Servers are pushing the limits with more cores and more memory. PCI Express has improved peripheral bandwidth, but the storage bottleneck has led to a hefty storage hierarchy that’s about to get a little higher (see “The Storage Hierarchy Gets More Complex” at electronicdesign.com).
Marvell’s DragonFly VSA (virtual storage accelerator) puts more RAM and flash storage between the operating system and the conventional storage devices using a combination of hardware and software (Fig. 1). It targets virtualized enterprise platforms that utilize NAS/SAN/DAS storage and employs Marvell’s HyperScale caching technology.
The board features a DDR3 DRAM cache with error correction coding (ECC) that’s backed up by on-board flash storage and a supercapacitor system that will copy the DRAM contents to 32 Gbytes of single-level cell (SLC) NAND flash in the event of a power loss. It only takes a minute to recharge the supercap.
This is similar in operation to many battery-backed SAS/SATA controllers, so the system can restart instantly after a power loss. It provides the first level cache. The board also has half a dozen 6-Gbit/s SAS/SATA interfaces designed to be paired with solid-state disk (SSD) drives that are used to provide the second level cache.
The other piece to the puzzle is the VSA driver on the host that adds a thin shim between other storage device drivers and the operating system. The interface essentially provides a transparent, multilevel cache to the storage hierarchy. Application programming interface (API) calls are redirected to the VSA driver that manages access to the DragonFly’s storage, making calls to the actual device drivers as necessary.
Local storage includes other SAS/SATA controllers. Network storage can utilize protocols like NFS, iSCSI, FC, and FCoE. Marvell supports all major operating systems including Linux, Windows Server, and VMware.
SAS/SATA controllers often use a similar architecture with some on-board DRAM, usually less than 1 Gbyte. Some can employ attached SSDs as a cache for hard drives that are also attached to the controller. Like the DragonFly, these controllers typically have PCI Express Gen 2 by eight interfaces.
The DragonFly comes in a number of different configurations but the minimum has 1 Gbyte of DRAM (Fig. 2). The other low-profile configuration has 2 Gbytes while the full-height, half-length cards have dual-inline memory-module (DIMM) sockets supporting 4 or 8 Gbytes.
The system is designed to provide a tenfold read and write performance improvement. It uses the caches to provide write buffering and can handle re-ordered coalescing. It supports high-availability synchronous mirroring for single point of failure (SPOF) protection. Also, the DragonFly normally uses less than 14 W. It will grab just under 25 W when recharging the supercap.