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Updated 4.x user guide caching page #500

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17 changes: 11 additions & 6 deletions _docs-4/administration/caching.md
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Expand Up @@ -4,21 +4,21 @@ category: administration
order: 6
---

Accumulo [tablet servers][tserver] have **block caches** that buffer data in memory to limit reads from disk.
Accumulo Scan Servers and [Tablet Servers][tserver] have **block caches** that buffer data in memory to limit reads from disk.
This caching has the following benefits:

* reduces latency when reading data
* helps alleviate hotspots in tables

Each tablet server has an index and data block cache that is shared by all hosted tablets (see the [tablet server diagram][tserver]
Each Scan / Tablet Server has an index and data block cache that is shared by all hosted tablets (see the [tablet server diagram][tserver]
to learn more). A typical Accumulo read operation will perform a binary search over several index blocks followed by a linear scan
of one or more data blocks. If these blocks are not in a cache, they will need to be retrieved from [RFiles] in HDFS. While the index
block cache is enabled for all tables, the data block cache has to be enabled for a table by the user. It is typically only enabled
for tables where read performance is critical.

## Configuration

The {% plink tserver.cache.manager.class %} property controls which block cache implementation is used within the tablet server. Users
The {% plink general.block.cache.manager.class %} property controls which block cache implementation is used within the server. Users
can supply their own implementation and set custom configuration properties to control its behavior (see org.apache.accumulo.core.spi.cache.BlockCacheManager$Configuration.java).

The index and data block caches are configured for tables by the following properties:
Expand All @@ -27,22 +27,27 @@ The index and data block caches are configured for tables by the following prope
* {% plink table.cache.index.enable %} - enables index block cache on the table (default is `true`)

While the index block cache is enabled by default for all Accumulo tables, users must enable the data block cache by
setting {% plink table.cache.block.enable %} to `true` in the shell:
setting {% plink table.cache.block.enable %} to `true` in the shell at the namespace or table level:

config -ns mynamespace -s table.cache.block.enable=true
config -t mytable -s table.cache.block.enable=true

Or programmatically using [TableOperations.setProperty()][tableops]:
Or programmatically using [NamespaceOperations.setProperty()][namespaceops] or [TableOperations.setProperty()][tableops]:

```java
client.namespaceOperations().setProperty("mynamespace", "table.cache.block.enable", "true");
client.tableOperations().setProperty("mytable", "table.cache.block.enable", "true");
```

The size of the index and data block caches (which are shared by all tablets of tablet server) can be changed from
The size of the index and data block caches (which are shared by all tablets of the server) can be changed from
their defaults by setting the following properties:

* {% plink sserver.cache.data.size %}
* {% plink sserver.cache.index.size %}
* {% plink tserver.cache.data.size %}
* {% plink tserver.cache.index.size %}

[tserver]: {% durl getting-started/design#tablet-server-1 %}
[RFiles]: {% durl getting-started/design#rfile %}
[namespaceops]: {% jurl org.apache.accumulo.core.client.admin.NamespaceOperations#setProperty-java.lang.String-java.lang.String-java.lang.String- %}
[tableops]: {% jurl org.apache.accumulo.core.client.admin.TableOperations#setProperty-java.lang.String-java.lang.String-java.lang.String- %}
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