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SSA technology breaks 3000 IO bottlenecks

New arbitrated storage topology speeds disk access for MPE/iX

By David A. Lethe
Compass Corporate Systems

[Editor’s note: As this issue went to press, we heard rumors that the HP 3000 division was about to approve SSA disk technology for use in the 3000 systems it supports. We first spotted SSA in the IBM marketplace three years ago, where it was making its debut. As SSA comes of age for MPE/iX, we asked one of the suppliers of these systems to explain how it works and how it can maximize HP 3000 disk operations.]

MPE/iX is one of the best operating systems there is for disk-intensive applications. This is because it hardly ever accesses the slowest device in a computer, the disk subsystem. MPE/iX only accesses disk when data isn’t in main memory, or when it determines that the data needs to be flushed to disk drives.

Over the last decade, however, advances in CPU technology and massive increases in storage requirements have been creating problems for HP 3000 users. Even though CPU power doubles roughly every 18 months, the disk industry hasn’t been able to keep pace. Many HP 3000s are now IO bound, rather than CPU bound. As storage requirements continue to increase 20 to 40 percent annually, the problem continues to get worse. If you throw money at a CPU upgrade on an IO-intensive system, you are just throwing money away.

Serial SCSI is the industry’s answer to IO problems. In the early 90s, IBM recognized that they not only had to make disks faster, they had to make the interface smarter. They invented and patented two new SCSI interfaces: FibreChannel and SSA. Both use the SCSI command set and are open ANSI standards. Since no vendor yet offers a fibre-channel solution for HP 3000s, this article will discuss the merits of incorporating SSA technology.

Serial Storage Architecture subsystems (SSA) offer these architectural advantages:

• SSA on HP systems is very fast. If your applications are IO-intensive, it can typically double your throughput as well as your IOs per second, with any block size.

• Data flows in full-duplex operation, with two 20Mb/second transmit channels and two 20Mb/second receive channels.

• Copper cables can extend your range to 100 meters, optical cables to 2.4 kilometers. This distance is between devices, so disk subsystems or adapters can be used as repeaters to extend beyond this.

• You don’t need switches or terminators. SSA drives are self-configuring, as they each have their own unique serial number. They use low-cost twisted-pair wiring (but not the same cabling used for Ethernet). The two cables each have four pairs of wires, compared to 68 wires for Fast/Wide SCSI. Drives can be hot-swapped.

• The architecture supports up to 127 devices per loop, and MPE/iX has been stress-tested to 80 drives. Multiple systems can be attached to the same loop, and multiple loops can be attached to the same system.

• The loop has built-in reliability. You can break the loop to add or remove drives, subsystems, hosts, or even loops. While the loop is broken into a string, data still flows (at half-speed) and no operator intervention is required.

• As additional adapters are added to the loop, aggregate performance increases proportionally as adapters are added.

• Each device can detect opens and shorts, and new paths are automatically routed to restore data flow. This happens without additional host overhead.

• Each frame of data is CRC checked and acknowledged. SCSI and FibreChannel can’t do this. When there is an error in either of those environments, it can take a significant amount of clock time and host overhead to react accordingly. Only in the event of a non-recoverable error is the host notified.

• SSA drives use the standard SCSI-2 instruction set for complete transparency to applications, hardware, and device drivers.

SSA’s history on the HP 3000

IBM shipped the first SSA subsystem in August 1995. Since that date, over three petabytes (3,000 terabytes) of the storage have been shipped on a variety of platforms. In 1996, Vicom Technologies developed a universal SSA adapter that allowed SCSI-based systems to attach to SSA drives. It only supported several Unix variants, including HP-UX.

This bridge not only provided servers with all of the topological innovations of SSA, but also serendipitously doubled the real-world throughput capability of the SCSI port. This was primarily facilitated because SSA data streams aren’t arbitrated. Multiple drives can concurrently process SCSI requests. Furthermore, the overhead for the bus phases required for arbitration was removed.

SCSI arbitration is the reason why performance starts to trail off with 3-4 drives on a single-ended SCSI, and 6-7 drives on a fast-wide SCSI port. Every request requires a drive to gain control of the SCSI chain. Out of the 11 bus phases required to process a read or write request, 8 are required for arbitration. SSA doesn’t need these 8 phases, which results in significantly faster response in high transaction environments (OLTP), as well as faster throughput (data warehousing). In fact, SSA subsystems on HP 3000/HP 9000 systems can perform over 1600 IOs per second and 18.5 Mb/sec sustained per adapter port. As adapters are added, performance increases proportionally.

In May of 1997, I worked with Forrest Wilson of IBM and Sam Tam of Vicom to develop the drive firmware, which enabled HP 3000s to recognize the SSA drives. It took about six months to certify the subsystems for a variety of customers – including credit unions, major airlines, insurance companies and even HP – in field tests.

The subsystem was announced and displayed at last year’s HP World conference. I contracted with Michael Hensley of Allegro to port the HP-UX service and diagnostic routines to MPE/iX. Although SSA for the HP 3000 was introduced a year ago, its rollout was hindered by legal issues, which have been resolved. SSA is ready for mass distribution as well as worldwide technical and on-site support.

Attaching SSA Drives to a 3000

An external RAID controller mounts in your HP 19-inch rack. The controller has up to four host adapter ports, and four SSA disk subsystem ports. The host ports connect via the standard HP cable to your HP fast-wide SCSI adapter cards (28696A), or to your HSC-SCSI adapter. The adapters can be on the same host, or on different hosts. Additional controllers can be added as required.

The other controller port attaches to SSA disk subsystems. The IBM 7133 subsystem holds up to 16 hot-swap SSA drives, and the Polaris subsystem holds up to 12 hot-swap IBM SSA disk drives. Drives of 4.5Gb and 9.1Gb capacity are shipping now, and an 18 GB drive should be available by the time this article is published. Any combination of drive capacity can be used in a subsystem. In addition, subsystems can be daisy-chained to the same host adapters, and a subsystem can be logically partitioned between different host adapters.

Once everything is physically attached and cabled, then you must configure the controller. This may be done by setting some dip switches on the controllers, or attaching a Windows-based PC for a GUI. Configuration options include controlling write-cache, setting up hardware mirroring, assigning global spares, and defining RAID-0 or RAID-1 groups. Although RAID-5 configurations are not possible at this time, it’s under development.

Why consider SSA

SSA drives are fast. In your most IO-intensive applications, you can typically double the throughput of HP disks like Model 20s, or even EMC’s Symmetrix subsystems. With the SSA topology and RAID options, you can also do things like

• Attach more than 16 drives (that’s not a typo, you could attach up to 64 drives) to a single fast-wide SCSI adapter – great if you are slot-bound.

• Attach more than the hard limit of 254 drives to a single HP 3000 (using hardware RAID-0)

• Mirror systems’ data remotely, without any host overhead. The mirrored disks can be concurrently physically attached to a disaster recovery server several kilometers away. Even when there is a hardware failure, there still isn’t any host overhead, or bandwidth degradation.

• Add/remove/migrate storage online. You can even add new subsystems without interruption or reboots, even if you don’t have any new fast-wide SCSI adapters.

• Instantly backup terabytes – define three-way hardware mirroring, then split a volume off the mirror, mount it on your backup server, and copy the data. This effectively allows you to backup/recover at 80Mb/second without affecting network overhead. When the backup is complete, then resync the two-way into three-way mirrors.

• SSA drives also support dual-porting. That is, the same LDEV can be attached to two different HP SCSI adapters. We support this today on other operating systems, including HP-UX. HP will support this in MPE/iX 6.0.

• Concurrently attach multiple drives to multiple systems, although you can’t mount the same LDEV on two systems. This will work under MPE/iX 6.0.

Configuring for MPE/iX

All MPE/iX programs including MAPPER, SYSGEN and IOCONFIG will recognize the SSA drives as standard SCSI disk drives. If you are running MPE/iX 5.5, use IOCONFIG to add/remove devices online. Assuming you have a fast-wide adapter at hardware address 4/8, then the drives will be addressed in the following order: 4/8.0.0, 4/8.1.0 … 4/8.15.0, 4/8.0.1, 4/8.1.1, 4/8.2.1 … Just assign the LDEVs, and a drive type of ST15150W, and you are in business.

If you are configuring hardware mirroring, then MPE/iX will only see half the drives. You may also wish to configure a hot spare which will also be invisible to the host. In the event of a failure, you will get audio and visual warning of the failed drive, and the hot spare will take over for it. Just replace the hot spare with a new drive, and that one will become the next hot spare.

Since the SSA drives appear as standard HP/Seagate drives, then all software including Mirror/iX, Predictive Support, SwitchOver, HP Diagnostics and third-party applications work perfectly. No patches, drivers or modifications are required.

HP support

HP is currently supporting HP 3000 users with SSA drives attached. In addition, IBM and other companies provide on-site support for the subsystems. It is also important to know that HP CSY and many end-users have gone through stringent testing and certification procedures to validate their performance and compatibility.


An HP 3000-specific RAID adapter for SSA lists for $15,600 in late July. A 16-drive x 9.1Gb disk IBM 7133 disk subsystem lists for $69,000 ($0.47/Mb). My company’s Polaris SSA subsystem using the same IBM 9.1Gb disk drives goes for about half of that.

Subsystems scale from 36Gb to terabytes, and you can put over a terabyte in a single 19-inch rack footprint. Copper SSA cables are priced about the same as HP SCSI cables. Optical SSA cables are significantly more.

What’s next

Shortly after this article publishes we will begin to offer 160 Mb/second drives, 10,000 RPM drives, and 18Gb drives. When HP supports their FibreChannel adapter for the HP 3000, then we will ship our FC-to-SSA adapter. This will give you a 100 Mb/second disk to subsystem interface, one that uses SSA drives with a 160Mb/sec interface. You can compare that to HP’s Model 30 subsystem, which offers a 100Mb/second host interface that uses 20 Mb/sec SCSI drives internally. With these new additions, we expect price reductions soon.

Copyright 1998 The 3000 NewsWire. All rights reserved