Hybrid Virtualization

October 17, 2012, Storage Network World, Santa Clara, CA—Philip Black from Nexsan presented a perspective on hybrid storage systems to accelerate virtualization environments. The changes in technologies and user demands for higher performance call for new strategies in the data center.

Users are continuing to ask for more performance and capacity in their storage systems. Four possible responses include, adding more flash memory into the apps servers. This memory will provide fast responses for reference data, but the challenge is to maintain high availability and serviceability. In addition, sharing this expensive memory causes increases in latency for all users.

Second, convert the storage system to pure SSD. This fills the data center with lots of fairly low capacity and expensive solid-state drives that are very fast. Third, develop a flash cache between the app server and main store. The challenges are the user must self install these memories, and the system will lack important integration.

And finally, consider a hybrid that mixes SSDs and HDDs in the storage system. The different device types can be used as cache, tier, or both. This solution is especially useful for mixed applications, and allow the user to adjust the amounts of solid-state and hard disk drives. Tiering or cache will include RAID calculations on all paths. If cache is used before RAID, then the read can be moved to the cache.

Comparing and contrasting the approaches shows that a cache structure allows the use of policies and certifications, block copies, and retains memory hierarchies. A tiered approach leaves policies undefined, copies the full data set, and puts hot data into DRAM, hard copies into nonvolatile RAM, and online data into the SSDs. The solid-state drives need to use SLC for the high-volume traffic due to the need to have rigorous leveling algorithm control.

In a cache configuration, the high IOPS functions can use eMMC. Slower data requirements can go through SATA drives because there is little or no benefit in using SAS drives. The software for this memory sub-system should include management capabilities for the metadata. By changing from an HDD only configuration to a hybrid, the user can see significant improvements in IOPS without expending too much more for the SSDs.

So the best use of the various technologies is to put nonvolatile DRAM on the fastest changing data. Get new SLC or eMMC storage for data that requires high I/O rates, but this function is very control dependent. You can use a small SSD cache for 2 to 10 times improvement in throughput, with 4 to 5 times improvement being the most likely range. Technologies change quickly and often so a modular solution is better than a highly structured one. One area to consider is to change interfaces from the normal drive connections to PCIe for much higher data speeds through the system.