The basic premise of the article is this – you can achieve high availability with Exchange 2010 without the use of Database Availability Groups (DAG) by using a combination of VMware HA and Symantec’s ApplicationHA.  Application HA leverages APIs exposed in vSphere 4.1 that let third party applications interact and communicate with VMware HA to perform various functions such as restarting the guest if the entire application has failed.  ApplicationHA can also be used to restart individual guest services, so it can provide a level of application awareness beyond what VMware HA can do alone.

Joe and I agree that the blog is technically correct in that you can get high availability for Exchange 2010 without using DAGs if you use these tools.  Where we don’t agree with the author is in the usefulness of this kind of solution compared to the functionality that a DAG actually provides.

More than just HA

DAGs provide high availability of the Exchange environment but they do much more than that.  They facilitate DR scenarios by including a native replication technology that lets organizations easily replicate their email data to another site that can be brought online quickly and safely.  You can also easily move databases between Mailbox servers within the same site for routine system maintenance, Windows patches, etc.  DAGs can also detect a failure and activate another database copy quickly, most times (but not always) quicker than a VM can reboot.  Additionally, because a reboot can be required with an ApplicationHA/VMware HA solution, there is technically an outage to the end users and the end user experience associated with a reboot repopulating the Exchange database cache isn’t ideal.

DAGs provide high availability at the storage layer

Since a DAG allows up to 16 copies of any mailbox database, it can provide high availability at the storage layer in addition to just protecting the server and Exchange services.  This allows you to put your databases on completely separate storage platforms for a higher degree of availability and protection from many kinds of storage-related failures.  Since Exchange 2010 enables and encourages larger mailboxes and databases, recovery time in the event of catastrophic server failure can be significant if some kind of storage high availability or resiliency isn’t in place.

Management separation

In many firms, the engineering team responsible for Exchange may not be the same as the team responsible for the virtual environment.  In these cases, it is important to understand that leveraging a DAG would allow the Exchange team to be wholly responsible for high availability of the Exchange application.  Leveraging ApplicationHA/VMware HA to protect Exchange would require coordination between the Exchange and virtualization teams for any high availability issues/troubleshooting and that may not be appropriate or optimal in many environments.

Microsoft supported technology

Whenever you go down the road of protecting Microsoft applications with tools that are provided by third party vendors, you run the risk of having issues with Microsoft support.  This used to be more of an issue in the earlier days when host based replication tools were used to protect Exchange, SQL, etc., and is likely less of a problem these days.  I suspect that Symantec has done the right thing here and ApplicationHA does not cause issues with Microsoft support, but having never used it I can’t say for sure.

Cost

When it comes to protecting Tier-1 applications like Exchange cost is important but is not often the driving factor in making a technology decision.  Email has become one of the most important applications in the organization so firms are more willing to pay up to protect it.  That said, the cost of Symantec’s ApplicationHA seems high compared to the costs of implementing a DAG.  With a DAG, you’re just paying for an extra Windows license (which may not have a capital cost depending on your Windows licensing model) and an additional Exchange license.  That may come out cheaper than ApplicationHA, which is licensed in packs of 5 protected servers.  Again cost is usually not the most important factor, but paying less and getting more functionality out of a DAG makes it an attractive option.

Final thoughts

I strongly believe that vSphere provides a great platform for virtualizing Tier-1 applications like Exchange.  Joe and I have worked with clients that have virtualized Exchange 2010 for thousands of users on top of vSphere with great success.  Yet we both feel that the value that DAGs add to Exchange 2010 makes using them on your virtualized Exchange 2010 deployment well worth the added planning, storage requirements, and restrictions.

In my opinion there are some areas where using ApplicationHA and VMware HA guest level monitoring might be useful, such as:

• Small environments where there is no budget for additional storage purchases to accommodate multiple database copies.
• There is a requirement for using vMotion on Mailbox servers (which is not supported in combination with DAGs).  I personally cannot think of a use case where I would want to use vMotion over moving databases between nodes in a DAG, but I recognize there may be situations where this may be desirable.
• An organization already has an investment in a host based or storage based replication product for Exchange but still wants server high availability.  Again, with the native replication functionality of Exchange 2010 (and the 3rd party replication API for the DAG) there are less reasons to use features other than what the DAG provides natively.

I believe that ApplicationHA and using VMware HA for guest level monitoring is a huge step in the right direction.  There are many other use cases for this technology and I think we’ll start to see more people enabling this for nearly all workloads, particularly the native VM monitoring provided by VMware HA.  In the case of Exchange 2010, I think the benefits of DAGs outweigh the benefits that using ApplicationHA provide.

In VMware vSphere, there are several native Path Selection Policies (PSP) that handle how the ESX or ESXi hosts connect to the storage infrastructure.  For best performance, most use VMware’s native for Round Robin PSP for iSCSI MPIO.  This allows you to better utilize all of your NICs rather than keeping the paths in an active/standby configuration.  In addition to the native policies, VMware has also opened this up to storage vendors to write their own PSPs to take better advantage of their storage arrays.

Previously only available in vSphere Enterprise Plus, with the release of vSphere 4.1 VMware has reduced the licensing requirement to use vendor supplied PSPs to the Enterprise level.  At the same time, Dell released their own PSP for best performance when using EqualLogic SANs.

So why does it make sense to use the MEM and specifically if you’re using Windows Failover Clusters?  Here are a few reasons:

1) VMware does not support virtualized Failover Clusters using the native Round Robin policy.  They also technically do not support Microsoft clusters on iSCSI storage either, but VMware has stated that that restriction will be going away soon.  See this link for more info on virtualizing Microsoft clusters (PDF link).

2) Dell has shown improved performance when using the MEM on virtualized clusters, particularly with Microsoft Exchange 2010 VMs.

3) Installation and configuration of the MEM is very easy and can be configured on a per VMFS volume level.  This would allow you to only use the MEM for the VMFS partitions where your clustered VMs reside.  That said, the benefits of the MEM extend to non-clustered VMs and should probably be used for all VMFS partitions.

Today I decided to take that concept and extend it to systems that you might manage alongside your VI3/vSphere environment.  Storage management seemed like the obvious first choice.

I created an XML file called EqualLogic.xml in C:\Program Files\VMware\Infrastructure\Virtual Infrastructure Client\Plugins\SAN Management.  The contents of the file are as follows (you would replace the <url> section with the IP or DNS name of your SAN):

<scriptConfiguration version=”1.0.0″>
<key>EqualLogic</key>
<description>EqualLogic SAN Management</description>
<view parent=”Inventory.HostSystem”>
<title locale=”en”>EqualLogic</title>
<url>http://10.1.97.30</url>
</view>
</scriptConfiguration>

This gave me a nice way to manage my SAN from the same interface that I use to manage my vSphere environment.  It is simply opening a browser window within the vSphere Client and letting me manage the SAN.

The code above will make the EqualLogic tab visible only when clicking on an ESX/ESXi host.  If you wanted to extend that to other objects, you can simply adjust the “<view parent=” section.  For example, to also make this available at the cluster level you would include the following:

<view parent=”Inventory.Cluster”>
<title locale=”en”>EqualLogic</title>
<url>http://10.1.97.30/</url>
</view>

Similarly you could add Inventory.Global, Inventory.VirtualMachine, Inventory.Datacenter, etc.

This is a really simple way to make it easy to manage any web interface (not just EqualLogic) from within the vSphere/VI Client.  It’s not a new trick and has been out there for a while but I had never used it for this until today.

In my recent post on the effects of ASLR in vSphere the comments turned into a discussion about TPS on modern processors. And there are countless posts about this issue on the VMTN forums where folks are looking for a fix. In reality nothing is broken and there is no need to fix the issue.

VMware has published a KB article that gives more information on TPS with Nehalem processors and why it appears TPS isn’t working (this affects modern AMD processors also). The short version is that TPS uses small pages (4K), and Nehalem processors utilize large pages (2MB). The ESX/ESXi host keeps track of what pages could be shared, and once memory is over-comitted it breaks the large pages into small pages and begins sharing memory.

Many people think this is a bug in ESX that needs to be fixed. This likely started because when vSphere 4 was released there was a bug around memory usage on ESX hosts with Nehalem processors. In reality the bug was that vCenter was triggering high memory usage alarms for virtual machines running in this configuration. Nothing was actually wrong but because the host was using all of the assigned memory for the VM, vCenter was incorrectly triggering the alarm. That behavior has since been fixed with a patch and is no longer an issue.

So what does this actually look like? When a VM is powered up on an ESX host with Nehalem processors, the amount of host memory in use will not drop down as the VM uses less memory or becomes idle. Those of us that have been using ESX for a long time likely found this scenario disturbing.

From vSphere Client (red highlighted section shows guest taking all of the 2GB assigned memory, yet memory usage in the guest is very low):

From esxtop (red highlighted section shows almost no memory being shared with page sharing):

The above screenshots show a host that is under-committed on memory and so no page sharing is occurring.  If the host gets over-commited page sharing kicks in automatically by breaking up large pages into small pages.  You can force the use of small pages on all guests all the time by changing the value of the advanced option Mem.AllocGuestLargePage to 0.  I don’t really see any reason to do this – remember that TPS isn’t broken and what you see in the above screenshots is normal and expected.

Once host memory is over-committed (or if you use the advanced option), memory sharing kicks in and things look like they normally do when page sharing is taking place.

From the vSphere Client (red highlighted section shows guest taking very little of the assigned 2GB memory as page sharing has kicked in):

From esxtop (red highlighted sections show a large amount of shared memory and the host is over-commited on memory by 48%):

A quick note on the esxtop screenshot above – it was taken from a VDI environment where all workloads are identical so that explains the high amount of shared pages.  It was also overcommitted more than normal as it was taken during host maintenance.

I hope this clears up some of the confusion around TPS on modern Intel/AMD processors.  In short, don’t get hung up on the fact that TPS isn’t kicking in like it did with older processors.  Nothing is broken, TPS is working as expected, and it will kick in when you actually need it.

I’m working on a vSphere implementation using Dell EqualLogic SANs and wanted to configure Round Robin on all of my datastores. Dell has a great whitepaper on how to set this up, but unfortunately the document fails to mention one key thing: this doesn’t change the default path selection plugin (PSP) from Fixed to Round Robin.   That means that you’ll have to set the multipathing policy to Round Robin on all of your existing datastores and will have to remember to do that on all future datastores. When you’ve got multiple ESX hosts with lots of  datastores this can quickly become a pain.

Luckily there is a way to force the default multipathing policy to Round Robin. The following commands can be used to change the default PSP to Round Robin as well as configure round robin specifically for the EqualLogic provider.  These commands can be entered at the Service Console or via the vSphere CLI 4.0:

esxcli nmp satp setdefaultpsp –satp VMW_SATP_DEFAULT_AA –psp VMW_PSP_RR
esxcli nmp satp setdefaultpsp –satp VMW_SATP_EQL –psp VMW_PSP_RR
esxcli corestorage claimrule load
esxcli corestorage claimrule run

Note that “satp” and “psp” are preceded by two dashes and not a single dash as it appears in this blog post.

Once you enter those commands (no rebooting required) any volume you add, either new or existing, will use Round Robin MPIO by default.

Address Space Layout Randomization (ASLR) is a security feature that randomizes the position of data in memory, making it more difficult for attackers to predict where data can be found while in memory. This feature has been enabled in Windows since Windows Vista, and other operating system such as Linux and MacOS implement this in some form as well.

Since ASLR randomizes information in memory it makes sense that it would be more difficult for TPS to find identical memory pages and thus memory sharing would be reduced. But just how much of a difference does it make? I decided to try and find out. Here are the specs from my test environment:

Server: HP DL385 G1 (AMD Opteron 275)
ESX: 4.0.0 build 244038
Guest OS: Windows Server 2008 R2
Guest RAM: 2.5GB

All guests were cloned from the same template and have the same software installed. On guests TESTSRV1 and TESTSRV3, I left the default settings. On TESTSRV2 and TESTSRV4, I disabled ASLR using the following regkey:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management]
“MoveImages”=dword:00000000

In all of my testing, including leaving VMs idle and also running memory tests, I found no measurable difference in the amount of memory shared with TPS. I also looked at esxtop to see how much memory was actually being shared and I saw virtually no difference whatsoever between VMs that had ASLR enabled and those that had it disabled.

Host Memory Usage:

esxtop statistics:

The SHRD and SHRDSVD columns represent how much memory is being shared with TPS and the total memory savings. Clearly there is quite a bit of memory sharing going with or without ASLR enabled.

Why would this be the case since it makes sense that TPS would be hurt by ASLR? ASLR requires applications to “opt-in” to have their memory randomized, and I suspect that much of Windows 2008 R2 is not opted in. Perhaps applications will come out in the future that are written to take advantage of ASLR, but at the moment that doesn’t appear to be the case.

Of course this is by no means a definitive test as it wasn’t run with production systems and real users running real applications. That said, I think it shows that ASLR does not dramatically reduce the amount of memory shared with TPS. I did also look at production systems left at the default settings (ASLR enabled) and saw similar memory sharing gains. I’m curious if others have seen similar results in their environments, so drop me a line if you’ve done any similar testing.

More info:

What is ASLR (Wikipedia)
Interpreting esxtop statistics

FT is often compared to Microsoft Windows Failover Clusters, formerly Microsoft Cluster Server (MSCS), and in fact many have talked about how FT can replace Microsoft clustering altogether.  Rather than jump to conclusions like this, it is important to understand the use cases for both technologies.  In addition, there are several limitations to FT that need to be considered. Here are some important points to remember about FT:

1) FT only supports a single vCPU, limiting its usefulness for some applications (this will probably change in the future).

2) FT is meant to protect against host level failures only, such as physical server failures.

3) FT keeps protected virtual machines in complete lockstep, meaning whatever happens on the Primary VM also happens on the Secondary VM.  Why is this important to understand?  Guess what happens when the Primary VM bluescreens.

4) FT VMs share the same virtual disk file, meaning a storage level failure affects both.

Microsoft Failover Clustering, on the other hand, can help protect against application and operating system level failures in addition to physical server failures.  Clusters also make it easier to patch the underlying operating system with minimal downtime.  Finally, FT has a limited set of hardware that it is compatible with (see this link to check if your system is compatible) so that may limit its usefulness for some organizations using older hardware.

All of that said, FT is a great feature and definitely has a place in your virtual infrastructure.  Best of all it is available in vSphere Advanced and above, making it an affordable feature that many organizations may already own.  So what are good use cases for FT over Microsoft clusters?  Here are some thoughts:

• Provide hardware level protection to applications that don’t natively support any clustering functionality.  There are many examples here – web servers, application servers, etc.
• Provide hardware level protection to applications where clustering support may be available but is either expensive or requires a special license.  Document management servers, indexers, etc., may make good candidates here.
• Provide extra protection against hardware failures for critical applications during specific business periods where downtime simply cannot be tolerated, such as accounting servers during end of month processing.
• Your specific virtual infrastructure configuration precludes you from using Microsoft clusters due to Microsoft or VMware supportability.

VMware FT is a great new feature that can help provide extra uptime to virtual machines in your environment.  It is available in most versions of vSphere and should definitely be considered as part of a virtual infrastructure design.  Just make sure you understand the use cases for it and don’t rule out Microsoft clusters where they are appropriate.

See also:

FT Frequently Asked Questions

Third party storage and backup vendors frequently call the vCenter API to issue a snapshot creation of running virtual machines before they grab a copy of the virtual machine’s hard disk – VMDK files – which are located on the VMware VMFS datastore. This process is typically followed by an immediate deletion of the snapshot file which will merge the changes back into the initial VMDK file. Problems can occur when this process does not complete successfully leaving you with an “open” snapshot. This problem is compounded when it occurs multiple times against the same guest.

Each open snapshot can significantly degrade virtual machine performance and also contribute to poor storage utilization. Unless you monitor each of your virtual machines on a daily basis, you could quickly be left with an uncomfortable situation on your hands.

The script below can be easily customized for your environment and provide regular email reports for open snapshots.

#Initialize VI Toolkit

Add-PSSnapin VMware.VimAutomation.Core

#Configure VC credentials

$Username = ‘service_account_name’

$password = ‘PASSWORD’

$server = ‘vcenter_host.domain.local’

#Setup email client

$SmtpClient = New-Object system.net.mail.smtpClient

$MailMessage = New-Object system.net.mail.mailmessage

$SmtpClient.host = “smtp_host.domain.local”

$MailMessage.from = “virtualcenter@domain.com”

$MailMessage.To.add(“admin@domain.com”)

$MailMessage.IsBodyHtml = 1

$MailMessage.Subject = “List of current VMware Snapshots”

#Get snapshots from all servers

Connect-VIServer $server -User $Username -Password $password

$Snaps = Get-VM | Get-Snapshot | Select VM,Name,Created

# Send email

$MailMessage.body = $Snaps | ConvertTo-Html

$SmtpClient.Send($MailMessage)

To execute the script, you will need to install Powershell and the VI Toolkit. Good luck!