Virtunet Write-Back Caching Improves the Performance of EMC VNX Appliance Without Requiring Any Appliance or SAN Upgrade
Accelerating EMC VNX appliances is a popular use case for Virtunet’s VirtuCache host side caching software.
EMC’s VNX appliances have 3 shortcomings that VirtuCache fixes without touching the SAN and Appliance,
- They are expensive in terms of $/IOPS
- Even the high-end VNX 7500 can accommodate only 2TB of SSDs per appliance
- SSDs in the VNX that are used for tiering data from HDDs do not cache writes, hence writes are not accelerated
There are two ways to improve the performance of VNX. One is to install EMC’s FastCache SSDs in the appliance, the other is to use VirtuCache host side caching software to cache data from VNX to in-host SSDs.
VirtuCache is installed in the VMware physical server along with any SSD in the same host. It then automatically caches frequently used data (reads) and all recent writes from/to any SAN based storage appliance to this in-host SSD. Consequently, we improve storage performance from our customer’s existing storage appliance considerably, thus allowing higher hardware consolidation ratios, and improving application performance from within VMs.
Below is a table comparing these two options.
| EMC FastCache SSDs in the VNX | Virtunet VirtuCache caching from VNX to in-host SSD |
|
Higher latency because • SSDs behind controller and shared network • Only 2/4 Controllers (CPUs) involved in caching |
Lower latency because • SSD closer to CPU on a dedicated local SATA Bus • Each VMware host CPU involved in caching. Hence more CPU used in caching. |
| Accelerates frequently used reads and reads that are changed in memory | Accelerates frequently used reads and all recent writes |
| Caching granularity is 64KB block size, meaning that even if the application retrieves data in smaller chunks, FastCache will allocate 64KB space for this data | VirtuCache caches data at the granularity of data requested. Hence space on the SSD is used optimally. This results in higher cache hit ratio with VirtuCache |
| Maximum SSD that can be installed in a VNX is only 100GB in VNX 5100, 500GB in VNX 5300, 1TB in VNX 5500, 1.5TB in VNX 5700, and 2.1TB in VNX 7500 | Maximum SSD that can be used in VirtuCache is 4TB per Host, considerably more than VNX |
| Expensive – SSDs in VNX cost $20-40/GB | Affordable – The same SSDs that go in the VNX if bought retail costs $0.50/GB and if bought from the server vendor costs $5/GB, so substantially cheaper than EMC SSDs. Virtunet cost (including SSD cost) works out to $3-6/GB |