The fio Pareto parameter allows us to create a workload, which references a very large dataset, but specify a hotspot for the access pattern. Here’s an example using the same setup as the ILM experiment, but using a Pareto value of 0:8. My fio file looks like this..
random_distribution=pareto:0.8 The experiment shows that with the access pattern as a Pareto ratio 0:8, meaning 20% of the overall dataset is “hot” the ILM process happens much faster as the hotspot is smaller, and is identified faster than a 100% uniform random access pattern. We would expect a similar shape for any sort of caching mechanism.
At some point potential Hyper-converged infrastructure (HCI) users want to know – “How fast does this thing go?”. The real question is “how do we measure that?”.
The simplest test is to run a single VM, with a single disk and issue a single IO at a time. We see often see this sort of test in bake-offs, and such a test does answer an important question – “what’s the lowest possible response time I can expect from the storage”.
However, this test only gives a single data point. Since nobody purchases a HCI cluster to run a single VM, we also need to know what happens when multiple VMs are run at the same time. This is a much more difficult test to conduct, and many end-users lack access and experience with tools that can give the full picture.
In the example below, the single VM, single vdisk, single IO result is at the very far left of the chart. Since it’s impossible to read I will tell you that the result is about 2,500 IOPS at ~400 microseconds. (in fact we know that if the IOPS are 2,500 the response time MUST be 400 microseconds 1/2,500 == .0004 seconds)
However with a single VM, the cluster is mostly idle, and has capacity to do much more work. In this X-Ray test I add another worker VM doing the exact same workload pattern to every node in the cluster every 5 minutes.
By the time we reach the end of the test, the total IOPS have increased to around 600,000 and the response time only increased by an additional 400 microseconds.
In other words the cluster was able to achieve 240X the amount of work measured by the single VM on a single node with only a 2X increase in response time, which is still less than 1ms.
The overall result is counter-intuitive, because the rate of change in IOPS (240X) is way out of line with the increase in response time (2X). The single VM test is using only a fraction of the cluster capacity.
When comparing HCI clusters to traditional storage arrays – you should expect the traditional array to outperform the cluster at the far left of the chart, but as work scales up the latent capacity of the HCI cluster is able to provide huge amounts of IO with very low response times.
Specifically a customer wanted to see how performance changes (and how quickly) as data moves from HDD to SSD automatically as data is accessed. The access pattern is 100% random across the entire disk.
In a hybrid Flash/HDD system – “cold” data (i.e. data that has not been accessed for a long time) is moved from SSD to HDD when the SSD capacity is exhausted. At some point in the future – that same data may become “hot” again, and so we want to make sure that the “newly hot” data is quickly moved back to the SSD tier. The duration of the above chart is around 5 minutes – and we see that by, around the 3 minute mark the entire dataset is resident on the SSD tier.
This X-ray test uses a couple of neat tricks to demonstrate ILM behavior.
Edit container preferences to send sequential data immediately to HDD
Overwriting data with NUL/Zero bytes frees the underlying data on Nutanix filesystem
To demonstrate ILM from HDD to SSD (ad ultimately into the DRAM cache on the CVM) we first have to ensure that we have data on the HDD in the first place. By default Nutanix OS will always try to write new data to SSD. To circumvent that behavior we can edit the container preferences. We use the fact that the “prefill” will be a sequential workload, while the measured workload will be a random workload. To make the change, use “ncli” to change the ” Sequential I/O Pri Order” to be HDD only.
In my case I happened to call my container “xray” since I didn’t want to change the default container. Now, when X-Ray executes the prefill stage, the data will land on HDD. As a second requirement, we want to see what happens when IO with different size blocks are issued so that we can get a chart similar to this: To achieve the desired behavior, we need to make sure that, at the beginning of each test, the data, again resides on HDD. The problem is that the data is up-migrated during the test. To do this we do an initial overwrite of the entire disk with “NULL” bytes using a parameter in fio “zero_buffers”. This causes the data to be freed on the Nutanix filesystem. Then we issue a normal profile with random data. Once the data is freed, then we know that the new initial writes will go to HDD – because we edited the container to do so. The overall test pattern looks like this
Create and clone VMs
Prefill with random data (Data will reside on HDD due to container edit)
Read disk with 16KB block size
Zero out the disks – to remove/free the up-migrated data