functions/Test-DbaMaxMemory.ps1

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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
   Allows you to specify a comma separated list of servers to query.
 
  .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: Memory
   dbatools PowerShell module (https://dbatools.io, clemaire@gmail.com)
   Copyright (C) 2016 Chrissy LeMaire
   License: GNU GPL v3 https://opensource.org/licenses/GPL-3.0
 
  .LINK
   https://dbatools.io/Test-DbaMaxMemory
 
  .EXAMPLE
   Test-DbaMaxMemory -SqlInstance sqlcluster,sqlserver2012
 
   Calculate the 'Max Server Memory' settings for all servers within the SQL Server Central Management Server "sqlcluster"
 
  .EXAMPLE
   Test-DbaMaxMemory -SqlInstance sqlcluster | Where-Object { $_.SqlMaxMB -gt $_.TotalMB } | 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(Position = 0, Mandatory = $true, ValueFromPipeline = $True)]
        [Alias("ServerInstance", "SqlServer", "SqlServers")]
        [DbaInstance[]]$SqlInstance,
        [PSCredential]$SqlCredential,
        [PSCredential]$Credential,
        [switch][Alias('Silent')]$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 "Failure" -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-DbaSqlService -ComputerName $instance -Credential $Credential -EnableException
                }
                else {
                    $serverService = Get-DbaSqlService -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.SqlMaxMB
            $totalMemory = $serverMemory.TotalMB

            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  = $serverMemory.ComputerName
                InstanceName  = $serverMemory.InstanceName
                SqlInstance   = $serverMemory.SqlInstance
                InstanceCount = $instanceCount
                TotalMB       = [int]$totalMemory
                SqlMaxMB      = [int]$maxMemory
                RecommendedMB = [int]$recommendedMax
            } | Select-DefaultView -Property ComputerName, InstanceName, SqlInstance, InstanceCount, TotalMB, SqlMaxMB, RecommendedMB
        }
    }
}