VSAN performanceOne of my goals in testing VSAN was to compare the level of performance available on low-cost hardware (the Lenovo three-node cluster) against the higher-end (the Supermicro four-node cluster) and to attempt to identify hardware-specific issues that could be improved with an upgrade. I measured performance by using the VMware I/O Analyzer, a freely downloadable tool from VMware Labs that makes the process of measuring storage performance easier by combining a commonly available tool (Iometer) with nifty, Web-based control magic.
Version 1.6 of the VMware I/O Analyzer (IOA) consists of a 64-bit Suse Linux Enterprise Server 11 SP2 virtual machine with two attached virtual disks. The first disk contains the operating system and testing software, while the second disk serves as the target for the tests. All Iometer traffic targets the second disk in raw mode, so it will write to the device directly, bypassing any file system.
Figure 2: The VSAN Observer dashboard displays all of the relevant statistics for VSAN nodes and drives.
In order to generate large amounts of traffic, VMware suggests using multiple I/O Analyzer VMs on each node in the VSAN cluster. To test both the four-node Supermicro cluster and the three-node Lenovo cluster, I used eight VMs on each node — for a total of 32 worker VMs on the four-node cluster, and 24 on the three-node cluster — with an additional I/O Analyzer VM on each serving as the controller node.
I/O Analyzer comes with a list of different workload types supporting a wide range of I/O sizes from 512 bytes to 512KB. Iometer provides the ability to specify the types and percentage of I/O operations, reads, and writes, along with the amount of time to run each test.
To compare my two clusters, I ran two different I/O Analyzer workloads to measure high write performance and a mixture of reads and writes. The Max IOPS test used a 512KB block size for 100 percent sequential read, while the combo test used 4KB blocks and a mix of 70 percent reads and 30 percent writes. The results of the two tests tell two different stories. Whereas the three-node cluster held its own against the four-node cluster in the Max IOPS test (roughly 154K vs. 190K maximum total IOPS), the four-node cluster proved vastly superior (yielding roughly double the performance) in the mixed workload test. The results of the mixed workload test are presented in the chart below.
With more RAM, more CPU, larger SSD, and 10GbE networking, the three-node Supermicro cluster more than doubled the read and write performance of the three-node Lenovo cluster.
The single most important factor in VSAN performance will be the size of the SSD cache. If the data your workload requires is not found in the flash cache, but must be accessed from rotating disk, then I/O latency will shoot up and IOPS will fall dramatically.
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