docs: add docs for new queue.compression setting

Author: yaauie

docs/reference/logstash-settings-file.md

@@ -68,6 +68,7 @@ The `logstash.yml` file includes these settings.
 | `queue.checkpoint.acks` | The maximum number of ACKed events before forcing a checkpoint when persistent queues are enabled (`queue.type: persisted`). Specify `queue.checkpoint.acks: 0` to set this value to unlimited. | 1024 |
 | `queue.checkpoint.writes` | The maximum number of written events before forcing a checkpoint when persistent queues are enabled (`queue.type: persisted`). Specify `queue.checkpoint.writes: 0` to set this value to unlimited. | 1024 |
 | `queue.checkpoint.retry` | When enabled, Logstash will retry four times per attempted checkpoint write for any checkpoint writes that fail. Any subsequent errors are not retried. This is a workaround for failed checkpoint writes that have been seen only on Windows platform, filesystems with non-standard behavior such as SANs and is not recommended except in those specific circumstances. (`queue.type: persisted`) | `true` |
+| `queue.compression` | Set a persisted queue compression goal, which allows the pipeline to spend CPU to reduce the serialized size on disk. Acceptable values are `speed`, `balanced`, and `size`.  | `none` |
 | `queue.drain` | When enabled, Logstash waits until the persistent queue (`queue.type: persisted`) is drained before shutting down. | `false` |
 | `dead_letter_queue.enable` | Flag to instruct Logstash to enable the DLQ feature supported by plugins. | `false` |
 | `dead_letter_queue.max_bytes` | The maximum size of each dead letter queue. Entries will be dropped if they  would increase the size of the dead letter queue beyond this setting. | `1024mb` |

docs/reference/persistent-queues.md

@@ -81,6 +81,11 @@ If you want to define values for a specific pipeline, use [`pipelines.yml`](/ref
 
     To avoid losing data in the persistent queue, you can set `queue.checkpoint.writes: 1` to force a checkpoint after each event is written. Keep in mind that disk writes have a resource cost. Setting this value to `1` ensures maximum durability, but can severely impact performance. See [Controlling durability](#durability-persistent-queues) to better understand the trade-offs.
 
+`queue.compression`
+:   Sets the event compression goal for use with the persisted queue. Default is `none`. Acceptable values include:
+    * `speed`: optimize for fastest compression operation
+    * `size`: optimize for smallest size on disk, spending more CPU
+    * `balanced`: a balance between the `speed` and `size` settings
 
 
 ## Configuration notes [pq-config-notes]