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replication backlogs update #1395

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Jun 15, 2021
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replication backlogs update #1395

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39 changes: 32 additions & 7 deletions content/rs/administering/database-operations/updating-configurations.md
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Expand Up @@ -60,26 +60,51 @@ If a participating cluster that was removed forcefully returns attempts to re-jo
it will have an out of date on Active-Active database membership.
The joined participating clusters reject updates sent from the removed participating cluster.

## Replication backlog
## Database replication backlog

Active-Active and Active-Passive databases maintain a replication backlog to synchronize the database instances between clusters.
By default, the replication backlog is set to one percent (1%) of the database size and ranges between 1MB to 250MB per shard.
Use the [crdb-cli]({{< relref "rs/references/crdb-cli-reference.md" >}}) and the [rladmin]({{< relref "rs/references/rladmin.md" >}}) utilities to control the size of the replication backlog. You can set it to `auto` or set a specific size.
Redis databases that use [replication for high availability]({{< relref "rs/concepts/high-availability/replication.md" >}}) maintain a replication backlog (per shard) to synchronize the primary and replica shards of a database.
By default, the replication backlog is set to one percent (1%) of the database size divided by the database number of shards and ranges between 1MB to 250MB per shard.
Use the [rladmin]({{< relref "rs/references/rladmin.md" >}}) and the [crdb-cli]({{< relref "rs/references/crdb-cli-reference.md" >}}) utilities to control the size of the replication backlog. You can set it to `auto` or set a specific size.

The syntax varies between Active-Active and Active-Passive databases.
The syntax varies between regular and Active-Active databases.

For a regular Redis database:
```text
rladmin tune db <db:id | name> repl_backlog <Backlog size in MB or 'auto'>
```

For an Active-Active database:
```text
crdb-cli crdb update --crdb-guid <crdb_guid> --default-db-config "{\"crdt_repl_backlog_size\": <SIZE_IN_BYTES | 'auto'>}"
crdb-cli crdb update --crdb-guid <crdb_guid> --default-db-config "{\"repl_backlog_size\": <size in MB | 'auto'>}"
```

## Active-Active CRDT replication backlog

In addition to the database replication backlog, Active-Active databases maintain a CRDT replication backlog (per shard) to synchronize the database instances between clusters.
By default, the CRDT replication backlog is set to one percent (1%) of the Active-Active database size divided by the database number of shards and ranges between 1MB to 250MB per shard.
Use the [crdb-cli]({{< relref "rs/references/crdb-cli-reference.md" >}}) utility to control the size of the CRDT replication backlog. You can set it to `auto` or set a specific size:

```text
crdb-cli crdb update --crdb-guid <crdb_guid> --default-db-config "{\"crdt_repl_backlog_size\": <size in MB | 'auto'>}"
```

## Active-Passive replication backlog

In addition to the database replication backlog, Active-Passive databases maintain a replication backlog (per shard) to synchronize the database instances between clusters.
By default, the replication backlog is set to one percent (1%) of the database size divided by the database number of shards and ranges between 1MB to 250MB per shard.
Use the [rladmin]({{< relref "rs/references/rladmin.md" >}}) utility to control the size of the replication backlog. You can set it to `auto` or set a specific size.

For an Active-Passive database:
```text
rladmin tune db <db:id | name> repl_backlog <Backlog size in MB or 'auto'>
```

{{< note >}}
On an Active-Passive database, the replication backlog configuration applies to both the replication backlog for shards synchronization and for synchronization of database instances between clusters.
{{< /note >}}

**For Redis Software versions earlier than 6.0.20:**
The replication backlog defaults to 1MB and cannot be set dynamically with 'auto' mode.
The replication backlog and the CRDT replication backlog defaults are set to 1MB and cannot be set dynamically with 'auto' mode.
To control the size of the replication log, use [rladmin]({{< relref "rs/references/rladmin.md" >}}) to tune the local database instance in each cluster.
```text
rladmin tune db <db:id | name> repl_backlog <Backlog size in MB (or if ending with bytes, KB or GB, in the respective unit)>
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