Disk I/O priority
Disk I/O priority enables workload priorities to be set at the bucket level.
Bucket priority settings can be specified at the bucket-level. The bucket disk I/O priority can be set as either high or low, whereas, low is the default. Bucket priority settings determine whether I/O tasks for a bucket are enqueued in either low or high priority task queues. Threads in the global pool poll the high priority task queues more often compared to the low priority task queues. Bucket latency and I/O operations are impacted by the setting value. When a bucket has a high priority, its I/O tasks are picked up at a high frequency and, thus, are able to processed faster compared to tasks belonging to a low priority bucket.
The default buckets settings can be configured during the initial setup and then edited after the setup. However, changing bucket priority after the setup results in a restart of the bucket and resetting of the client connections.
When upgrading from a 2.x release to a 3.x release, Couchbase converts an existing thread value to either a low or a high priority based on the following:
- Buckets allocated six to eight (6-8) threads in Couchbase Server 2.x are marked high priority in bucket setting after the upgrade to 3.x or later.
- Buckets allocated three to five (3-5) threads in Couchbase Server 2.x are marked low priority in bucket settings after the upgrade to 3.x or later.