Tag Archives: vSAN

How to Architect a VSA , Nutanix or VSAN solution for >=N+1 availability.

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How to architect a VSA, Nutanix or VSAN solution for the desired level of availability (i.e.: N+1 , N+2 etc) is a question I am asked regularly by customers and contacts throughout the industry.

This needs to be addressed in two parts.

1. Compute
2. Storage

Firstly, Compute level resiliency, As a cluster grows, the chances of a failure increases so the percentage of resources reserved for HA should increase with the size of the cluster.

My rule of thumb (which is quite conservative) is as follows:

1. N+1 for clusters of up to 8 hosts
2. N+2 for clusters of >8 hosts but <=16
3. N+3 for clusters of >16 hosts but <=24
4. N+4 for clusters of >24 hosts but <=32

The above is discussed in more detail in : Example Architectural Decision – High Availability Admission Control Setting and Policy

The below table highlights in Green my recommended HA percentage configuration based on the cluster size, up to the current vSphere limit of 32 nodes.

HApercentages

Some of you may be thinking, if my Nutanix or VSAN cluster is only configured for RF2 or FT1 for VSAN, I can only tolerate one node failure, so why am I reserving more than N+1.

In the case of Nutanix, after a node failure, the cluster can restore itself to a fully resilient state and tolerate subsequent failures. In fact, with “Block Awareness” a full 4 node block can be lost (so an N-4 situation) which if this is a requirement, needs to be considered for HA admission control reservations to ensure compute level resources are available to restart VMs.

Next lets talk about the issue perceived to be more complicated, Storage redundancy.

Storage redundancy for VSA, Nutanix or VSAN is actually not as complicated as most people think.

The following is my rule of thumb for sizing.

For N+1 , Ensure you have enough capacity remaining in the cluster to tolerate the largest node failing.

For N+2, Ensure you have enough capacity remaining in the cluster to tolerate the largest TWO nodes failing.

The examples below discuss Nutanix nodes and their capacity, but the same is applicable to any VSA or VSAN solution where multiple copies of data is kept for data protection, as opposed to RAID.

Example 1 , If you have 4 x Nutanix NX3060 nodes configured with RF2 (FT1 in VSAN terms) with 2TB usable per node (as shown below), in the event of a node failure, 2TB is no longer available. So the maximum storage utilization of the cluster should be <75% (6TB) to ensure in the event of any node failure, the cluster can be restored to a fully resilient state.

4node3060

Example 2 , If you have 2 x Nutanix NX3060 nodes configured with RF2 (FT1 in VSAN terms) with 2TB usable per node and 2 x Nutanix NX6060 nodes with 8TB usable per node (as shown below), in the event of a NX6060 node failure, 8TB is no longer available. So the maximum storage utilization of the cluster should be 12TB to ensure in the event of any node failure (including the 8TB nodes), the cluster can be restored to a fully resilient state.

4nodemixed

For environments using Nutanix RF3 (3 copies of data) or VSAN (FT2) the same rule of thumb applies but the usable capacity per node would be lower due to the increased capacity required for data protection.

Specifically for Nutanix environments, the PRISM UI shows if a cluster has sufficient capacity available to tolerate a node failure, and if not the following is displayed on the HOME screen and alerts can be sent if desired.

CapacityCritical

In this case, the cluster has suffered a node failure, and because it was sized suitably, it shows “Rebuild Capacity Available” as “Yes” and advises an “Auto Rebuild in progress” meaning the cluster is performing a fully automated self heal. Importantly no admin intervention is required!

If the cluster status is normal, the following will be shown in PRISM.

CapacityOK

In summary: The smaller the cluster the higher the amount of capacity needs to remain unused to enable resiliency to be restored in the event of a node failure, the same as the percentage of resources reserved for HA in a traditional compute only cluster.

The larger the cluster from both a storage and compute perspective, the lower the unused capacity is required for HA, so as has been a virtualization recommended practice for years….. Scale-out!

Related Articles:

1. Scale Out Shared Nothing Architecture Resiliency by Nutanix

2. PART 1 – Problems with RAID and Object Based Storage for data protection

3. PART 2 – Problems with RAID and Object Based Storage for data protection

Cost vs Reward for the Nutanix Controller VM (CVM)

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I hear a lot of FUD (Fear Uncertainty and Doubt) getting thrown around about the Nutanix Controller VM (CVM) being a resource (vCPU/vRAM) hog.

So I thought I would address this perceived issue.

For those of you who are car people, you will understand the benefits of a Supercharger increasing performance of an engine.

The supercharger does this by attaching a belt to a pulley connected to the motor which spins the supercharger to force more air into the combustion chambers. This allows more fuel to be added to the mix to produce higher horsepower from the same engine displacement (engine capacity, ie: 2.0 Litres)

What is downside of a Supercharger?

The supercharger belt connected to the pulley can require even hundreds of horsepower to simply drive the supercharger. As such, a 300HP engine may have to use half of its power to just drive the supercharger.

So for example, a 300HP engine less 60HP (25%) to drive the supercharger equates to only 240HP remaining. But, as a result of the supercharger forcing more air into the engine, the engine now produces an additional 200HP.

So the “cost” of running the supercharger is 60HP, but the overall benefit is 200HP, resulting in the engine now producing 440HP.

Let’s now relate this back to the Nutanix Controller VM (CVM).

The CVM provides the storage features,functionality,excellent scalability and performance for the Virtual Machines. For example, reducing the latency thanks to Data Locality keeping data local to the compute node running the VM for faster reads and writes.

The faster the reads and writes, the less time VMs spend in a “CPU wait” state waiting for I/Os to be acknowledged by the storage which means the CPUs are being more efficiently utilized. This is a small part of the value the Nutanix CVM provides.

In Summary, the CVM does use some compute resources from the host (which depend on the node type and performance required) but like a Supercharger to an engine, the Nutanix CVM delivers significantly higher value to the VMs than the resources it uses.

Related Articles:

1. Rule of Thumb: Sizing for Storage Performance in the new world.

2. Is VAAI beneficial with Virtual Storage Appliance (VSA) based solutions ?

3. PART 1 – Problems with RAID and Object Based Storage for data protection

PART 2 – Problems with RAID and Object Based Storage for data protection

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Following on from Part 1, this post will discuss hyper-converged Distributed File Systems (i.e,: Nutanix) and compare with traditional SAN/NAS RAID and  hyper-converged solutions using Object storage for data protection.

The below diagram shows a 4 node hyper-converged solution using a Distributed File System with the same 4 x 4TB SATA drives with data protection using replication with 2 copies. (Nutanix calls this Resiliency Factor 2)

HyperconvergedDFSNormal

The first difference you may have noticed, is the data is much more granular than the Hyper-Converged Object store example in Part 1.

The second less obvious difference is the replicated copies of the data (i.e.: The data with Purple letters) on node 1 do not reside on a single other node, but are distributed throughout the cluster.

Now lets look at a drive failure example:

Here we see Node 1 has lost a Drive hosting 8 granular pieces of data 1MB in size each.

HyperconvergedDFSRecovery

Now the Distributed File System detects that the data represented by A,B,C,D,E,I,M,P has only a single copy within the cluster and starts the restoration process.

Lets walk through each step although these steps are completed concurrently.

1. Data “A” is replicated from Node 2 to Node 3
2. Data “B” is replicated from Node 2 to Node 4
3. Data “C” is replicated from Node 3 to Node 2
4. Data “D” is replicated from Node 4 to Node 2
5. Data “E” is replicated from Node 2 to Node 4
6. Data “I” is replicated from Node 3 to Node 2
7. Data “M” is replicated from Node 4 to Node 3
8. Data “P” is replicated from Node 4 to Node 3

Now the cluster has restored resiliency.

So what was the impact on each node?readwriteiorecovery

The above table shows a simplified representation of the workload of restoring resiliency to the cluster. As we can see, the workload (being 8 granular pieces of data being replicated) was distributed across the nodes very evenly.

Next lets look at the advantages of a Hyper-Converged Solution with a Distributed File System (which Nutanix uses).

  1. Highly granular distribution using 1MB extents not large Objects.
  2. The work required to restore resiliency after one drive (or node) failure was distributed across all drives and nodes in the Cluster leveraging all drives/nodes capability. (i.e.: Not constrained to the <100 IOPS of a single drive)
  3. The restoration rebuild is a low impact activity as the workload is distributed across the cluster and not dependant on source/destination pair of drives or nodes
  4. The rebuild has a low impact on the virtual machines running on the distributed file system and consistent performance is maintained.
  5. The larger the cluster the quicker and lower impact the rebuild is as the workload is distributed across a higher number of drives/nodes for the same size (Gb) worth of restoration.
  6. With Nutanix SSDs are used not only for Read/Write cache but as a persistent storage tier, meaning the recovering data will be written to SSD and where the data being recovered is not in cache (Memory or SSD tiers) it is still possible the data will be in the persistent SSD tier which will dramatically improve the performance of the recovery.

Summary:

As discussed in Part 1, Traditional RAID used by SAN/NAS and Hyper-converged solutions using Object based storage both suffer similar issues when recovering from drive or node failure.

Where as Nutanix Hyper-converged solution using the Nutanix Distributed File System (NDFS) can restore resiliency following a drive or node failure faster and with lower impact thanks to its highly granular and distributed architecture, meaning more consistent performance for virtual machines.