Tag Archives: decision

ESXi Host Isolation Response and custom isolation address configuration.

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I was reviewing a vSphere design recently and I came across an interesting design choice which I thought I would share.

The architect selected the isolation response of “Leave Powered On” and disabled  “das.usedefaultisolationaddress”  (which is by default enabled) and configured multiple custom isolation addresses using the “das.isolationadressX” advanced setting.

The architect explained that this was done to minimize the chance of a false positive isolation event. In many environments such as ones using IP storage or where the ESXi Management VMKernel default gateway is not highly available, this can be a very good idea.

In this environment, the storage was provided via FC and the default gateway was highly available.

So was there a benefit in changing the default setting of “das.usedefaultisolationaddress” and configuring custom isolation addresses?

The short answer is No.

This is because the isolation response is configured with “Leave Powered On” so regardless of the host being isolated or not, the Virtual Machines will remain powered on.

So keep it simple, if your isolation response is “Leave Powered On” there is no need to change either of these advanced settings.

The below articles show examples of isolation response and custom isolation addresses configurations for IP Storage, FC storage and Hyper-converged environments.

Related Articles

1. Host Isolation Response for IP Storage
2. Host isolation response for FC based Storage
3. Host Isolation Response for a Nutanix Environment

Example Architectural Decision – Default Virtual Machine Compatibility Configuration

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Problem Statement

In a VMware vSphere 5.5 environment, what is the most suitable configuration for Virtual Machine Compatibility setting at the Datacenter and Cluster layers?

Assumptions

1. vSphere Flash Read Cache is not required.
2. VMDKs of greater than 2TB minus 512b are not required.

Motivation

1. Reduce complexity where possible.
2. Maximize supportability.

Architectural Decision

Configure the vSphere Datacenter level “Default VM Compatibility” as “ESXi 5.1 or later” and leave the vSphere Cluster level “Default VM Compatibility” as “Use datacenter setting and host version” (default).

Justification

1. Avoid limiting management of the environment to the vSphere Web Client.
2. The Default VM Compatibility only needs to be set once at the datacenter layer and then all clusters within the datacenter will inherit the desired setting.
3. Reduce the dependency of the Web Client in the event of a disaster recovery.
4. As vFRC and >2TB VMDKs and vGPU are not required, there is no significant advantage to HW Version 10.
5. Ensuring a standard virtual machine compatibility level is maintained throughout the environment and reducing the chance of mismatched VM version types in the environment.
6. Simplicity.

Implications

1. Virtual Machine Hardware Compatibility automatic update must be DISABLED to prevent the VM hardware being automatically upgraded following a shutdown.
2. vSphere Flash Read Cache (vFRC) cannot be used.
3. VMDKs will remain limited at 2TB minus 512b.

Alternatives

1. Virtual Machine HW Version 10 (vSphere 5.5 onwards).
2. Virtual Machine HW Version 8 (vSphere 5.0 onwards).
3. Virtual Machine HW Version 7 (vSphere 4.1 onwards).
4. Older Virtual machine HW versions.

 

Competition Example Architectural Decision Entry 4 – vCloud Allocation Pool Usable Memory

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Name: Prasenjit Sarkar
Title: Senior Member of Technical Staff
Company: VMware
Twitter: @stretchcloud
Profile: VCAP-DCD4/5,VCAP-DCA4/5,VCAP-CIA,vExpert 2012/2013

Problem Statement

When using an Allocation Pool with 100% memory reservation, due to the VM memory overhead, the usable memory is less than what is expected by the users. What is the best way to ensure users can use the entire memory assigned to the Allocation pool.

Assumptions

1. vCD 5.1.2 is in use

2. vSphere 5.1 or later is in use

3. Org VDC created with Allocation Pool

Constraints

1. vCD 5.1.2 has to be used

2. Allocation Model only VDCs are affected

Motivation

1. Need to use 100% memory allocated to the VDC with Allocation Pool model

2. Optimal use of Memory assigned to Org VDC and then to the VM

Architectural Decision

Due to the “by design” fact of VM memory overhead, we cannot use the entire allocated memory and this will be solved by enabling Elastic Allocation Pool in the vCloud System level and then set a lower vCPU Speed value (260 MHz). This will allow VMs to use the entire allocated memory (100% guarantees) in the Org VDC.

Alternatives

1. Over allocate resources to the customer but only reserve the amount they purchased.

Historically VM overhead ranges in between <=5% to 20%. Most configurations have an overhead of less than 5%, if you assume such you could over allocate resources by 5% but only reserve ~95%. The effect would be that the customer could consume up to the amount of vRAM they purchased and if they created VMs with low overhead (high vRAM allocations, low vCPU) they could possibly actually consume more than they “purchased”. In the case of a 20GHz/20GB purchase we would have to set the Allocation to 21GHz but set the reservation to 95%.

Justification

VM memory overhead is calculated with so many moving targets like the model of the CPU in the ESXi host the VM will be running on, whether 3D is enabled for MKS, etc. So you cannot use the entire allocated memory at any point in time.

By selecting the Elastic VDC, we are overwriting this behavior and still not allowing more VMs to power on from what they have entitled to. Also Elastic VDC gives us an opportunity to set a custom vCPU speed and lowering the vCPU speed will allow you to deploy more vCPUs without being penalized. Without setting this flag, you cannot overcommit the vCPU, which is really bad.

260MHz is the least vCPU speed we can set and thus this has been taken to allow system administrators to overcommit the vCPUs in a VDC with Allocation Pool.

 

Implications

1. One of the caveat is not having any memory reservation for any VMs. Due to the nature of OrgVDCs, it does not allow an Org Admin to set the resource reservation for the VMs (unlike Reservation Pool) and thus any VMs with Elasticity on will not have any reservation which will be marked as overkill for the customer’s high I/O VMs (like DB or Mail Server).

You can easily overwrite the resource reservation using the vSphere but that is not the intent. Hence, we flag it as RISK as it will hamper customer’s VM performance for sure.

If we say we are reserving 100% memory and thus spawning the VMs will get equal memory and can’t oversubscribe the memory as the limit is still what the customer has bought, then also if there is a contention of memory within those VMs, I don’t have an option to prefer those VMs which are resource hungry. In a nutshell all of the VMs will get equal share.

Equal shares will distribute the resource in a RP equally and thus there will not be any guarantee that a hungry VM can get more resource on demand.

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