functions/Test-DbaMaxMemory.ps1

function Test-DbaMaxMemory {
    <#
    .SYNOPSIS
        Calculates the recommended value for SQL Server 'Max Server Memory' configuration setting. Works on SQL Server 2000-2014.
 
    .DESCRIPTION
        Inspired by Jonathan Kehayias's post about SQL Server Max memory (http://bit.ly/sqlmemcalc), this script displays a SQL Server's: total memory, currently configured SQL max memory, and the calculated recommendation.
 
        Jonathan notes that the formula used provides a *general recommendation* that doesn't account for everything that may be going on in your specific environment.
 
    .PARAMETER SqlInstance
        The target SQL Server instance or instances.
 
    .PARAMETER SqlCredential
        Windows or Sql Login Credential with permission to log into the SQL instance
 
    .PARAMETER Credential
        Windows Credential with permission to log on to the server running the SQL instance
 
    .PARAMETER EnableException
        By default, when something goes wrong we try to catch it, interpret it and give you a friendly warning message.
        This avoids overwhelming you with "sea of red" exceptions, but is inconvenient because it basically disables advanced scripting.
        Using this switch turns this "nice by default" feature off and enables you to catch exceptions with your own try/catch.
 
    .NOTES
        Tags: MaxMemory, Memory
        Author: Chrissy LeMaire (@cl), netnerds.net
 
        Website: https://dbatools.io
        Copyright: (c) 2018 by dbatools, licensed under MIT
        License: MIT https://opensource.org/licenses/MIT
 
    .LINK
        https://dbatools.io/Test-DbaMaxMemory
 
    .EXAMPLE
        PS C:\> Test-DbaMaxMemory -SqlInstance sqlcluster,sqlserver2012
 
        Calculate the 'Max Server Memory' for SQL Server instances sqlcluster and sqlserver2012
 
    .EXAMPLE
        PS C:\> Get-DbaRegServer -SqlInstance sqlcluster | Test-DbaMaxMemory
 
        Calculate the 'Max Server Memory' settings for all servers within the SQL Server Central Management Server "sqlcluster"
 
    .EXAMPLE
        PS C:\> Get-DbaRegServer -SqlInstance sqlcluster | Test-DbaMaxMemory | Where-Object { $_.MaxValue -gt $_.Total } | Set-DbaMaxMemory
 
        Find all servers in CMS that have Max SQL memory set to higher than the total memory of the server (think 2147483647) and set it to recommended value.
 
    #>

    [CmdletBinding()]
    param (
        [parameter(Mandatory, ValueFromPipeline)]
        [DbaInstanceParameter[]]$SqlInstance,
        [PSCredential]$SqlCredential,
        [PSCredential]$Credential,
        [switch]$EnableException
    )

    process {
        foreach ($instance in $SqlInstance) {
            Write-Message -Level VeryVerbose -Message "Processing $instance" -Target $instance

            try {
                $server = Connect-SqlInstance -SqlInstance $instance -SqlCredential $SqlCredential
            } catch {
                Stop-Function -Message "Error occurred while establishing connection to $instance" -Category ConnectionError -ErrorRecord $_ -Target $instance -Continue
            }

            Write-Message -Level Verbose -Target $instance -Message "Retrieving maximum memory statistics from $instance"
            $serverMemory = Get-DbaMaxMemory -SqlInstance $server
            try {
                Write-Message -Level Verbose -Target $instance -Message "Retrieving number of instances from $($instance.ComputerName)"
                if ($Credential) {
                    $serverService = Get-DbaService -ComputerName $instance -Credential $Credential -EnableException
                } else {
                    $serverService = Get-DbaService -ComputerName $instance -EnableException
                }
                $instanceCount = ($serverService | Where-Object State -Like Running | Where-Object InstanceName | Group-Object InstanceName | Measure-Object Count).Count
            } catch {
                Write-Message -Level Warning -Message "Couldn't get accurate SQL Server instance count on $instance. Defaulting to 1." -Target $instance -ErrorRecord $_
                $instanceCount = 1
            }

            if ($null -eq $serverMemory) {
                continue
            }
            $reserve = 1

            $maxMemory = $serverMemory.MaxValue
            $totalMemory = $serverMemory.Total

            if ($totalMemory -ge 4096) {
                $currentCount = $totalMemory
                while ($currentCount / 4096 -gt 0) {
                    if ($currentCount -gt 16384) {
                        $reserve += 1
                        $currentCount += -8192
                    } else {
                        $reserve += 1
                        $currentCount += -4096
                    }
                }
                $recommendedMax = [int]($totalMemory - ($reserve * 1024))
            } else {
                $recommendedMax = $totalMemory * .5
            }

            $recommendedMax = $recommendedMax / $instanceCount

            [pscustomobject]@{
                ComputerName     = $server.ComputerName
                InstanceName     = $server.ServiceName
                SqlInstance      = $server.DomainInstanceName
                InstanceCount    = $instanceCount
                Total            = [int]$totalMemory
                MaxValue         = [int]$maxMemory
                RecommendedValue = [int]$recommendedMax
                Server           = $server # This will allowing piping a non-connected object
            } | Select-DefaultView -Property ComputerName, InstanceName, SqlInstance, InstanceCount, Total, MaxValue, RecommendedValue
        }
    }
}