Tag Archives: design

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.

Back to Competition Main Page or Competition Submissions

Data Centre Migration Strategies – Part 2 – Lift and Shift

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Continuing on from Data Centre Migration Strategies Part 1 – Overview, Part 2 focuses on the “Lift and Shift” method.

I’m sure your reading this and already thinking, “this is the least interesting migration strategy, tell me about vMSC and SRM!” and well, your right, BUT it is important to understand the pros and cons so if you are ever in a situation where you have to use this method (I have on numerous occasions) that the migration is successful.

So what are the pros and cons of this method.

Pros

1. No need to purchase equipment for the new data centre
2. The environment should perform as it did at the original data centre following relocation
3.The approach is simple from a technical perspective ie: No new products are required
4. Low direct cost (Note: Point 8 in Cons)
5. Achieves a Recovery Point Objective (RPO) of zero (0).

Cons

1. The entire environment needs to be fully shut-down
2. The outage for the environment starts from when the servers are shut-down, until completion of operational verification testing at the new datacenter. Note: This may take several days depending on the size of the environment.
3. This method is high risk as the ability to fail back to the original datacenter requires all equipment be physically relocated back. This means the Recovery Time Objective (RTO) cannot be low.
4. The Lift and shift method cannot be tested until at least a significant amount of equipment has been physical relocated
5. In the event of an issue during operational verification at the new data centre, a decision needs to be made to proceed and troubleshoot the issues, OR at what point to fail back.
6. Depending on your environment, a vendor (eg: Storage) may need to revalidate your environment
7. Your migration (and schedule) are heavily dependant on the logistical side of the relocation which may have many factors (eg: Traffic / Weather) which are outside your control which may lead to delays or failed migration.
8. Potentially high indirect cost eg: Downtime, Loss of Business , productivity etc

When to use this method?

1. When purchasing equipment for the new data centre is not possible
2. When extended outages to the environment are acceptable
3. When you have no other options

Recommendations when using “Lift and Shift”

1. Ensure you have accurate wiring and rack diagrams of your datacenter
2. Be prepared with your vendor support contact details on hand as it is common following relocation of equipment to have hardware failures
3. Ensure you have an accurate Operational Verification document which tests every part of your environment from Layer 1 (Physical) all the way to Layer 7 (Application)
4. Label EVERYTHING as you disconnect it at the original datacenter
5. Prior to starting your data centre  migration, discuss and agree on a timeline for the migration and at what point and under what situation do you initiate a fail back.
6. Migrate the minimum amount of physical equipment that is required to get your environment back on-line and do your Operational Verification, then on successful completion of your Operational Verification migrate the remaining equipment. This allows for faster fail-back in the event Operational Verification fails.

In Part 3, we discuss Data centre migrations using VMware Site Recovery Manager. (Coming soon)

 

Data Centre Migration Strategies – Part 1 – Overview

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After a recent twitter discussion, I felt a Data Centre migration strategies would be a good blog series to help people understand what the options are, along with the Pros and Cons of each strategy.

This guide is not intended to be a step by step on how to set-up each of these solutions, but a guide to assist you making the best decision for your environment when considering a data centre migration.

So what’s are some of the options when migrating virtual machines from one data centre to another?

1. Lift and Shift

Summary: Shut-down your environment and Physically relocate all the required equipment to the new location.

2. VMware Site Recovery Manager (SRM)

Summary: Using SRM with either Storage Replication Adapters (SRAs) or vSphere Replication (VR) to perform both test and planned migration/s between the data centres.

3. vSphere Metro Storage Cluster (vMSC)

Summary: Using an existing vMSC or by setting up a new vMSC for the migration, vMotion virtual machines between the sites.

4. Stretched vSphere Cluster / Storage vMotion

Summary: Present your storage at one or both sites to ESXi hosts at one or both sites and use vMotion and Storage vMotion to move workloads between sites.

5. Backup & Restore

Summary: Take a full backup of your virtual machines, transport the backup data to a new data centre (physically or by data replication) and restore the backup onto the new environment.

6. Vendor Specific Solutions

Summary: There are countless vendor specific solutions which range from Storage layer, to Application layer and everything in between.

7. Data Replication and re-register VMs into vCenter (or ESXi) inventory

Summary: The poor man’s SRM solution. Setup data replication at the storage layer and manually or via scripts re-register VMs into the inventory of vCenter or ESXi for sites with no vCenter.

Each of the above topics will be discussed in detail over the coming weeks so stay tuned, and if you work for a vendor with a specific solution you would like featured please leave a comment and I will get back to you.