Dynamic scheduling of resources is important in virtualization (or HCI). Generally, virtualization platforms optimize clusters’ availability and performance through a dynamic change of VM distribution. VMware vSphere Distributed Resource Scheduler (DRS) is the most well-known solution for this feature. Due to its simple design and useful functions, VMware’s DRS has quickly gained popularity among users and even named this feature. 

As a leading HCI solution provider, SmartX has also introduced the DRS feature in the latest HCI software version SMTX OS 5.1 (hereinafter referred to as SMTX OS). Unlike VMware 6.x and other mainstream virtualization platforms, SMTX OS enhances DRS implementation with an innovative scoring system, which can fully adapt to the containerized environment, improving resource utilization and O&M efficiency. 

In this article, we will delve into SmartX HCI’s innovation in DRS and make a comparison with that of other mainstream virtualization platforms.

DRS in Mainstream Virtualization Platform: Mainly Serve VMs in Traditional Virtualization

Mainstream virtualization platforms’ DRS is similar to that in VMware vSphere 6*. The main goal of DRS is to ensure resource and load balancing on the host through the dynamic change of VM placement. In addition, this feature can support more application scenarios when combined with other virtualization features, for example:

  • Improve VM SLAs through resource balancing.
  • Improve host efficiency and reduce resource consumption by centralizing VMs with low I/O.
  • Automatically evaluate and initialize resource placement when creating workloads.
  • Automatically balance workloads to maximize host performance.
  • Automatically migrate VMs when the host is under maintenance, reducing the complexity of O&M.
  • Fix affinity constraints.

*VMware vSphere 7 introduces a new implementing mechanism for the DRS system.

DRS system can be roughly divided into two parts:

  • Scoring system: Periodically collects the resource usage of hosts and VMs in the cluster, and determines whether the cluster is under resource contention or unbalanced resource distribution according to its scores.
  • VM migration: The DRS algorithms generate VM migration recommendations if there is a load imbalance. Users can choose to apply the recommendations manually or automatically to balance the cluster load.

Therefore, the scoring system matters a lot to the entire DRS feature. In VMware 6.x, DRS’s scoring system focuses on cluster status by checking whether the resources in the hosts need to be rebalanced. This is because, in the cluster, resource consumption between hosts can differ significantly. If it can be improved, DRS will apply hot migration of VMs (from hosts with heavy workloads vMotion to those with less workloads).

Essentially, this DRS implementation focuses on balancing the CPU and memory utilization between hosts. It is effective for business workloads in traditional virtualization. In traditional virtualization, as one VM only supports a single application, the CPU and memory consumption, and even the VM numbers are relatively fixed. Therefore, changing VM distribution to balance workloads between hosts can be a feasible way. 

However, as more enterprises use containers to support business applications, workloads’ working patterns have also changed; one VM can support multiple containers, and the number of containers changes dramatically, which results in an intensive fluctuation of resource consumption on a host. 

In this situation, DRS’s scoring system for traditional virtualization can have negative impacts; VMs are likely to be migrated repeatedly and pointlessly due to the frequent changes of containers, causing degradation to application performance. In the cloud era, it is necessary to enhance the legacy DRS scoring system and make it more container-friendly. 

DRS in SMTX OS 5.1: Innovating DRS Scoring System for Modernized Applications 

In the upgraded SmartX HCI software – SMTX OS 5.1 – DRS is innovated with a new scoring system that comprehensively evaluates both the VM resource contention and host resource sufficiency to adapt to modernized workloads.

DRS Scoring System in SMTX OS 5.1

In SMTX OS 5.1, DRS works in the following steps:

  • DRS periodically scores VMs and hosts.
  • A VM’s DRS score is calculated based on its CPU, memory, and storage usage. The higher the score, the more sufficient the resources allocated to it. The lower the score, the more intensely it contends for resources.
  • A host’s DRS score is calculated based on its CPU, memory, and storage usage. The higher the DRS score, the more sufficient the host’s resources are. The lower the score, the more insufficient the host’s resources are.
  • VM with the lowest score will be migrated to the host with the highest score. This will be carried out after an evaluation of migration cost-efficiency. The VM migration repeats until the cluster’s resources are balanced between hosts.

VM DRS Score

In SmartX HCI, a VM’s overall DRS score is calculated by adding 80% of its CPU and memory scores with 20% of its storage score. The current version does not take the VM network into account.

  • VM CPU score
    DRS system monitors the Steal Time of the VM CPU and calculates the VM CPU score according to the formula. VM with more CPU resource contention gains a lower CPU score.
  • VM memory score
    When there is no memory overcommitment, this VM’s memory score should be 100% as there is no contention for memory resources. If the memory is overcommitted, the system will monitor the usage of shared memory and give a lower score to VMs that consume more shared memory resources.
  • VM storage score
    As SMTX OS supports I/O locality, VMs will first turn to the replica on the local host to read I/O. When scoring VM storage, the DRS system will evaluate how many replica data blocks of that VM are owned by the node where the VM is located. If the host has a complete replica of that VM, the storage score should be 100%. However, if the host only contains a partial replica, which means that the VM needs to read across the network, the VM’s storage score should be reduced.

Host DRS Score

Similar to the VM DRS score, a host’s DRS score is calculated based on its CPU, memory, and storage usage.

  • Host CPU score
    The DRS system evaluates the host’s idle time. Hosts with more idle time gain higher scores.
  • Host memory score
    The DRS system evaluates the host’s available memory proportion and scores based on whether the host has a memory overcommitment. When there is no memory overcommitment, hosts with more available memory gain higher scores.
  • Host storage score
    Because the storage engine of SMTX OS supports automatic balancing of storage capacity, host storage scores are not evaluated based on the host’s local storage utilization, but on the proportion of VM replica data blocks the host contains. Hosts with higher ratios gain higher scores.

Evaluation of VM Migration Cost-Effectiveness 

To achieve the dynamic scheduling of VMs, the DRS system should also consider the VM migration cost; a VM will be migrated only when its benefits outweigh its costs. For example, if a VM has a large memory and needs to migrate a large amount of data, the DRS system may not generate migration recommendations. 

DRS VM Migration Recommendation

Migration Threshold

With SMTX OS, users can customize the VM migration threshold in three modes.

Automation Level

In SMTX OS, VM migration recommendations generated by the DRS system can be performed automatically or manually.

  • Manual migration
    In this mode, DRS only recommends VM migrations but does not apply them automatically, allowing users to fully evaluate the application scenario (for example, users who want to avoid unexpected VM migration for mission-critical applications can use this mode). Notably, users need to check migration recommendations regularly and make decisions independently.
  • Automatic migration
    DRS automatically migrates VMs after generating migration recommendations to meet the threshold. This mode does not involve human intervention with resources automatically balanced through the DRS system. For large-scale VM clusters, this mode can significantly reduce the O&M burden.

Benefits and Values

Given the innovative scoring system and O&M-friendly design, DRS in SMTX OS 5.1 can fully improve the HCI cluster’s resource utilization in the containerized environment, further optimizing cluster performance and enhancing O&M efficiency. 

Compared with the DRS system for legacy virtualization, DRS in SMTX OS 5.1 brings more benefits, such as

  • Richer application scenarios: With a VM-centric scoring system, SMTX OS’ DRS feature can support more application scenarios.
  • Smarter scheduling strategy: The scoring system comprehensively evaluates the cluster’s resource distribution. It also fully adapts to HCI architecture by involving storage scores and the evaluation of migration cost-effectiveness. This further optimizes decision-making and reduces VM overhead.
  • Simpler O&M: SMTX OS’ DRS system allows users to customize migration threshold and automation level, which reduces O&M burden while improving O&M flexibility. 

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