12.2.1 SSA backend interconnection

The Storage Serial Architecture (SSA) connectivity with the SSA loops in the lower level of the storage server or backend imposed RAID rank saturation and reached their limit of 40 MB/second for a single stream file I/O. IBM decided not to pursue SSA connectivity, despite its ability to communicate and transfer data within an SSA loop without arbitration.

12.2.2 Arrays across loops

Advancing from the F20 to the 800 model, the internal structures and buses increased two fold and so did the back end with striping logical volumes across loops (AAL). This increased the sequential throughput on an SSA loop from 40 MB/sec to 80 MB/sec for a single file operation, distributing the back-end I/O across two loops. Because of the increasing requirements to serve more data and at the same time reduce the application I/O response time, the SSA loop throughput was not sufficient and surfaced more often with RAID rank saturation.

12.2.3 Switch from ESCON to FICON ports

The front end got faster when it moved from ESCON at 200 Mbps to FICON at 2 Gbps with an aggregated bandwidth from 32 ESCON ports x 200 Mbps at 6.4 Gbps to 16 FICON ports with 2 Gbps each, yielding 32 Gbps. Note that the pure technology, like 2 Gbps, is not enough to provide good performance. The 2 Gbps FICON implementation in the ESS HA proved to provide industry leading throughput in MB/sec as well as I/Os per second. An ESS FICON port, even today, has the potential to exceed the throughput capabilities of other vendor’s FICON ports.

When not properly configured (for example, with four FICON ports in the same HA), these powerful FICON ports have the potential to saturate the respective host bay. Spreading FICON ports evenly across all host bays puts increased pressure on the internals of the ESS below the HA ports.

12.2.4 PPRC over Fibre Channel links

With PPRC over ESCON links there was some potential bottleneck when the HA changed from ESCON to FICON. Despite a smart overlap and utilizing multiple PPRC ESCON links for PPRC, the speed difference between FICON/FCP and ESCON channels introduced some imbalance in the ESS. The ESS 800 finally introduced PPRC over FCP links with 2 Gbps. Again this enhancement proves that the move to 2 Gbps technology is only half of the story. With a smart implementation in the FCP port connection for PPRC, the performance of an ESS FCP PPRC port is still not matched today by other implementation efforts.

These performance enhancements into and out of the ESS shifted potential bottlenecks back into the internals of the ESS for very high write I/O rates with 15,000 write I/Os and more per second.

12.2.5 Fixed LSS to RAID rank affinity and increasing DDM size

Another growing concern was the fixed affinity of logical subsystems (LSS) to RAID ranks and the respective volume placement. Volumes had to reside within a single SSA loop and even within the same RAID array; later in the A-loop and B-loop, but still bound to a single device adapter (DA) pair.

Even more serious was the addressing issue with the 256 device limit within an LSS and the fixed association to a RAID rank. With the growing disk drive module (DDM) size and

256DS8000 Series: Concepts and Architecture

Page 278
Image 278
IBM DS8000 manual SSA backend interconnection, Arrays across loops, Switch from Escon to Ficon ports