"As you probably know there is a trend in enterprise computing towards networked storage. This is illustrated by the emergence during the past few years of standards like SRP (SCSI RDMA Protocol), iSCSI (Internet SCSI) and iSER (iSCSI Extensions for RDMA)," began Bart Van Assche, proposing that SCST be merged into the mainline kernel. He suggested that while similar to the STGT project which has been part of the mainline kernel since 2.6.20, "SCST is superior to STGT with respect to features, performance, maturity, stability, and number of existing target drivers. Unfortunately the SCST kernel code lives outside the kernel tree, which makes SCST harder to use than STGT."
SCSI subsystem maintainer, James Bottomley, was not convinced, explaining:
"The two target architectures perform essentially identical functions, so there's only really room for one in the kernel. Right at the moment, it's STGT. Problems in STGT come from the user<->kernel boundary which can be mitigated in a variety of ways. The fact that the figures are pretty much comparable on non IB networks shows this. I really need a whole lot more evidence than at worst a 20% performance difference on IB to pull one implementation out and replace it with another. Particularly as there's no real evidence that STGT can't be tweaked to recover the 20% even on IB."
"I'm pleased to announce [the] fourth release of the distributed storage subsystem, which allows [you] to form a storage [block device] on top of remote and local nodes, which in turn can be exported to another storage [block device] as a node to form tree-like storage [block devices]," Evgeniy Polyakov stated on the Linux Kernel mailing list. The new release includes a new configuration interface and several bug fixes.
Network device driver and SATA subsystem maintainer, Jeff Garzik, was not impressed with the concept, "[distributed block devices] are not very useful, because it still relies on a useful filesystem sitting on top of the DBS." He went on to explain the problem, "it devolves into one of two cases: (1) multi-path much like today's SCSI, with distributed filesystem arbitrarion to ensure coherency, or (2) the filesystem running on top of the DBS is on a single host, and thus, a single point of failure (SPOF)." He proposed instead that time would be better spent developing a POSIX-only distributed filesystem, "in contrast, a distributed filesystem offers far more scalability, eliminates single points of failure, and offers more room for optimization and redundancy across the cluster." Jeff went on to caution, "a distributed filesystem is also much more complex, which is why distributed block devices are so appealing :)" When Lustre was pointed out as an existing option, Jeff noted, "Lustre is tilted far too much towards high-priced storage, and needs improvement before it could be considered for mainline."
Evgeniy Polyakov, listed as the connector and w1 subsystem maintainer, announced the first release of his distributed storage subsystem, "which allows [you] to form storage on top of remote and local nodes, which in turn can be exported to another storage as a node to form tree-like storages." He describes the features of this new block device: "zero additional allocations in the common fast path not counting network allocations; zero-copy sending if supported by device using sendpage(); ability to use any implemented algorithm (linear algo implemented); pluggable mapping algorithms; failover recovery in case of broken link; ability to suspend remote node for maintenance without breaking dataflow to another nodes (if supported by algorithm and block layer) and without turning down main node; initial autoconfiguration (ability to request remote node size and use that dynamic data during array setup time); non-blocking network data processing; support for any kind of network media (not limited to tcp or inet protocols); no need for any special tools for data processing (like special userspace applications) except for configuration; userspace and kernelspace targets."
In his blog, Evgeniy noted a similarity to the recently discussed DRBD. In the recent announcement he compares his solution to iSCSI and NBD noting the following advantages: "non-blocking processing without busy loops; small, pluggable architecture; failover recovery (reconnect to remote target); autoconfiguration; no additional allocations; very simple; works with different network protocols; and storage can be formed on top of remote nodes and be exported simultaneously".
A recent posting to the lkml announced that the Open-iSCSI and Linux-iSCSI projects have merged into a single effort. iSCSI stands for the Internet Small Computer Systems Interface, a transport protocol that works on top of TCP and allows the SCSI family of protocols to function over a network instead of only over a local bus. Regarding the recent merging of efforts, the announcement explains:
"After some dialog with the developers on each team, it was decided that although each team started out with independent code and some differences in architecture, it was worth considering the possibility of combining the two efforts into one. Alternatives were considered for the combined architecture of the two projects, including adding an option for a kernel control plane. After discussions, it was decided by consensus that the open-iscsi architecture and code would be the best starting point for the "next gen" linux-iscsi project.
"The advantages of this starting point include open-iscsi's optimized I/O paths that were built from scratch, as well as incorporation of well tested iscsi-sfnet components for the control plane and userspace components. The combined open-iscsi and linux-iscsi teams believe this will result in the highest performing and most stable iSCSI stack for Linux."