Adding Additional Search Crawl database to a SP2013 Search farm

Problem: By adhereing to Microsoft recommendations for search farms, a threshold that is passed fairly quickly is that the Search Crawl database should not exceed 20 million items per database.  I have a 12 server search farm with four partitions (recommendation 10 million items per index partion).  I need to add a second Crawler DB.

Initial Hypothesis: Figure out your current crawl databases.

Add additional Search Crawl databases

 Review the existing crawl search databases

I proved this on a test system by resetting my index and recrawling, the results after adding 3 content db's are shown below.

Resolution:  Add more crawl database to the search service application.

$SSA = Get-SPEnterpriseSearchServiceApplication$searchCrawlDBName = "SP_Search_CrawlStore2"
$searchCrawlDBServer = "SP2013-SQL3" # SQL Aliase could be the Conn str
$crawlDatabase = New-SPEnterpriseSearchCrawlDatabase -SearchApplication $SSA -DatabaseName $searchCrawlDBName -DatabaseServer $searchCrawlDBServer
$crawlStoresManager = new-Object Microsoft.Office.Server.Search.Administration.CrawlStorePartitionManager($SSA)
$crawlStoresManager.BeginCrawlStoreRebalancing()

Powershell to see what is going on:

cls
$SSA = Get-SPEnterpriseSearchServiceApplication
$crawlStoresManager = new-Object Microsoft.Office.Server.Search.Administration.CrawlStorePartitionManager($SSA)
Write-Host "CrawlStoresAreUnbalanced:" $crawlStoresManager.CrawlStoresAreUnbalanced()
Write-Host "CrawlStoreImbalanceThreshold:" $ssa.GetProperty("CrawlStoreImbalanceThreshold")
Write-Host "CrawlStoresAreUnbalanced:" $crawlStoresManager.CrawlStoresAreUnbalanced()
Write-Host "CrawlPartitionSplitThreshold:" $ssa.GetProperty("CrawlPartitionSplitThreshold")
$crawlLog = New-Object Microsoft.Office.Server.Search.Administration.CrawlLog $SSA
$dbInfo= $crawlLog.GetCrawlDatabaseInfo()
Write-Host "Number of Crawl Databases:"
$dbInfo.Count$dbInfo.Values

More Info:
http://blogs.msdn.com/b/sharepoint_strategery/archive/2013/01/28/powershell-to-rebalance-crawl-store-dbs-in-sp2013.aspx