Nimble Storage offers a flash-enabled hybrid storage solution that combines MLC flash with high capacity disk. CASL (Cache Accelerated Sequential Layout) - the architecture backing Nimble arrays, is a log structure file system. In other words CASL coalesces random writes coming into the system and writes them out in a full RAID stripe to disk. There is an efficient garbage collection mechanism at the back end that ensures that full stripes are available to write to at all times (unlike hole-filling file systems such as ZFS and WAFL).
Nimble uses flash as a read cache (no writes are staged here even though some writes go to cache based on the IO profile) - this allows us to eliminate the overhead of RAID protecting flash. Also the metadata is always stored in cache so even if there is a cache miss, the access is accelerated. In addition, compression is turned on at all times. This is a LZ variant and natively supports variable sized blocks. So there's no need to incur the read-modify-write overhead that comes with space saving techniques typically.
So what does the above mean for VDI? There are a lot of read path enhancements for VDI including the more recent VSA accelerator from VMware. This is all good. In addition, VDI places higher loads on the storage array to address write demands and that is where CASL shines. The read storms are addressed using the flash cache and the write peaks using the coalescing techniques outlined above.
With the launch of the CS420-x2 array, Nimble expects to be able to support north of 2000 users within a single array (3U form factor) for a moderate workload profile (12-16 IOPS steady state), delivering very competitive storage cost/desktop as well as desktop density.
