Yellow Bricks

by Duncan Epping

ESX

Network loss after HA initiated failover

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I had a discussion with one of my readers last week and just read this post on the VMTN community which triggered this article.

When you create a highly available environment take into account that you will need to have enough vSwitch ports available when a failover needs to occur. By default a vSwitch will be created with 56 ports and in general this is sufficient for most environments. However when two of your hosts fail in a 10 host cluster you might end up with 60 or more VMs running on a single host. If this would happen several VMs would not have a vSwitch port assigned.

The most commonly used command when creating an automated build procedure probably is:

esxcfg-vswitch -a vSwitch1

This would result in a vSwitch named “vSwitch1” with the default amount of 56 ports. Now it is just as easy to set it up with 128 ports for instance:

esxcfg-vswitch -a vSwitch1:128

Always design for a worst case scenario. Also be aware of the overhead, some ports are reserved for internal usage. You might want to factor in some additional ports for this reason as for instance in the example above you will have 120 ports available for your VMs and not the 128 you specified.

Scale UP!

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Lately I am having a lot of discussions with customers around sizing of their hosts. Especially Cisco UCS(with the 384GB option) and the upcoming Intel Xeon 5600 series with six cores per CPU takes the “Scale Up” discussion to a new level.

I guess we had this discussion in the past as well when 32GB became a commodity. The question I always have is how many eggs do you want to have in one basket. Basically do you want to scale up(larger hosts) or scale out(more hosts).

I guess it’s a common discussion and a lot of people don’t see the impact sizing your hosts correctly. Think about this environment, 250 VMs in total with the need of roughly 480GB of memory:

  • 10 Hosts, each having 48GB and 8 Cores, 25 VMs each.
  • 5 Hosts, each having 96GB and 16 Cores, 50 VMs each.

If  you look at it from an uptime perspective; Would a failure occur in scenario 1 you will lose 10% of your environment. If you look at scenario 2 this is 20%. Clearly the associated cost with the down time for 20% of your estate is higher than for 10% of your estate.

Now it’s not only the associated cost with the impact of a host failure it is also for instance the ability of DRS to load balance the environment. The less hosts you will have the smaller the chances are DRS will be able to balance the load. Keep in mind DRS uses a deviation to calculate the imbalance and simulates a move to see if it results in a balanced cluster.

Another thing to keep in mind is HA. When you design for N+1 redundancy and need to buy an extra host the costs associated for redundancy is high for a scale up scenario. Not only the costs associated are high, the load when the fail-over needs to occur will also increase immense. If you only have 4 hosts and 1 host fails the added load on the 3 hosts will have a higher impact than it would have on for instance 9 hosts in a scale out scenario.

Licensing is another often used argument for buying larger hosts but for VMware it usually will not make a difference. I’m not the “capacity management” or “capacity planning” guru to be honest but I can recommend VMware Capacity Planner as it can help you to easily create several scenarios. (Or Platespin Recon for that matter.) If you have never tried it and are a VMware partner check it out and run the scenarios based on scale up and scale out principles and do the math.

Now, don’t get me wrong I am not saying you should not buy hosts with 96GB but think before you make this decision. Decide what an acceptable risk is and discuss the impact of the risk with your customer(s). As you can imagine for any company there’s a cost associated with down time. Down time for 20% of your estate will have a different financial impact than down time for 10% of your estate and this needs to be weighted against all the pros and cons of scale out vs scale up.

VM powered on Alarm?

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One of my readers(Thanks Andrzej!) emailed me something that I thought might be interesting for those who are closely monitoring their environment.

Did you know that there are two similar VM event triggers in Alarms in vCenter?

  1. VM powered on
  2. DRS – VM powered on

The first only works for VMs outside of DRS enabled clusters. The second only works for VMs inside DRS enabled clusters. Now that’s definitely something you should be aware off when enabling Alarms / Event triggers. Imagine you want to know when a VM has been powered on and you enable the first even trigger but didn’t notice it will only sent an alarm when the VMs are not part of DRS cluster… You could be waiting for a very long time before you receive a single event alarm.

Just when I wanted to click “Publish” I received an email from one of my colleagues. Horst Mundt wrote an excellent article about Alarms and created a very handy spreadsheet which contains all alarms / events.

vSphere alarm triggers
In terms of alarms, vCenter 4 has much more to offer than vCenter 2.5. There is a whole range of default alarms available when you install vCenter 4, and they will give you a very good first shot for monitoring your vSphere  environment. If you’ve never wondered what exactly the default alarms mean, or how to tune them – that’s fine. If you’re interested in a bit more detail – read the attached PDF.

Make sure to visit the VMTN source page and leave a comment or rate the article.

Single Initiator Zoning, recommended or not?

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A question we receive a lot is what kind of zoning should be implemented for our storage solution? The answer is usually really short and simple: at least single initiator zoning.

Single initiator zoning is something we have always recommend in the field (VMware PSO Consultants/Architects) and something that is clearly mentioned in our documentation… at least that’s what I thought.

On page 31 of the SAN Design and Deploy guide we clearly state the following:

When a SAN is configured using zoning, the devices outside a zone are not visible to the devices inside the zone. When there is one HBA or initiator to a single storage processor port or target zone, it is commonly referred to as single zone. This type of single zoning protects devices within a zone from fabric notifications, such as Registered State Change Notification (RSCN) changes from other zones. In addition, SAN traffic within each zone is isolated from the other zones. Thus, using single zone is a common industry practice.

That’s crystal clear isn’t it? Unfortunately there’s another document floating around which is called “Fibre Channel SAN Configuration Guide” and this document states the following on page 36:

  • ESX Server hosts that use shared storage for virtual machine failover or load balancing must be in one zone.
  • If you have a very large deployment, you might need to create separate zones for different areas of functionality. For example, you can separate accounting from human resources.

So which one is correct and which one isn’t? I don’t want any confusion around this. The first document, the SAN Design and Deploy guide is correct. VMware recommends single initiator zoning. Of course if you want to do “single initiator / single target” that would even be better, but single initiator is the bare minimum. Now let’s hope the VMware Tech Writers can get that document fixed…

CPU/MEM Reservation Behavior

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Again an interesting discussion we had amongst some colleagues (Thanks Frank, Andrew and Craig! Especially Craig as most text below comes from The Resource Guru). The topic was CPU/Memory reservations and more specifically the difference in behavior of these two.

One would expect that both a CPU and Memory reservation would have the same behavior when it comes to claiming and releasing resources but unfortunately this is not the case. Or should we say fortunately?

The following is taken from the resource management guide:

CPU Reservation:
Consider a virtual machine with reservation=2GHz that is
totally idle. It has 2GHz reserved, but it is not using any of
its reservation. Other virtual machines cannot reserve these 2GHz. Other virtual machines can use these 2GHz, that is, idle
CPU reservations are not wasted.

Memory Reservation:
If a virtual machine has a memory reservation but has not yet accessed its full reservation, the unused memory can be reallocated to other virtual machines. After a virtual machine has accessed its full reservation, ESX Server allows the virtual machine to retain this much memory, and will not reclaim it, even if the virtual machine becomes idle and stops accessing memory.

The above paragraph is a bit misleading , as it seems to imply that a VM has to access its full reservation. What it should really say is “Memory which is protected by a reservation will not be reclaimed by ballooning or Host-level swapping even if it becomes idle,” and “Physical machine memory will not be allocated to the VM until the VM accesses virtual RAM needing physical RAM backing.” Then that pRAM is protected by the reservation and won’t be reclaimed by ballooning or .vswp-file swapping. If there is any .vswp memory at all as no .vswp is created when the reservation is equal to the provisioned memory.

Note, however, that even if pRAM is not allocated to the VM to back vRAM because the VM hasn’t accessed corresponding vRAM yet, the whole reservation is reserved, but the pRAM could still be used  This gets really confusing. But I think of it thus:

  1. Reservations can be defined at the VM level or the Resource Pool level.
  2. Reservations at the RP level are activated or reserved immediately.
  3. Reservations at the VM level are activated or reserved when the VM is powered on.
  4. An activated reservation is removed from the total physical Resource “Unreserved” accounting.
  5. Reserving and using a resource are distinct: memory or CPU can be reserved but not used or used but not reserved.
  6. CPU reservations are friendly.
  7. Memory reservations are greedy and hoard memory.
  8. Memory reservations are activated at startup, yet pRAM is only allocated as needed. Unallocated pRAM may be used by others.
  9. Once pRAM is protected by a memory reservation, it will never be reclaimed by ballooning of .vswp-swapping even if the corresponding vRAM is idle.

Example: A VM has 4 GB of vRAM installed and a 3 GB memory reservation defined. When the VM starts, 3 GB of pRAM are reserved. If the host had 32 GB of RAM installed and no reservations active, it now has 29 GB “unreserved”.

However, if the VM accesses only 500 MB of vRAM, only 500 MB of pRAM are allocated (or granted) to it. Other VMs could use 2500 MB of RAM that you would think is part of the reservation. They cannot reserve that 2500 MB however. As soon as the VM accesses 3 GB of vRAM and so has 3 GB of pRAM backing it, no other VMs can use that 3 GB of pRAM even if the VM never touches it again, because that pRAM is now protected by the 3 GB Reservation.  If the VM uses 4 GB, it gets the 3 GB guaranteed never ballooned or swapped, but the remaining 1 GB is subject to ballooning or swapping.

Simple huh 😉