Dedicated swapfile datastores in VMware are often overlooked.   Here’s why you might use them, and how to size them easily with PowerCLI.

It’s very often advisable to create dedicated swapfile datastores in your VMware vSphere environment.   There are numerous benefits:

• Ensure there’s room to start a VM
• Use different storage type than what the working directory uses for performance or cost savings
• Reduce replication traffic when using storage based replication, because there’s no reason to replicate this storage
• You may want to snapshot storage that runs VMs for easy recoverability, but there’s no reason to snapshot swapfile

If you decide to create dedicated datastores, you want to use the following principles:

• Create datastores that are resilient, so that VMs can be started
• Have hosts that frequently have VMs VMotion between them, such as a cluster, use the same datastores to reduce vMotion network traffic
• Carefully monitor their space, and size them correctly, and allow for some overhead for growth.

The swapfile size for each VM is determined by the following:

• The VM’s defined RAM minus the RAM reservation for that VM.

For example, if a VM is defined as having 8GBs, but the reservation for RAM is set for 2GBs, a 6GB swapfile will be created.  By default, a VM has no reservation for RAM.

That means that this datastore space consumption can fluctuate as VMs are built, powered off and on, whenever RAM is added or removed from a VMs definintion, or if its memory reservation is adjusted.

This begs the question – How do you easily size for these datastores easily?  Harnass PowerShell by using PowerCLI!  Simply tune the $vms variable portion or what’s piping to it of the following to grab the VMs that will likely VMotion between the same hosts.  This would usually be by cluster.

$vms = get-cluster clustername | get-vm
$RAMDef = $vms | Measure-Object -Sum memoryGB | select-object sum -expand sum
$RAMResSum = $vms | get-VMResourceConfiguration | measure-object -sum memreservationGB | select-object sum -expand sum
$SwapDatastore = $RamDef - $RamResSum
Write-Host "Defined amount of RAM within VMs is $RAMDef GBs"
Write-Host "Memory reservation for VMs is $RamResSum GBs"
Write-Host "A datastore of at least $SwapDatastore GBs will be needed, plus overhead."

Output will look like this:

Defined amount of RAM within VMs is 218 GBs
Memory reservation for VMs is 0 GBs
A datastore of at least 218 GBs will be needed, plus overhead.

For overhead, you want to keep at least 25% free probably minimum just to keep datastore free space alarms from going off, plus any additional growth from the factors outlined above, mostly centered around new VMs being built.

Many customers balk when told how big the swapfile datastore will be, but you have to remember if you’re changing this within a customer’s environment, they’re going to gain back swapfile space within their existing datastores as swapfiles get placed on the dedicated datastore.

Also, think of the potential storage space savings you could get if you are storage snapshotting your VM datastores, and replicating, plus the bandwidth savings.  Let’s say you have VMs that in aggregate are defined with 500GBs of RAM with no memory reservation.  If you’re doing both snapshots and replication and didn’t dedicate a datastore to the swapfiles, you’re talking savings of 500GBs of replication space, and up to 1TB worth of space savings alone depending upon how much additional space the swapfiles are taking within your storage snapshots.  Pretty worth it!

How do you migrate existing swapfiles?

1. First, set your cluster to use the host’s swapfile setting instead of the cluster’s.
2. Set all your hosts to use the same datastore.

To do this in PowerCLI:

$cluster = "clustername"
$swapfiledatastore = "swapfiledatastorename"
get-cluster $cluster | set-cluster -VMSwapfilePolicy InHostDataStore

You’ll have to manually set the host’s cluster datastore with the web or thick client.  PowerCLI fails to set the heartbeat datastore if the host is in a cluster unfortunately.

You should see the swapfiles deleted from the VMs’ working directories and created in the new datastore as VMs are power cycled.