SCSI RDMA Protocol
Encyclopedia
In computing the SCSI RDMA Protocol (SRP) is a protocol that allows one computer to access SCSI
devices attached to another computer via remote direct memory access
(RDMA). The SRP protocol is also known as the SCSI Remote Protocol. The use of RDMA makes higher throughput and lower latency possible than what is possible through e.g. the TCP/IP communication protocol. RDMA is only possible with network adapters that support RDMA in hardware. Examples of such network adapters are InfiniBand
HCAs and 10 GbE network adapters with iWARP
support. While the SRP protocol has been designed to use RDMA networks efficiently, it is also possible to implement the SRP protocol over networks that do not support RDMA.
As with the ISCSI Extensions for RDMA
(iSER) communication protocol, there is the notion of a target (a system that stores the data) and an initiator (a client accessing the target) with the target performing the actual data movement. In other words, when a user writes to a target, the target actually executes a read from the initiator and when a user issues a read, the target executes a write to the initiator.
While the SRP protocol is easier to implement than the iSER protocol, iSER offers more management functionality, e.g. the target discovery infrastructure enabled by the iSCSI protocol. Furthermore, the SRP protocol never made it into an official standard. The latest draft of the SRP protocol, revision 16a, dates from July 3, 2002 .
In order to use the SRP protocol, an SRP initiator implementation, an SRP target implementation and networking hardware supported by the initiator and target are needed. The following software SRP initiator implementations exist:
The following SRP target implementations exist:
Bandwidth and latency of storage targets supporting the SRP or the iSER protocol should be similar. On Linux there is an SRP storage target implementation available that runs inside the kernel (SCST) and an iSER storage target implementation that runs in user space (STGT). Measurements have shown that the first implementation has a lower latency and a higher bandwidth than the second. This is probably because the RDMA communication overhead is lower for a component implemented in the Linux kernel than for a user space Linux process, and not because of protocol differences .
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
devices attached to another computer via remote direct memory access
Remote Direct Memory Access
In computing, remote direct memory access is a direct memory access from the memory of one computer into that of another without involving either one's operating system...
(RDMA). The SRP protocol is also known as the SCSI Remote Protocol. The use of RDMA makes higher throughput and lower latency possible than what is possible through e.g. the TCP/IP communication protocol. RDMA is only possible with network adapters that support RDMA in hardware. Examples of such network adapters are InfiniBand
InfiniBand
InfiniBand is a switched fabric communications link used in high-performance computing and enterprise data centers. Its features include high throughput, low latency, quality of service and failover, and it is designed to be scalable...
HCAs and 10 GbE network adapters with iWARP
IWARP
The Internet Wide Area RDMA Protocol is a computer networking protocol for transferring data efficiently.It is sometimes referred to simply as "RDMA", though RDMA is not a feature exclusive to iWARP.-History:...
support. While the SRP protocol has been designed to use RDMA networks efficiently, it is also possible to implement the SRP protocol over networks that do not support RDMA.
As with the ISCSI Extensions for RDMA
ISCSI Extensions for RDMA
The iSCSI Extensions for RDMA is a computer network protocol that extends the Internet Small Computer System Interface protocol to use Remote Direct Memory Access...
(iSER) communication protocol, there is the notion of a target (a system that stores the data) and an initiator (a client accessing the target) with the target performing the actual data movement. In other words, when a user writes to a target, the target actually executes a read from the initiator and when a user issues a read, the target executes a write to the initiator.
While the SRP protocol is easier to implement than the iSER protocol, iSER offers more management functionality, e.g. the target discovery infrastructure enabled by the iSCSI protocol. Furthermore, the SRP protocol never made it into an official standard. The latest draft of the SRP protocol, revision 16a, dates from July 3, 2002 .
In order to use the SRP protocol, an SRP initiator implementation, an SRP target implementation and networking hardware supported by the initiator and target are needed. The following software SRP initiator implementations exist:
- Linux SRP initiator, available since November 2005 (kernel version 2.6.15 ).
- Windows SRP initiator, available through the WinOF InfiniBand stack .
- VMWare SRP initiator, available since January 2008 through Mellanox' OFED Drivers for VMware Infrastructure 3 and vSphere 4 .
- Solaris 10 SRP initiator, available through Sun's download page .
- The IBM POWER virtual SCSI client driver for Linux (ibmvscsi), available since January 2008 (kernel version 2.6.24 ). Virtual SCSI allows client logical partitions to access I/O devices (disk, CD, and tape) that are owned by another logical partition.
The following SRP target implementations exist:
- A Linux SRP target implementation for RDMA networks, distributed via the SCST project .
- The IBM POWER virtual SCSI target driver for Linux (ibmvstgt), available since January 2008 (kernel version 2.6.24 ).
Bandwidth and latency of storage targets supporting the SRP or the iSER protocol should be similar. On Linux there is an SRP storage target implementation available that runs inside the kernel (SCST) and an iSER storage target implementation that runs in user space (STGT). Measurements have shown that the first implementation has a lower latency and a higher bandwidth than the second. This is probably because the RDMA communication overhead is lower for a component implemented in the Linux kernel than for a user space Linux process, and not because of protocol differences .