I needed a way to have new mailboxes be automatically distributed across a list of databases on multiple servers, and I recalled seeing a script somewhere.  The blog post with those details can be found here.  As the author points out, it can take a long time to build the hash table, too long for my requirements.  For example, one of my longer queries builds the table for 9 databases with over 5000 mailboxes in 58 seconds.  That is just way too  long for someone to feed a user account into to get a mailbox created.

So I broke down the query being used to find where efficiency could be improved.  The Get-ExchangeServer cmdlet slows the script down and can be avoided altogether by feeding the server list directly into the Get-MailboxDatabase cmdlet.  Additionally, I discovered just how inefficient the Exchange cmdlets are.  I already knew they were based on running other cmdlets for targets servers across a WAN that return not a lot of data, but this reinforced those findings.

To get away from the Exchange cmdlets, I query AD directly by instantiating ADSI in .NET manually.  Instead of slowly building the hash table I just take the record count of the search results and populate the table entry at once.  This reduced the time to completion from 58 seconds to 24 seconds.  These findings are consistent across different database counts and mailbox totals: 60% reduction in time.  While still too long for my taste, it certainly is much better than before and is within reason for the people at my company that make use of the script.

Since my script encompasses more than just the load balancing part (I create a GUI form where the location for the mailbox is selected, which feeds a list of appropriate servers to the function), this is just the part that creates the hash table with the database counts and returns the database with the lowest count:

$mbxServers = @()
$mailboxcount = @{}
$domain = [System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain()
$root = [ADSI]"GC://$($domain.Name)"
$mbxServers | %{Get-MailboxDatabase -server $_} | ?{$_.recovery -eq $FALSE} | %{
$filter = "(&(objectcategory=user)(homeMDB=" + ($_.DistinguishedName) + "))"
$search = new-Object System.DirectoryServices.DirectorySearcher($root,$filter)
$search.PropertiesToLoad.Add("homeMDB") | Out-Null
$result = $search.FindAll()
$mailboxcount["$($_.Identity)"] = $result.count
}
($mailboxcount.GetEnumerator() | sort value | select -first 1).key