Posts Tagged ‘pool’

Load Balancing Ansible Tower Using NSX

February 1, 2020

Disclamer: this configuration is not validated by either VMware or Red Hat. Make sure it is applicable to your use case and thoroughly test before implementing in production.


If you landed on this page I trust you already know what Ansible is. It’s a great configuration management tool centred around using YAML to describe the desired state configuration of your various infrastructure components. This desired state is captured in what Ansible calls playbooks, which once written, can then be used in a repeatable way to deploy brand new components or enforce configuration on already deployed ones.

Ansible can be installed and used from CLI, which is usually a good starting point. If you have multiple people using Ansible in your organization, you can also deploy AWX. It’s a free GUI add-on to Ansible, which makes managing concurrent user access to Ansible easier, by adding projects, schedules and credentials management. On top of that there is Ansible Tower. Ansible Tower is a paid version of AWX and gives you additional enterprise features and services like clustering, product support, validated upgrade paths, etc. In this article we will be focusing on Ansible Tower version of the product.

Also worth mentioning that this configuration will be based on Ansible Tower cluster feature, which lets you run all nodes as active/active. Prior to version 3.1 it was called redundancy and worked only in active/passive mode. Redundancy feature is deprecated and is outside the scope of this blog post.


Deploying multiple Ansible Tower nodes in a cluster already gives you redundancy. If one of the nodes fails you can connect to another node, by just changing your browser URL. The benefit of having a load balancer is that you have one URL you can hit and if a node goes down, such situation is handled by load balancer automatically.

In this example we will be deploying a VMware NSX load-balancer in the following topology:


Deploying an NSX load-balancer for HTTPS port 443 is simple, you can find numerous examples of how to create application profiles, monitors, pools and VIPs in official VMware documentation or out on the Internet. But with Ansible there’s one catch. If you try to use the default HTTPS monitor that NSX load balancer comes with, you will find HTTP 400 code in Ansible nginx logs: - - [20/Jan/2020:04:50:19 +0000] "GET / HTTP/1.0" 400 3786 "-" "-" "-" - - [20/Jan/2020:04:50:24 +0000] "GET / HTTP/1.0" 400 3786 "-" "-" "-" - - [20/Jan/2020:04:50:29 +0000] "GET / HTTP/1.0" 400 3786 "-" "-" "-"

And an error in NSX load balancer health check:

As it turns out, when you make a HTTP request to Ansible Tower, specifying HTTP “Host” header is a requirement. Host header simply contains the hostname of the server you’re making a request to. Browsers add this header automatically, that’s why you’re not going to see any errors, when accessing Ansible Tower Using Firefox or Chrome. But NSX doesn’t add this header to the monitor checks by default, which makes Ansible Tower upset.

Here is the trick you need to do to make Tower happy:

Now nginx logs show success code 200: - - [21/Jan/2020:22:54:42 +0000] "GET / HTTP/1.0" 200 11337 "-" "-" "-" - - [21/Jan/2020:22:54:47 +0000] "GET / HTTP/1.0" 200 11337 "-" "-" "-" - - [21/Jan/2020:22:54:52 +0000] "GET / HTTP/1.0" 200 11337 "-" "-" "-"

Load balancer health check is successful:

And pool members are up and reachable:

Note: technically the host header should contain the hostname of the Tower node we’re making a health check on. But since NSX monitor is configured per pool and not per pool member, we have to use a fake hostname “” as a workaround. When I was testing it, Tower didn’t complain.


Even though I said that the rest of the load-balancer configuration is standard, I still think having screenshots for reference is helpful if you need to validate configuration. So find the full list of settings below.

Screenshot 1: Application Profile

Screenshot 2: Service Monitor

Screenshot 3: Pool

Screenshot 4: Virtual Server

Overview of NetApp Replication and HA features

August 9, 2013

NetApp has quite a bit of features related to replication and clustering:

  • HA pairs (including mirrored HA pairs)
  • Aggregate mirroring with SyncMirror
  • MetroCluster (Fabric and Stretched)
  • SnapMirror (Sync, Semi-Sync, Async)

It’s easy to get lost here. So lets try to understand what goes where.



SnapMirror is a volume level replication, which normally works over IP network (SnapMirror can work over FC but only with FC-VI cards and it is not widely used).

Asynchronous version of SnapMirror replicates data according to schedule. SnapMiror Sync uses NVLOGM shipping (described briefly in my previous post) to synchronously replicate data between two storage systems. SnapMirror Semi-Sync is in between and synchronizes writes on Consistency Point (CP) level.

SnapMirror provides protection from data corruption inside a volume. But with SnapMirror you don’t have automatic failover of any sort. You need to break SnapMirror relationship and present data to clients manually. Then resynchronize volumes when problem is fixed.


SyncMirror mirror aggregates and work on a RAID level. You can configure mirroring between two shelves of the same system and prevent an outage in case of a shelf failure.

SyncMirror uses a concept of plexes to describe mirrored copies of data. You have two plexes: plex0 and plex1. Each plex consists of disks from a separate pool: pool0 or pool1. Disks are assigned to pools depending on cabling. Disks in each of the pools must be in separate shelves to ensure high availability. Once shelves are cabled, you enable SyncMiror and create a mirrored aggregate using the following syntax:

> aggr create aggr_name -m -d disk-list -d disk-list

HA Pair

HA Pair is basically two controllers which both have connection to their own and partner shelves. When one of the controllers fails, the other one takes over. It’s called Cluster Failover (CFO). Controller NVRAMs are mirrored over NVRAM interconnect link. So even the data which hasn’t been committed to disks isn’t lost.


MetroCluster provides failover on a storage system level. It uses the same SyncMirror feature beneath it to mirror data between two storage systems (instead of two shelves of the same system as in pure SyncMirror implementation). Now even if a storage controller fails together with all of its storage, you are safe. The other system takes over and continues to service requests.

HA Pair can’t failover when disk shelf fails, because partner doesn’t have a copy to service requests from.

Mirrored HA Pair

You can think of a Mirrored HA Pair as HA Pair with SyncMirror between the systems. You can implement almost the same configuration on HA pair with SyncMirror inside (not between) the system. Because the odds of the whole storage system (controller + shelves) going down is highly unlike. But it can give you more peace of mind if it’s mirrored between two system.

It cannot failover like MetroCluster, when one of the storage systems goes down. The whole process is manual. The reasonable question here is why it cannot failover if it has a copy of all the data? Because MetroCluster is a separate functionality, which performs all the checks and carry out a cutover to a mirror. It’s called Cluster Failover on Disaster (CFOD). SyncMirror is only a mirroring facility and doesn’t even know that cluster exists.

Further Reading