Archive: Posts

EMC Fast Cache Storage Controller Based Caching for VNX Appliances versus VirtuCache Host Side Caching for Any SAN based Appliance

In case of FastCache or any storage appliance based caching, the SSD is more expensive than if the same SSD were bought retail. Also the SSD performs better in the host than in the storage appliance, since the SSD in the appliance is constrained by the network and controller.

EMC FastCache is SSD based caching functionality sold by EMC for their VNX appliances. It tiers frequently used data, mainly reads, to SSDs in the appliance from slower HDDs on the appliance.

VirtuCache is software sold by Virtunet (and competing with EMC’s FastCache) that is installed in the VMware kernel along with a SSD in the same VMware host, and it caches frequently and recently used data, both reads and writes, to the in-host SSD from any SAN based storage appliance.

This blog article lists the differences between VirtuCache and EMC’s FastCache.

1. FastCache SSDs are constrained by the controller and storage network: In the case of FastCache, the SSD is behind the storage network and behind the controllers (CPUs) on the storage appliance. In the case of VirtuCache, the SSD is in the VMware host and hence on a high speed dedicated SATA or SAS link to the host CPU that consumes hot data. Hence SSDs in the case of VirtuCache are not constrained by the storage network speed and storage controllers on the appliance.

2. Higher cache hit ratio since more CPUs are used with VirtuCache than FastCache: Since SSDs are over a 100X higher performing than HDDs, to get the most out of these SSDs, high powered CPUs are required. In the case of FastCache, the CPUs involved in caching are the 2 or 4 CPUs on the storage appliance. In the case of VirtuCache, since VirtuCache is installed in the VMware kernel, every host CPU in the VMware cluster is involved in caching. Since a lot more CPUs are used with VirtuCache, caching efficiencies are higher than in the case of Fast Cache. Also for the same reason, a lot more SSDs can be used with VirtuCache versus FastCache.

Because of the above two reasons, the same SSD works better when installed in the host and used for caching with VirtuCache than when the SSD is deployed in the VNX appliance and used with FastCache.

3. Accelerating Reads and Writes – In the case of both VirtuCache and FastCache, all frequently used reads are cached to the SSD. However in the case of writes, with VirtuCache, all recent writes are written to the local in-host SSD. In the case of FastCache, it is primarily reads that are tiered to the SSD, and only the writes that modify reads are written to the SSD, else the writes are written to HDDs in the appliance.

4. Cost per GB and Cost per IOPS – FastCache SSDs are enterprise grade SSDs from SSD OEMs like Samsung or Intel, branded as EMC. These SSDs are much cheaper if bought retail or from server OEMs. FastCache SSDs cost $60/GB (accounting for Raid-1 of the SSDs). Since VirtuCache uses in-host SSDs, the same SSDs can be bought for cheaper. These same SSDs from retailers like amazon.com cost $0.50/GB and from the server vendor cost $5/GB. On a cost/IOPS basis, at $1/IOPS, FastCache is 10x more expensive than VirtuCache (cost/IOPS and cost/GB can be demonstrated in an evaluation of VirtuCache).

5. FastCache caches data at 64KB granularity, 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. This results in higher cache hit ratio for VirtuCache versus FastCache

6. Usable SSD capacity in the case of FastCache ranges from 100GB in VNX5100 to 2TB in VNX 7500. In the case of VirtuCache, a maximum of 4 TB of SSD can be installed in each VMware host.