Changing disk write queue quotas
One of the specific uses of cbepctl is to the change the default maximum items for a disk write queue. This impacts replication of data that occurs between source and destination nodes within a cluster. Both data that a node receives from client applications, and replicated items that it receives are placed on a disk write queue. If there are too many items waiting in the disk write queue at any given destination, Couchbase Server will reduce the rate of data that is sent to a destination. This is process is also known as backoff.
By default, when a disk write queue contains one million items, a Couchbase node will reduce the rate it sends out data to be replicated. You can change this setting to be the greater of 10% of the items at a destination node or a number you specify. For instance:
> ./cbepctl 10.5.2.31:11210 -b bucket_name -p bucket_password set tap_param tap_throttle_queue_cap 2000000
Be aware that this tool is a per-node, per-bucket operation. That means that if you want to perform this operation, you must specify the IP address of a node in the cluster and a named bucket. If you do not provided a named bucket, the server will apply the setting to any default bucket that exists at the specified node. If you want to perform this operation for an entire cluster, you will need to perform the command for every node/bucket combination that exists for that cluster.
In this example we specify that a replica node send a request to backoff when it has two million items or 10% of all items, whichever is greater. You will see a response similar to the following:
setting param: tap_throttle_queue_cap 2000000
In this next example, we change the default percentage used to manage the replication stream. If the items in a disk write queue reach the greater of this percentage or a specified number of items, replication requests will slow down:
> ./cbepctl 10.5.2.31:11210 -b bucket_name -p bucket_password set tap_param tap_throttle_cap_pcnt 15
In this example, we set the threshold to 15% of all items at a replica node. When a disk write queue on a replica node reaches this point, it will request replication backoff. For more information about replicas, replication and backoff from replication. The other command options for tap_param are:
| Parameter | Description |
|---|---|
| tap_keepalive | Seconds to hold a named tap connection. |
| tap_throttle_queue_cap | Max disk write queue size when tap streams will put into a temporary, 5-second pause. ‘Infinite’ means there is no cap. |
| tap_throttle_cap_pcnt | Maximum items in disk write queue as percentage of all items on a node. At this point tap streams will put into a temporary, 5-second pause. |
| tap_throttle_threshold | Percentage of memory in use when tap streams will be put into a temporary, 5-second pause. |