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Fewer than 5% of servers have a flash tier deployed today and cost is likely a major reason for that small figure. In some cases, the cost of the flash is a significant percentage of the blade it serves. Even so, adding solid-state storage can help improve the density of a server environment. If the physical server is I/O bound with a certain number of VMs installed yet has more CPU capacity available, flash will enable more VMs to be deployed on that server.
EMC’s recently released VFCache 1.5 has added data deduplication technology, leveraged from other EMC products. Deduplication can be a game changer for some IT organizations because its benefits to flash are twofold. It decreases the cost per GB stored, an important metric to most IT organizations; and it requires fewer write operations to the flash card, thereby extending the life of the physical solid-state storage. This reduces the tradeoff IT organizations must make to balance the higher cost per GB of SSD versus the lower cost per IOPS of SSD. Similarly, VeloBit’s HyperCache enables compression on flash. Compression stores more data in the same physical media, also lowering the cost per GB. “Your mileage may vary” always applies, but IT organizations shouldn’t expect the 20:1 deduplication/ compression ratios achieved with backup appliances. EMC suggests a more conservative 20% expectation for deduplication of VFCache.
Proximal Data takes, perhaps, a more conventional
Maintaining data coherence
Because virtual server and virtual desktop environments are the key targets for server flash, IT managers need to understand how a solid-state software product will function in that environment. One key value of a VM is its ability to migrate to recover from hardware failures or for performance improvement. How solid-state software behaves during a migration is an important factor in determining the best product for a given situation.
Key features to look for in solid-state software
- Seamless support of vMotion for business continuity.
- Compression and/or deduplication to reduce the cost per GB stored.
- Deployment flexibility in device type and workload targeting.
- “Hot” data algorithm and cache hit rate.
Of the factors needed to support virtual environments, perhaps the most important is data coherence, or consistency, between the server flash and back-end storage. The solid-state software algorithms copy the “hot” data from the primary storage to the flash and ensure the data remains consistent between the two repositories. Although some vendors plan to support active-active clustering in the future, support is currently limited to active-passive configurations. As such, data consistency issues arise when data is modified out-of-band (i.e., a snapshot restore) or when a volume is unmounted/remounted as may occur during a VMDK move.
EMC VFCache supports live migration with vMotion in a VMware environment. As such, it doesn’t need to “re-warm” the cache when a VM is migrated and it fully supports business continuity. NetApp Flash Accel caches the VMDK at a block level and uses what the company calls “persistent endurability” that avoids cache re-warming in the event of a reboot or VM failure. VeloBit HyperCache currently requires a re-warm, though it has persistency on the roadmap. Proximal Data AutoCache algorithms adapt in real-time to move metadata with the VM to “pre-warm” the cache.
This was first published in October 2012