How to successfully Virtualize MS Exchange – Part 17 – Virtual Machine Storage Configuration

In addition to Part 16 where we discussed Virtual Disk Provisioning options and recommendations, In this part we will cover how to optimally configure a Virtual Machine for an Exchange MBX/MSR workload from a virtual storage controller perspective.

Once you have made the decision on storage platform, and assuming you have chosen to use VMFS or NFS datastores (and not iSCSI in-Guest or RDMs), then this article is for you.

Virtual Machines just like physical servers, have SCSI controllers (albeit virtual) and ESXi has a number of options to choose from which include:

1. BusLogic Parallel
2. LSI Logic Parallel
3. LSI Logic SAS
4. Paravirtual SCSI (PVSCSI)
5. AHCI SATA Controller

By default when creating a new virtual machine the default adapter for Windows 2008 and 2012 is “LSI Logic SAS” because Windows does not have the PVSCSI driver by default.

BusLogic ParallelLSI Logic Parallel adapters are not recommended for Windows 2008/2012 as they are legacy controllers with lower performance, as such I will not cover these in any more detail as they are irrelevant to Exchange deployments.

Instead I will cover the LSI Logic SASAHCI SATA Controller and Paravirtual SCSI (PVSCSI) adapters.

Starting with LSI Logic SAS,

This is the default controller for Windows 2008/2012 VMs, as a result, it is very common to see Exchange deployments using this controller. It has good performance and works out of the box with a Windows install without requiring drivers.


1. The default Controller for Windows 2008/2012
2. No need for manually inserting drivers to install Windows
3. Higher performance than AHCI SATA controller


1. Lower performance than PVSCSI
2. Higher CPU overheads in Guest compared to PVSCSI
3. Higher latency than PVSCSI
4. Lower maximum number of VMDKs supported per controller (15) compared to AHCI SATA (30)

Next let’s discuss the AHCI SATA Controller.

The AHCI SATA controller is new in vSphere 5.5 and is only supported in Virtual Machines with Hardware version 10. The SATA controller can be used on its own or in addition to LSI or PVSCSI controllers to provide additional VMDKs / Capacity which increases a single VMs maximum capacity from ~3.7PB to over 11PB.


1. Can support 30 VMDKs per Controller (120 total) compared to 15 for LSI / PVSCSI
2. Can be used in addition to PVSCSI controllers to provide more storage performance and capacity per Exchange VM (if required)
3. High capacity supported per controller than LSI Logic / PVSCSI


1. Higher CPU utilization per IO compared to LSI / PVSCSI options
2. Lower overall performance compared to LSI and PVSCSI
3. Higher latency compared to LSI and PVSCS

And Finally the Paravirtual SCSI Controller.

The PVSCSI controller is the highest performing controller and has been supported since ESXi 4.0 and are design for high performance storage environments and are available for virtual machines running hardware version 7 and later.


1. Performance , Performance , Performance. Oh yeah and did I mention performance?
2. Lower Latency and Higher IOPS compared to other controllers
3. Lower CPU overhead on the Guest OS (and therefore ESXi)
4. More CPU is available for Exchange due to lower CPU overheads


1. Windows Failover Clustering is not supported, but this has no impact on MS Exchange including DAG deployments.
2. PVSCSI is not the default and requires inserting drivers into the Windows installation OR the VM to be built on LSI Logic SAS and once VMware Tools is installed, swapping to PVSCSI.
3. Lower maximum VMDKs supported per controller (15) compared to AHCI SATA (30)

Performance Comparison

From a performance perspective, Michael Webster (VCDX#66) wrote this great post “VMware vSphere 5.5 Virtual Storage Adapter Performance” and produced the following graph showing a comparison between SATA, LSI Logic SAS and PVSCSI controllers from an IOPS, Latency perspective.


As we can see, the PVSCSI adapter has significantly lower latency and higher IOPS than the SATA and LSILogic SAS controllers even when running on the same underlying storage.

While the Microsoft Exchange team have managed to successfully reduce I/O throughout the versions (2007-2013) the performance advantages also have a positive benefit on vCPU utilization.

Michael’s post states:

It (PVSCSI Controller) also had the lowest CPU usage. During the 32 OIO test SATA showed 52% CPU utilization vs 45% for LSI Logic SAS and 33% for PVSCSI.

What this means is less CPU utilization is used for I/O and lower average latency means more CPU is available for MS Exchange along with less CPU WAIT time (where the CPU is waiting for IO to complete before continuing). This means your onto a winner especially considering Exchange 2013 is very CPU intensive.

Which Controller should be used for Exchange VMs?

VMware have published the KB article “Do I choose the PVSCSI or LSI Logic virtual adapter on ESX\ESXi 4.0 for non-IO intensive workloads? (1017652)” which in summary explains:

The test results show that PVSCSI is better than LSI Logic, except under one condition–the virtual machine is performing less than 2,000 IOPS and issuing greater than 4 outstanding I/Os. This issue is fixed in vSphere 4.1 and later version, so that the PVSCSI virtual adapter can be used with good performance, even under this condition.


As the one caveat prior to vSphere 4.1 where LSI Logic can outperform PVSCSI, there are no significant downsides to using the PVSCSI compared to LSI as such, I recommend always using (multiple) PVSCSI adapters.

Now that we have decided on the PVSCSI adapter, what’s next?

As with physical servers, Virtual SCSI controllers including PVSCSI have their limits in terms of performance and scalability. To ensure maximum scalability, performance and low latency, multiple PVSCSI adapters should be used with all VMDKs evenly spread over the PVSCSI adapters as recommended in Part 11.

To do this, when adding a VMDK to the Exchange VM, ensure you select a different SCSI controller (which are created automatically on demand) by using the drop down box “Virtual Device Node” and selecting for example SCSI (1:0) as shown below.


For subsequent VMDKs you must then select SCSI (2:0) as shown below.


And then SCSI (3:0)


For the forth VMDK, you then select SCSI (0:1) because SCSI (0:0) is taken by the VMDK used for the guest OS.


Repeat the above process until you have sufficient VMDKs for your Exchange server VM.

The following illustrates my recommended configuration showing how to configure a VM supporting 8 database drives and 8 log drives.PVSCSIVMDKs

The above configuration will ensure maximum storage performance and can be expanded in the same configuration to support more than 3 times the number of databases + logs shown above and as such it is suitable for even very large (scale-up) Exchange MBX/MSR VMs.

For example, if each VMDK in the above configuration was just 4TB in size it would give you 64TB usable capacity and the VM can be scaled more than 3x the number of VMDKs.

Note: VMDKs can scale to 62TB (from vSphere 5.5) each although this may result in reduced performance.

TIP: Don’t forget to spread VMDKs evenly across datastores as per the recommendation in Part 11.

Recommendations for Exchange VM Storage Configuration:

1. Use multiple Paravirtual SCSI (PVSCSI) Adapters.
2. Use one VMDK per Database or Logs
3. Spread VMDKs evenly across multiple PVSCSI adapters
4. Spread VMDKs evenly across multiple datastores when using VMFS datastores
5. Spread VMDKs evenly across multiple datastores when using NFS datastores ensuring NFS datastores are served via multiple NAS controllers
6. Use more VMDKs as opposed to fewer larger VMDKs
7. Format NTFS volumes with an Allocation Unit Size of 64k
8. Keep it simple, do not mix virtual SCSI controller types.

Back to the Index of How to successfully Virtualize MS Exchange.

How to successfully Virtualize MS Exchange – Part 16 – Virtual Disk Provisioning Types

Once you have made the decision on storage platform, and assuming you have chosen to use VMFS or NFS datastores, the next decision is how should my VMDKs be provisioned?

The VMware Exchange 2013 Best Practice Guide does not make mention of disk provisioning options nor does it make any recommendations, however you’re in luck as we will cover all the options along with pros and cons here.

For Exchange 2010, Microsoft state in Understanding Exchange 2010 Virtualization:

Virtual disks that dynamically expand aren’t supported by Exchange.

Virtual disks that use differencing or delta mechanisms (such as Hyper-V’s differencing VHDs or snapshots) aren’t supported.

However I have been unable to find confirmation if this has changed or not for Exchange 2013 in the Exchange 2013 storage configuration options document which does state Thin provisioning for Storage spaces is supported but it does not state that any other form of thin provisioning is or is not supported.

While technically not supported in 2010, there is plenty of experts who understand and recommend thin provisioning including MCM and MVP for Exchange Dustin Smith who in this video talks about some of the considerations and benefits of thin Provisioning for Exchange 2010.

Now on to the topic at hand:

When creating a Virtual Machine, VMDK/s can be provisioned in one of three ways, these are:

1. Thick Provisioned Lazy Zeroed
2. Thick Provisioned Eager Zeroed
3. Thin Provisioned

Starting with Thick Provisioned Lazy Zeroed this means that the VMDK is thick provisioned but only zeroed in a just in time fashion.

The advantages of Thick Provisioned Lazy Zeroed VMDKs include:

1. Faster VM creation time than Eager Zeroed Thick (Minimal if the storage supports VAAI Write Same primitive) 
2. The entire VMDKs capacity is reserved making capacity planning easier than Thin Provisioning

The disadvantages of Thick Provisioned Lazy Zeroed VMDKs include:

1. Slower provisioning that Thin Provisioning (although the different is generally minimal)
2. The entire VMDKs capacity is reserved and unavailable for use by other virtual machines.

With Thick Provisioned Eager Zeroed (EZT) the VMDK is thick provisioned and all blocked zeroed at the time of creation. Eager Zeroed Thick VMDKs are supported on all VMFS datastores and on NFS datastores which support the VAAI-NAS Reserve Space primitive.

The advantages of EZT VMDKs these days are really minimal but include:

1.  Supporting Oracle RAC and VMware Fault Tolerance (neither being applicable to Exchange)
2. Increased performance verses Lazy and Thin Provisioned VMDKs (but more on this topic later).

However there are a number of downsides to this method which include:

1. Slower VM creation times. The time depends on the size of the VMDK/s being created and the speed of your storage as every Gb needs to be zeroed, just like performing a Full (not quick) format on your physical server.

Note: Storage array’s who support VAAI with the “Write Same” primitive can offload the zeroing to the storage array to reduce the load on the ESXi host and speed up provisioning time dramatically.

2. Increased potential for wasted capacity on a datastore.

3. Free space within VMDKs cannot be shared with other VMs which requires every VMDK have some (generally >10% is recommended) free space per VMDK to ensure the VM does not run out of space.

Lastly there is  Thin Provision which means the VMDK only takes up the amount of space that data is written too and before each write the block must be zeroed.

The advantages of Thin Provisioning VMDKs include:

1. You can create larger VMDKs with no space utilization penalty making capacity planning and growth easier.
2. Reduce wasted or unused space on the storage
3. Allows for disk space to be overcommitted ensuring maximum utilization and flexibility.
4. Free space in VMDKs is not wasted on the datastore reducing capacity requirements compared to Eager and Lazy Zeroed VMDKs.
5. The impact of SCSI reservations (VMFS datastores ONLY) causing performance issues (increased latency) when thin provisioned virtual machines (VMDKs) grow is no longer an issue as the VAAI Atomic Test & Set (ATS) primitive alleviates the issue of SCSI reservations.
6. Thin provisioned VMs reduce the overhead for Storage vMotion , Cloning and Snapshot activities. Eg: For Storage vMotion it eliminates the requirement for Storage vMotion (or the array when offloaded by VAAI XCOPY Primitive) to relocate “White space”. Note: Storage vMotion should rarely if ever be required for Exchange VMs.
7. Thin provisioning leaves maximum available free space on the physical spindles which should improve performance of the storage subsystem as a whole.

The disadvantages of thin provisioning include:

1. Increased risk of running out of space on a datastore or underlying storage array.
2. Additional write penalty of zeroing a block before writing to it. (again more on performance later in this post).
3. Increased importance of monitoring storage capacity utilization.
4. Not supported for Exchange 2010. Note: However there is no technical inhibitor for using Thin Provisioning but supported options are obviously preferable.

All in all, @FrankDenneman (VCDX #29) sums it up perfectly with his article Thin or thick disks? – it’s about management not performance. I would also suggest considering all other workloads in the environment, not just Exchange when making decisions about Thin Provisioning as it can be very beneficial and a huge cost saving (especially CAPEX) when purchasing new equipment.

Which brings us to our next topic, Thin Vs Thick Provisioning Performance!

There have been many recommendations not to use Thin Provisioning due to the performance impact of Zeroing a block before writing to it. This recommendation has been around for a long time, and like the VMDK on NFS debate appears to have strong options on both sides.

Now for the facts!

From a performance perspective most people are surprised to learn there is no significant performance advantage to using Thick Provisioned (Eager or Lazy Zeroed) VMDKs compared to Thin Provisioned disks.

In addition to that, with the reduction of I/O from Exchange 2007 to 2010 being around 50%, and from 2010 to 2013 another 50% reduction in I/O, Exchange is no longer the huge storage I/O heavy monster it once was.

VMware conducted a Performance Study of VMware vStorage Thin Provisioning back in the ESXi 4.0 days (~2009) which I will briefly summarize.

On page 6 of the performance study the following graph shows the different in performance between Thin and Thick VMDKs during zeroing and post-zeroing.

As you can see the performance is almost identical.


The next chart shows also from Page 6 is a comparison of throughput between thin and thick VMDKs. Again we see the difference is insignificant.


As a result of there being no significant performance impact of using Thin Provisioning, Performance should no longer be considered an objection to using Thin Provisioning!

I recommend taking advantage of the flexibility of using Thin Provisioning and creating larger Thin Provisioned VMDKs which can help simplify capacity management from a VM/OS and application perspective as well as making growth easier for Exchange as mailbox sizes increase over time.


When using thin provisioning always ensure you have your alerting properly set-up with early warning on your vSphere environment AND underlying storage to advise when storage capacity of a datastore or underlying LUN/NFS mount or storage is running low so this can be remediated.

In an upcoming post I will discuss the underlying storage, including provisioning type for LUNs and NFS mounts (i.e.: Thin on Thick / Thin on Thin / Thick on Thick and Thick on Thin).

Recommendations for VMDK provisioning:

1. Check with your storage vendor and unless they have solid justification for not using Thin Provisioning OR you have an operational constraint preventing it, use Thin Provisioned VMDKs. (The pros outweigh the cons in my opinion)
2. When using Thin Provisioning create larger VMDKs to simplify capacity management at the VM and OS/Application layer.
3. When using Thick or Thin provisioning, ensure you test performance using Jetstress and LoadGen with the same provisioning type.
4. Ensure alerting is configured and working to monitor capacity utilization especially when using thin provisioned VMDKs.

Back to the Index of How to successfully Virtualize MS Exchange.

More Information on VMDK and Datastore provisioning options:

1. Example Architectural Decision – Datastore (LUN) and Virtual Disk Provisioning (Thin on Thin)

2. Example Architectural Decision – Datastore (LUN) and Virtual Disk Provisioning (Thin on Thick)

Back to the Index of How to successfully Virtualize MS Exchange.

How to successfully Virtualize MS Exchange – Part 11 – Types of Datastores

Datastores are a logical construct which allows DAS,SAN or NAS storage is presented to ESXi. In the case of SAN and NAS storage it is generally “shared storage” which enabled virtualization features such as HA, DRS and vMotion.

When storage is presented to ESXi from DAS or SAN (block based) storage it is formatted with VMFS (Virtual Machine File System) and when storage is presented via file storage (NFS), it is presented to ESXi as an NFS mount.

Regardless of datastores being presented via block (iSCSI,FC,FCoE) or file based (NFS) protocols, they both host VMDKs (Virtual Machine Disks) which are block based storage. In the case of NFS, the SCSI commands are emulated by the hypervisor. This process is explained in Emulation of the SCSI Protocol and can be compared to Hyper-V SMB 3.0 (File) storage with VHDX which also emulates SCSI commands over File (SMB 3.0) storage.

The following diagram is courtesy of and shows “host1″ and “host2″ runnings VMs across VMFS (block) and NFS (file) datastores. Note the VMs residing on datastore1 and datastore2 all have .vmx and .vmdk files and operate in the exact same way from the perspective of the VM, Guest OS and applications.


The next paragraph is controversial and may be hotly debated, but to the best of my knowledge and the countless industry experts (from several different vendors) I have investigated this with over the last year including VMware’s formal position, it is completely true and I welcome any credible and detailed evidence to the contrary! (I even asked this question of Microsoft here).

Using either VMFS or NFS datastores meets the technical requirements for Exchange, being Write Ordering, Forced Unit Access (FUA) and SCSI abort/reset commands and because drives within Windows are formatted with NTFS which is a journalling file system as such the requirement to protect against Torn I/O is also achieved.

With that being said, Microsoft currently do not support Exchange running in VMDKs on NFS datastores.

The below is a quote from Exchange 2013 storage configuration options outlining the storage support statement for MS Exchange with the underlined section applying to NFS datastores.

All storage used by Exchange for storage of Exchange data must be block-level storage because Exchange 2013 doesn’t support the use of NAS volumes, other than in the SMB 3.0 scenario outlined in the topic Exchange 2013 virtualization. Also, in a virtualized environment, NAS storage that’s presented to the guest as block-level storage via the hypervisor isn’t supported.

If your interested in finding out more information about MS Exchange running in VMDKs on NFS datastores, see the links at the end of this post.

Now let’s discuss the limitations of Datastore’s and what impact they have on vSphere environments with MS Exchange deployments and why.

Number of LUNs / NFS Mounts : 256

This can be a significant constraint when using one or multiple datastore/s per Exchange MBX/MSR VM however in my opinion this should not be necessary nor is it recommended.

Generally Exchange VMDKs can be mixed with other VMs in the same datastore providing there is not a performance constraint. As such keep high I/O VMs (including other Exchange VMs) in different datastores.

As discussed in Part 10, if legacy per LUN snapshot based backup solutions are being used then in-Guest iSCSI may have to be used but for new deployments especially where new storage will be purchased, per LUN solutions should not be considered!

Number of Paths per ESXi host : 1024

If using VMFS datastores, the simple fact is 4 paths per LUN is the maximum you can use if you plan to reach or near the limit of 256 datastores. This is not a performance limiting factor with any enterprise grade storage solution.

Number of Paths per LUN : 32

If you configure 32 paths per LUN, you straight away restrict yourself to 32 LUNs per ESXi host (and vSphere cluster) so don’t do it! As mentioned earlier 4 paths per LUN is the maximum if you plan to reach 256 datastores. Do the math and this limit is not a problem.

Number of Paths per NFS mount : N/A

NFS mounts connect over IP to the Storage Controllers via vNetworking so there is no maximum as such although with NFS v3 only one physical NIC will be used at a time per IP subnet. This topic will be covered in a future post on vNetworking for MS Exchange.

VMFS Datastore Maximum Size : 64TB

256 LUNs x 64TB = 16,384TB per vSphere HA cluster. This is not a problem.

NFS Datastore Maximum Size : Varies depending on vendor

The limit depends on vendor but its typically higher than the VMFS limit of 64TB per mount, with some vendors not having a limit.

So you’re safe to assume >=16.384TB but always check with your current or potential storage vendor.

ESXi hosts per Volume (Datastore) : 64 (Note: HA cluster limit of 32)

As a vSphere cluster is limited to 32 currently, this limitation isn’t really an issue. With vSphere 6.0 it is expected cluster size will increase to 64 but ESXi hosts per Volume is not a maximum I have ever heard of being reached.


1. If you require a fully supported configuration use VMFS datastores.
2. Maximum 4 paths per LUN to ensure maximum scalability (if required).
3. Consider the underlying storage configuration and type of a datastore before deploying MS Exchange.
4. Do not deployment MS Exchange VMDKs onto datastores with other high I/O workloads
5. When mixing workloads on a datastore, enable SIOC to ensure fairness between workloads in the event of storage contention.
6. Spread Exchange VMDKs across multiple datastores for maximum performance and resiliency. e.g.: 12 VMDKs per Exchange MBX/MSR VM across 4 mixed workload datastores.
7. Do not use dedicated datastore/s per MS Exchange database or VM. (This is unnecessary from a performance perspective)
8. If choosing to use NFS Datastores, purchase Premier Support from Microsoft and negotiate support for NFS. Microsoft do provide support for many customers with Premier Support with Exchange running on NFS datastores although it is not their preference.

On a final note, in future posts I will be discussing in detail the underlying storage from a performance and availability perspective with Database availability Groups in mind.

Thank you to @mattliebowitz for reviewing this post. I highly recommend his book Virtualizing Business Critical Applications by VMware Press. I purchase and reviewed this book mid 2014, well worth a read!

Back to the Index of How to successfully Virtualize MS Exchange.

Articles on MS Exchange running in VMDK on NFS datastores

1. “Support for Exchange Databases running within VMDKs on NFS datastores

2. Microsoft Exchange Improvements Suggestions Forum – Exchange on NFS/SMB

3. What does Exchange running in a VMDK on NFS datastore look like to the Guest OS?

4. Integrity of I/O for VMs on NFS Datastores Series

Part 1 – Emulation of the SCSI Protocol
Part 2 – Forced Unit Access (FUA) & Write Through
Part 3 – Write Ordering
Part 4 – Torn Writes
Part 5 – Data Corruption