The core issue is that search heads are waiting for data from indexers across a slow, low-bandwidth WAN link. Splunk's multisite clustering has a built-in feature called "search affinity," where a search head preferentially searches indexers on its local site to minimize such cross-site traffic. However, for search affinity to be effective, a searchable copy of the data must be available on the local site. The sitesearchfactor attribute in server.conf on the cluster manager is used to guarantee that each site maintains a searchable copy of all buckets. By configuring this, local search heads can find the data they need locally, avoiding the network bottleneck and significantly improving search performance. This is a simple configuration change, making it the least expensive and easiest solution.

B. Move all indexers and search heads in one of the data centers into the same site.

This is a major architectural change that would negate the benefits of a multisite deployment for disaster recovery and would not solve the performance issue for users.

C. Install a network pipe with more bandwidth between the two data centers.

While this would likely solve the performance issue, it involves significant hardware and infrastructure costs and is explicitly not the "least expensive" or "easiest" option.

D. Set the site setting on each indexer in the server.conf clustering stanza to be the same for all indexers regardless of site.

This would effectively disable the multisite awareness of the cluster, breaking search affinity entirely and likely worsening performance as searches would be randomly distributed across the slow WAN.

1. Splunk Enterprise Documentation, Managing Indexers and Clusters of Indexers, "Configure search affinity in a multisite cluster."

This section states, "For search affinity to work, a searchable copy of the data must reside on the search head's site... You enable search affinity by configuring the sitesearchfactor." It directly identifies that the solution to ensure local data availability for searches is to configure this specific attribute.

2. Splunk Enterprise Documentation, Admin Manual, "server.conf".

Under the [clustering] stanza, the documentation for sitesearchfactor explains its purpose: "Determines the number of searchable copies of buckets that the cluster maintains on each site." This confirms that option A is the correct mechanism to control the location of searchable data copies.

3. Splunk Enterprise Documentation, Managing Indexers and Clusters of Indexers, "Multisite indexer cluster deployment overview."

This overview discusses the fundamental reasons for multisite clustering, including disaster recovery and managing geographically dispersed data centers. It notes that search heads on a site primarily search peers on the same site, a concept that relies on the correct configuration of replication and search factors to be effective, especially over a WAN. This context invalidates option D, which would undo this core design principle.