the sidebar "On the OEM Front"). Besides 40-MBps throughput, all the adapters feature drivers for DOS/Windows, Windows NT, NetWare, OS/2, and Unix.
Each adapter features both internal and external 68-pin connectors for wide devices as well as a single 50-pin internal connector for 8-bit devices. (For reliable operation, however, you can connect to only two of the three available connectors.) Thanks to jumperless configuration via ROM-based, menu-driven setup programs, physical installation is relatively simple. You plug the card into an available PCI slot and apply power to the system. The PCI slot must support bus-master transfers. Automatic termination is also a standard feature that worked well on each of the adapters we tested.
Narrow SCSI is an 8-bit-wide bus that supports up to eight devices, one of which is the host adapter. At any one time, only two devices on the bus can exchange data at a rate of
5 MBps. Wide SCSI increases the width of the bus to 16 bits, doubling the throughput to 10 MBps. Fast SCSI increases the bus clock to 10 MHz, raising throughput to 10 MBps for 8-bit devices and 20 MBps for 16-bit devices.
UltraSCSI represents a further doubling of the bus clock for data transfers to 20 MHz. (For compatibility reasons, SCSI commands themselves still clock at 1 MHz.) Like fast SCSI, UltraSCSI comes in both 8- and 16-bit flavors, also known as Fast-20 and Fast-40, respectively. There are also differential and dual-channel UltraSCSI cards. The cards we test here are 16-bit, single-channel, single-ended devices. Physically, an UltraSCSI host adapter can support up to 15 devices, including up to seven 8-bit narrow devices if you provide the appropriate connectors. For wide UltraSCSI devices, the limit is currently eight.
The Adaptec
AHA-2940 Ultra Wide
ships with an extensive array of software and excellent documentation. Like the other cards we tested, it's a goo
d performer. On the downside, we found setup and use unnecessarily technical.
By default, the 2940 Ultra Wide is configured as SCSI ID 7, the highest priority on the SCSI bus (see the sidebar "SCSI IDS"). The card also supports the SCSI specification in its ordering of SCSI ID priorities (7-0 have higher priority than 15-8). Adaptec indicates that the card supports the SCSI Configured Automatically (SCAM) protocol to automatically assign SCSI IDs to compatible devices, but it disables this option by default. We did not test SCAM.
The 2940 Ultra Wide includes the ROM-based, menu-driven SCSISelect configuration utility with which you configure the card and run utilities to perform a surface scan or format on your SCSI disks. The card's default configuration enables support for wide devices but disables support for fast UltraSCSI bus speeds. To take advantage of full 40-MBps wide UltraSCSI bandwidth, you must manually set this option. Adaptec's assumption is that users will be attaching older SCSI de
vices.
Adaptec supplies the card with the complete set of drivers for its 7800 family of adapters, which includes the 2940 Ultra Wide. Installation of the drivers for NetWare 3.1x was straightforward except for one annoying aspect. To uniquely identify one of possibly multiple SCSI adapters installed in a system, you must manually calculate a slot number to provide as an argument to Adaptec's NetWare driver. The description of the procedure takes over a page in the user's manual, involves several sessions of hexadecimal-to-decimal conversion, and requires you to edit your STARTUP.NCF file after installation.
This procedure makes using the 2940 Ultra Wide needlessly complex and error-prone. Because the setup program knows the required information, the amount of additional code required for it to perform the slot-number calculation and display the result clearly on the setup screen would be trivial. Forcing you to perform this exercise even when installing a single host adapter is inexcusable.
BusLogic (now a division of Mylex) ships its
FlashPoint LW
card with driver support for a wide variety of OSes. The card performed similarly to the others, but several quirks in its setup and operation made configuration range from difficult to nearly impossible.
Like the other host adapters, the FlashPoint LW comes configured as SCSI ID 7 by default. However, unlike other true 16-bit SCSI devices, which you can set to any ID from 0 to 15, it is impossible to configure the FlashPoint LW for SCSI IDs greater than 7. A call to BusLogic's technical-support line did not produce an explanation of this limitation or a description of the proper priority relationship between UltraSCSI IDs.
AutoSCSI, the FlashPoint LW's ROM-based, menu-driven configuration utility, was difficult to use efficiently. You must manually enable support for 40-MBps wide UltraSCSI speed, for example. And to do so, you must forge your way through no fewer than five menu levels. After making any changes for a
single device, AutoSCSI requires that you back out of the menu system before you can save them. Switching to another device using PageUp or PageDown while in the View/Modify Device Configuration window, as prompted by the menu screen, discards all changes you've made for the current device--without warning. You must configure each device individually in this tedious manner.
QLogic's
Fast SCSI PCI Ultra-W
ships with a full range of drivers, and its straightforward configuration program made setup easy. For out-of-the-box compatibility with older SCSI devices, the Fast SCSI PCI Ultra-W comes configured as SCSI ID 7 but supports the full range of 16-bit SCSI IDs. The documentation gives no information relating SCSI ID to bus priority. This adapter does not support SCAM.
The QLogic card has the standard fast SCSI cable limits of 9.8 feet (3 meters) for four or fewer UltraSCSI devices and 4.9 feet (1.5 meters) for five or more UltraSCSI devices. Curiously, the installation guide
provides no upper limit for the number of supported UltraSCSI drives.
The Fast SCSI PCI Ultra-W's ROM-based setup program, Fast Util, saves time by setting the default value of all options to produce maximum performance with UltraSCSI devices. The installation guide lacks any screen shots of the setup program, however, to orient you when describing the available options. We also found some discrepancies in the default settings that are reported in the installation guide.
Driving Performance
To evaluate each adapter's ability to deliver data, we equipped a 133-MHz Gateway Pentium PCI system with 64 MB of RAM and Novell's NetWare 3.12 server software. (Testing under NetWare 4.01 gave essentially identical results.) A custom 1.5-meter UltraSCSI cable attached an eight-drive array of 2-GB Seagate ST32155W Hawk UltraSCSI hard drives to the test card's 68-pin internal SCSI connector. The first seven of these drives were assigned SCSI IDs 0-6. The eighth drive received SCSI ID 8. Host-
adapter IDs were left at their default value of 7. This arrangement assigns the host adapter the highest priority on the SCSI bus, according to the SCSI specification (see the sidebar "SCSI IDS").
We used Symbios Logic's IOBench SCSI benchmark program (the IODTest portion) to measure the performance of the host adapters. Although IOBench is a NetWare loadable module (NLM), it doesn't run across a network, and IODTest avoids use of the NetWare file system. As a result, limits of the network topology or file-system cache did not impact the results. IOBench ignores any partition information on the drives being tested. We set the maximum number of outstanding requests per drive to 10.
Our test matrix consisted of 12 configurations generated by changing the read/write mix (all writes, all reads, or half-and-half), size of the I/O request (4- or 64-KB blocks), and using sequential or random access. We tested each configuration once while varying the number of drives from one to eight.
Bus Sa
turation
Though we didn't expect to achieve the theoretical maximum UltraSCSI throughput of 40 MBps, we did get 36-MBps aggregate throughput under the ideal conditions of reading (or writing) the drive sequentially with large, contiguous 64-KB blocks (
see the graph
). Each of the UltraSCSI drives in our test setup can exchange data at about 5100 KBps under these conditions. Eight drives could potentially deliver just over 40 MBps.
For combinations of one through six drives, all the adapters turned in comparable performances, and throughput on the SCSI bus increased linearly by about 5100 KBps per drive, as expected. Output from the FlashPoint LW increased linearly when we added a seventh drive. The remaining three adapters, however, began to show signs of bandwidth saturation, dropping the average output per drive by between 2 percent and 4 percent. Adding the eighth drive to the bus caused the throughput to level off for all adapters, indicating that the bus was operat
ing at its maximum capacity.
Large-block sequential writes (not shown) provided slightly less throughput than reads. Aggregate throughput writing eight drives was around 1000 KBps less with writes.
With small-block transfers, the ultimate data transfer rate across a SCSI bus can be limited by the inability of the host adapter to process enough I/O requests in a timely manner. With random I/O, transactions per second scaled up linearly as we added drives to the test (both reads and writes), showing that the drives themselves limited performance, not the bus or host adapter. When hamstrung by neither bus-bandwidth or I/O-processing limitations, all four adapters were capable and turned in roughly equivalent performance.
When we configured IOBench to read data sequentially in 4-KB (eight-sector) units, something more interesting happened due to the cards' differing abilities to concatenate sequential I/O requests into larger extents. When reading from one, two, or three drives simultaneously, a
ll four adapters gave comparable performances, and the number of I/Os processed per second increased linearly by about 1280 I/Os per second per drive. (The SCSI bus is not processing nearly as many I/Os per second as the test indicates because of concatenation.) When we added the fourth drive, differences between the adapters became obvious.
The total number of apparent I/Os per second processed by the Fast SCSI PCI Ultra-W leveled off at approximately 4200 I/Os per second for four drives. Adding more drives decreased the I/Os per drive, but the total I/Os processed by the adapter remained nearly constant. With the 2940 Ultra Wide, total processed I/Os per second increased slightly to a maximum of 4416 when we added the fourth drive. However, adding additional drives decreased aggregate processing to a low of 3670 I/Os per second for eight drives.
The FlashPoint LW distinguished itself in this configuration, showing a linear increase in the number of I/Os processed as we connected up to five drive
s. The processing rate leveled out when we added six or more drives, reaching a maximum of 6589 apparent I/Os per second for eight drives. The Symbios adapter also turned in good performance in this configuration, slowing down sooner but not leveling off. The total number of I/Os processed begins to flatten out when the eighth drive is added. Nonetheless, the Symbios adapter is the winner in this test by processing 6811 apparent I/Os per second.
The short-block sequential results probably represent I/O saturation as opposed to throughput saturation. Even though they're interesting, they do not have much application to real-world performance.
What to Buy
If your hard drive use involves the typical mix of reads and writes in a range of block sizes, you'll be hard-pressed to measure any practical performance difference between the host adapters tested here. Choosing the wide UltraSCSI adapter that's right for you should boil down to a combination of price, support, and compatibilit
y with existing hardware. Here again, we're hard-pressed to make a decision. In all cases, we found setup software less than satisfactory for the tested adapters.
Adaptec's 2940 Ultra Wide includes the most complete list of features, but it also has the highest price. The lowest-priced card, BusLogic's FlashPoint LW, is feature-poor, while QLogic's Fast SCSI PCI Ultra-W is in the middle in terms of price and features.
Product Information
AHA-2940 Ultra Wide..................$375
(kit with cables, drivers, and EZ-SCSI software)
Adaptec, Inc.
Milpitas, CA
Phone: (800) 934-2766 or (408) 945-8600
Fax: (408) 262-2533
Internet:
http://www.adaptec.com
Circle 1081 on Inquiry Card.
UltraSCSI Doubles Speed
