VMware went all in with all-flash Virtual SAN with the launch of Virtual SAN 6.2 on February 10th 2016. The ability to dedupe and perform data compression are only available on the all flash configuration, and really help drive down the cost of storage. To fully take advantage of all these capabilities, though, you should look into optimizing your flash cache.
Within a Virtual SAN node, storage is divided into Disk Groups, and within each Disk Group there is a cache SSD and 1-7 capacity SSDs. For the all-flash configuration the cache is configured to 100% write, and reads are either from the capacity or cache SSDs (depending on where the data is).
When dedupe and compression are turned on for Virtual SAN, those actions occur when data is destaged from the cache to the capacity SSDs. To take full advantage of these features, you would want to have a cache as large as possible. That way you have:
- Extra space to destage and still have hot data available in the cache
- Take advantage of more cost effective SSDs for caching. Less need for a high performance (expensive) SSD for a cache, as you have the extra space before compression is necessary.
However, there is a limit on how big your cache SSD can be. Presently in Virtual SAN, there is a maximum limit of 600GB for a Disk Group cache. Now with the ability to support 5 Disk Groups/Node, that amounts to up to 3TB of cache per node, which is a pretty fair amount. The interesting point, though, is any storage above 600GB for your cache is ignored by Virtual SAN.
In other words, if you are buying an 800GB, 1TB, (or there are some Ready Nodes with a 1.6TB) SSD for caching, your $/GB is increased anywhere from 33% to as high as 167%, due to the storage that’s being ignored.
You can go with a smaller cache (200GB or 400GB), but then you could be compromising performance because you have less room before you need to destage data. Plus the price difference between a cache SSD and capacity SSD means that for the extra 200GB what are you really saving with a smaller cache? Turns out about $80.
Micron recognized this issue with the Virtual SAN 600GB cache limit, and has come out with an SSD optimized specifically for Virtual SAN – the M510DC-600GB. The M510DC-600GB meets the endurance requirements on an AFA-6 system (3.8PB TBW) and exceeds the write performance requirements for an AFA-6 system – which is good feature for a write cache.
When considering all flash Virtual SAN Ready Nodes, or configuring your own, you want to make sure your cache is optimized. To take advantage of all the capabilities of Virtual SAN 6.2, you want to have your cache SSD as large as possible, but not pay for wasted space. The Micron M510DC-600GB is the only SSD on the market that fulfills those requirements.
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