During the past year or so, we've seen a veritable explosion of interest in using server-side solid-state storage devices as performance-boosting caches. While many of the vendors in this market talk about using their cache software in a virtual environment, the truth is EMC's VFCache, VeloBit, SanDisk's FlashSoft, Nevex's CacheWorks and other currently available caching products actually run in the Windows or Linux guest operating system, and not in the hypervisor. This complicates using them along with workload mobility features of modern hypervisors like vMotion or Live Migration.
Startup Proximal Data came out of stealth this week with AutoCache, the first server-side cache product designed from the ground up for the hypervisor environment. AutoCache installs in vSphere 4.1 or 5.0 ESXi hosts through a single VMware Install Bundle. The caching software installs as a block I/O filter driver between ESXi VMKernel and the block I/O drivers for the host's back-end storage devices. From this location, AutoCache can not only accelerate read I/O from local, iSCSI or Fibre Channel disks, but it can also collect access statistics on the host's disk I/O for analysis that can help administrators select the most appropriate SSDs for their particular workloads.
Once it's installed, AutoCache is a write-through cache, passing write I/O immediately to the back-end storage. This means the cache never contains any unique data, so a server failure doesn't leave data trapped in the cache. It also means there's no need to coordinate cache flushing with storage-array features like snapshots and replication. The cache index of which blocks are cached and their heat level is stored in host memory, so AutoCache doesn't add significant latency to cache misses like products that store the index on the SSD can.
Not using guest OS components simplifies installation and avoids tying guests to flash drives. As a result, AutoCache doesn't get in the way of vMotion, DRS or other vSphere management features.
Workload migration via vMotion is the Achilles' heel for most other server-side caching products that either require manual steps to disable the caching before and re-enable it after the VM is migrated, or support vMotion only when shared SSDs are used. AutoCache detects the vMotion through its vCenter plugin, enabling it to mark a VM's cached data as invalid in the cache and use that space for other VMs.
In version 1.0 of AutoCache, a migrated VM will arrive at its new host, where it will run at the speed of the back-end disk storage until that host's cache can be populated. Proximal Data says it has figured out a method for warming (preloading) the cache on the destination host and will include that feature in a future release.
AutoCache is priced at $995 per host for up to 500 Gbytes of cache. While AutoCache can theoretically work with any SSD, Proximal is initially supporting both PCIe and SAS/SATA SSDs from LSI and Micron, with Intel and Fusion-IO support to follow soon.