush [] 113
2< srv:lock []  tservers/localhost:9997/zlock#1950397a-b2ca-4685-b70b-67ae3cd578b9#0000000000$10000bf4e0a0004
2< srv:time []  M1618325648093
2< ~tab:~pr []  \x00

Next, the Compactor runs the compaction successfully and reports the status back to the Coordinator. The Coordinator inserts a final state marker into the metadata table (below the logs).

2021-04-13T14:54:11,597 [compactor.Compactor] INFO : Received next compaction job: TExternalCompactionJob(externalCompactionId:ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c, extent:TKeyExtent(table:32, endRow:null, prevEndRow:null), files:[In
putFile(metadataFileEntry:hdfs://localhost:8020/accumulo/tables/2/default_tablet/F0000048.rf, size:0, entries:0, timestamp:0), InputFile(metadataFileEntry:hdfs://localhost:8020/accumulo/tables/2/default_tablet/A0000047.rf, size:0, entries
:0, timestamp:0)], priority:3, readRate:0, writeRate:0, iteratorSettings:IteratorConfig(iterators:[]), type:FULL, reason:USER, outputFile:hdfs://localhost:8020/accumulo/tables/2/default_tablet/A000004k.rf_tmp, propagateDeletes:false, kind
:USER)
2021-04-13T14:54:11,598 [compactor.Compactor] INFO : Starting up compaction runnable for job: TExternalCompactionJob(externalCompactionId:ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c, extent:TKeyExtent(table:32, endRow:null, prevEndRow:null)
, files:[InputFile(metadataFileEntry:hdfs://localhost:8020/accumulo/tables/2/default_tablet/F0000048.rf, size:0, entries:0, timestamp:0), InputFile(metadataFileEntry:hdfs://localhost:8020/accumulo/tables/2/default_tablet/A0000047.rf, size
:0, entries:0, timestamp:0)], priority:3, readRate:0, writeRate:0, iteratorSettings:IteratorConfig(iterators:[]), type:FULL, reason:USER, outputFile:hdfs://localhost:8020/accumulo/tables/2/default_tablet/A000004k.rf_tmp, propagateDeletes:
false, kind:USER)
2021-04-13T14:54:11,599 [compactor.Compactor] INFO : CompactionCoordinator address is: localhost:9100
2021-04-13T14:54:11,599 [coordinator.CompactionCoordinator] INFO : Compaction status update, id: ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c, timestamp: 1618325651599, state: STARTED, message: Compaction started
2021-04-13T14:54:12,601 [compactor.Compactor] INFO : Starting compactor
2021-04-13T14:54:12,601 [compactor.Compactor] INFO : Progress checks will occur every 1 seconds
2021-04-13T14:54:12,718 [ratelimit.SharedRateLimiterFactory] DEBUG: RateLimiter 'read_rate_limiter': 69,672 of 0 permits/second
2021-04-13T14:54:12,718 [ratelimit.SharedRateLimiterFactory] DEBUG: RateLimiter 'write_rate_limiter': 45,120 of 0 permits/second
2021-04-13T14:54:13,179 [compactor.Compactor] INFO : Compaction completed successfully ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c
2021-04-13T14:54:13,180 [compactor.Compactor] INFO : CompactionCoordinator address is: localhost:9100
2021-04-13T14:54:13,181 [coordinator.CompactionCoordinator] INFO : Compaction status update, id: ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c, timestamp: 1618325653180, state: SUCCEEDED, message: Compaction completed successfully
2021-04-13T14:54:14,182 [compactor.Compactor] INFO : Compaction thread finished.
2021-04-13T14:54:14,182 [compactor.Compactor] INFO : Updating coordinator with compaction completion.
2021-04-13T14:54:14,184 [coordinator.CompactionCoordinator] INFO : Compaction completed, id: ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c, stats: CompactionStats(entriesRead:100000, entriesWritten:100000, fileSize:12354)
2021-04-13T14:54:14,185 [coordinator.CompactionFinalizer] INFO : Writing completed external compaction to metadata table: {"extent":{"tableId":"2"},"state":"FINISHED","fileSize":12354,"entries":100000}
2021-04-13T14:54:14,223 [coordinator.CompactionFinalizer] INFO : Queueing tserver notification for completed external compaction: {"extent":{"tableId":"2"},"state":"FINISHED","fileSize":12354,"entries":100000}
2021-04-13T14:54:14,290 [coordinator.CompactionFinalizer] INFO : Notifying tserver localhost:9997[10000bf4e0a0004] that compaction {"extent":{"tableId":"2"},"state":"FINISHED","fileSize":12354,"entries":100000} has finished.

2< ecomp:ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c []   {"inputs":["hdfs://localhost:8020/accumulo/tables/2/default_tablet/F0000048.rf","hdfs://localhost:8020/accumulo/tables/2/default_tablet/A0000047.rf"],"tmp":"hdfs://localhost:8020/accumulo/tables/2/default_tablet/A000004k.rf_tmp","dest":"hdfs://localhost:8020/accumulo/tables/2/default_tablet/A000004k.rf","compactor":"localhost:9101","kind":"USER","executorId":"DCQ1","priority":288230376151711747}
2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/A0000047.rf []   12330,99000
2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/F0000048.rf []   1196,1000
2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/F000004j.rf []   1302,1000
2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/F000004l.rf []   841,1000
2< last:10000bf4e0a0004 []  localhost:9997
2< loc:10000bf4e0a0004 []   localhost:9997
2< srv:compact []   111
2< srv:dir []   default_tablet
2< srv:flush [] 114
2< srv:lock []  tservers/localhost:9997/zlock#1950397a-b2ca-4685-b70b-67ae3cd578b9#0000000000$10000bf4e0a0004
2< srv:time []  M1618325653080
2< ~tab:~pr []  \x00
~ecompECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c : []    {"extent":{"tableId":"2"},"state":"FINISHED","fileSize":12354,"entries":100000}

Finally, the TabletServer commits the compaction.

2021-04-13T14:54:14,290 [tablet.CompactableImpl] DEBUG: Attempting to commit external compaction ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c
2021-04-13T14:54:14,325 [tablet.files] DEBUG: Compacted 2<< for USER created hdfs://localhost:8020/accumulo/tables/2/default_tablet/A000004k.rf from [A0000047.rf, F0000048.rf]
2021-04-13T14:54:14,326 [tablet.CompactableImpl] DEBUG: Completed commit of external compaction ECID:de6afc1d-64ae-4abf-8bce-02ec0a79aa6c

2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/A000004k.rf []   12354,100000
2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/F000004j.rf []   1302,1000
2< file:hdfs://localhost:8020/accumulo/tables/2/default_tablet/F000004l.rf []   841,1000
2< last:10000bf4e0a0004 []  localhost:9997
2< loc:10000bf4e0a0004 []   localhost:9997
2< srv:compact []   112
2< srv:dir []   default_tablet
2< srv:flush [] 114
2< srv:lock []  tservers/localhost:9997/zlock#1950397a-b2ca-4685-b70b-67ae3cd578b9#0000000000$10000bf4e0a0004
2< srv:time []  M1618325653080
2< ~tab:~pr []  \x00

Logging

The names of compaction services and executors are used in logging. The log messages below are from a tserver with the configuration above with data being written to the ci table. Also, a compaction of the table was forced from the shell.

2020-06-25T16:34:31,669 [tablet.files] DEBUG: Compacting 3;667;6 on cs1.small for SYSTEM from [C00001cm.rf, C00001a7.rf, F00001db.rf] size 15 MB
2020-06-25T16:34:45,165 [tablet.files] DEBUG: Compacted 3;667;6 for SYSTEM created hdfs://localhost:8020/accumulo/tables/3/t-000006f/C00001de.rf from [C00001cm.rf, C00001a7.rf, F00001db.rf]
2020-06-25T16:35:01,965 [tablet.files] DEBUG: Compacting 3;667;6 on cs1.medium for SYSTEM from [C00001de.rf, A000017v.rf, F00001e7.rf] size 33 MB
2020-06-25T16:35:11,686 [tablet.files] DEBUG: Compacted 3;667;6 for SYSTEM created hdfs://localhost:8020/accumulo/tables/3/t-000006f/A00001er.rf from [C00001de.rf, A000017v.rf, F00001e7.rf]
2020-06-25T16:37:12,521 [tablet.files] DEBUG: Compacting 3;667;6 on cs2.medium for USER from [F00001f8.rf, A00001er.rf] size 35 MB config []
2020-06-25T16:37:17,917 [tablet.files] DEBUG: Compacted 3;667;6 for USER created hdfs://localhost:8020/accumulo/tables/3/t-000006f/A00001fr.rf from [F00001f8.rf, A00001er.rf]

Metrics

The numbers of major and minor compactions running and queued is visible on the Accumulo monitor page. This allows you to see if compactions are backing up and adjustments to the above settings are needed. When adjusting the number of threads available for compactions, consider the number of cores and other tasks running on the nodes.

The numbers displayed on the Accumulo monitor are an aggregate of all compaction services and executors. Accumulo emits metrics about the number of compactions queued and running on each compaction executor. Accumulo also emits metrics about the number of files per tablets. These metrics can be used to guide adjusting compaction ratios and compaction service configurations to ensure tablets do not have to many files.

For example if metrics show that some compaction executors within a compaction service are under utilized while others are over utilized, then the configuration for compaction service may need to be adjusted. If the metrics show that all compaction executors are fully utilized for long periods then maybe the compaction ratio on a table needs to be increased.

User compactions

Compactions can be initiated manually for a table. To initiate a minor compaction, use the flush command in the shell. To initiate a major compaction, use the compact command in the shell:

user@myinstance mytable> compact -t mytable

If needed, the compaction can be canceled using compact --cancel -t mytable.

The compact command will compact all tablets in a table to one file. Even tablets with one file are compacted. This is useful for the case where a major compaction filter is configured for a table. In 1.4, the ability to compact a range of a table was added. To use this feature specify start and stop rows for the compact command. This will only compact tablets that overlap the given row range.

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