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Improving Storage Performance of Dell VRTX

Dell’s PowerEdge VRTX hyper-converged appliance can either have all hard drive datastores or all SSD datastores, but you can’t have SSDs act as tiering or caching media for VRTX volumes / virtual disks. That’s where VirtuCache comes in.

VirtuCache Overview

VirtuCache is kernel mode software that you install in VMware vSphere, that automatically caches frequently and recently used data, reads and writes, from Datastores on VRTX shared storage to any SSD (or host RAM) in VRTX. By doing so, we improve the performance of shared VRTX storage.

VirtuCache deployment is seamless, in the sense that applications or end users don’t realize that most of the data is now read from and always first written to the in-host cache media, instead of shared storage. Deploying VirtuCache doesn’t require any storage, network, or host reconfiguration.

It’s extremely easy to deploy and manage. You can choose to evaluate VirtuCache by registering for VirtuCache download here.

Click here for technical details around how VirtuCache caches reads and writes; syncs writes to shared storage; and protects against data loss in case of host failure.

Cache Media in VRTX

There are three options for caching media with VirtuCache for VRTX.

  1. You can use a SAS SSD configured as RAID0 device using the VRTX shared PERC RAID controller.

  2. The option we recommend most often is to use an Intel P4600 2TB NVME SSD in a spare PCIe slot in VRTX, one SSD per VRTX blade. The P4600 SSDs are higher performing than SAS SSDs and also cheaper.

  3. A higher performing option is to use some amount of host RAM as cache media. Because of the high cost of RAM and the fact that RAM itself is a constrained resource in most VMware deployments, using RAM as cache is not price/performance competitive. Though one advantage of using RAM as cache media, is that you can evaluate VirtuCache using existing host RAM before investing in a PCIe or SAS SSD.

Sample Cost

VirtuCache costs $3000/host for a perpetual license, add another $1000 for the 2TB Intel P4600 NVME SSD. All prices as of 2019. So the total cost works out to $16000 for 8TB SSD cache capacity + 4 VirtuCache licenses, for 4 VRTX blades. 8TB cache capacity is sufficient to cache up to 80TB of VRTX shared storage.

VRTX RAID Controller Hardware Cache is VERY DIFFERENT from VirtuCache

The VRTX shared PERC RAID controller documentation lists support for caching, but that’s very different from VirtuCache. VRTX RAID controller is capable of caching to only 1GB of battery backed RAM on the RAID controller. VirtuCache, on the other hand, can cache to much larger amounts of RAM or Flash installed in VRTX. Also VirtuCache protects against data loss in case of host / SSD failure by replicating data to cache media in another host and not with battery backed cache media.

Before/After Tests with VirtuCache

The images below (click on images to view those in higher resolution) show before/after results for Iometer tests, first without VirtuCache and then with VirtuCache caching to an Intel P4600 NVME SSD, and a third iteration with VirtuCache caching to host RAM.

Iometer test specs: 100% Random IO, 75-25% Read-Write ratio, 4KB block size, 128 simultaneous IO requests hitting storage, with the Iometer test file residing on a hard drive based VRTX datastore.

Latency (ms) Results for Iometer Tests for VirtuCache Caching from 7200 RPM SAS HDDs in VRTX to NVME SSD and RAMThroughput (MBps) Results for Iometer Tests for VirtuCache Caching from 7200 RPM SAS HDDs in VRTX to NVME SSD and RAM