SCSI and Network Throughput

My friend Scott L. Barber does something I didn't understand for a long time: he recommends using a Mac Plus, SE, or whatever your slowest Mac is as a file server!

He recommends this for small office setups, not graphic departments. I can see where it's an inexpensive solution, but couldn't understand why it was a good solution.

Now it makes sense to me.

Although some of us still use poky old LocalTalk for networking, the standard in business is 10Mbps (megabits per second, note lower case b) ethernet using coaxial or twisted-pair wiring. Although this is rated as a 10Mbps protocol, real world performance rarely hits that level.

Looking over my Retrospect backup logs, I note that top throughput is rarely over 5 Mbps using TCP/IP. The less efficient AppleTalk protocol hits about half that.

Granting that there is overhead for reading and writing data during backup, plus keeping a backup catalog, normal ethernet can maybe hit 8 Mbps with TCP/IP and 5 Mbps with AppleTalk.

There are two factors that determine network throughput: how fast a computer can prepare the data for transmission (i.e., read it from the hard drive) and how many bits per second the networking software and hardware allow.

SCSI Throughput

Mac users love SCSI because it has great bandwidth. Even the original SCSI standard supported up to 5 MBps (megabytes per second, note upper case B), although the earlier Macs couldn't achieve that speed. Today's Ultra SCSI is four times faster, although few drives can read or write beyond 12 MBps - and most are in the 6-8 MBps range. This is also true of better IDE drives. (Older drives, like older computers, tend to be slower.) All of these are faster than ethernet: IDE, SCSI-1, USB, and 16X or faster CD-ROM players.

Converted to the language of networking (bits per second, not bytes), a good hard drive today can read and write data at 40-100 Mbps.

This is well beyond the capacity of 10Mbps ethernet and helps explain the growing popularity of 100 Mbps ethernet.

Older Macs

The Mac II had a decent SCSI port in its day, providing 1.35 MBps throughput or 11 Mbps* - slightly faster than regular ethernet and as nearly fast as the new USB (universal serial bus) in the iMac. This means any Mac with a 68020 or 68030 CPU has the potential to saturate a 10Mbps ethernet network. (You also need a drive that supports 1.35 MBps throughput, which a lot of old drives couldn't do. The February 1990 MacUser tested 101 hard drives, none of which hit 7 Mbps. Today's faster mechanisms with large onboard caches unleash the full potential of the SCSI bus on these older Macs.)

In terms of network throughput (and only in terms of network throughput), there is nothing to be gained from using a Quadra or Power Mac server on a normal ethernet network.

But Scott recommends less powerful computers, which baffled me. The Mac Plus has very poor SCSI throughput, something on the order of 2 Mbps (0.25 MBps).* But unless you're saving large files, even that may be adequate.

The problem with the Plus is putting it on ethernet, which requires either a LocalTalk-to-Ethernet adapter (such as Farallon's EtherMac, which also drives the serial port faster than usual) or a SCSI-to-ethernet adapter. Maximum throughput with EtherMac is about 0.4 Mbps, making this a poor choice. But the SCSI adapter will run at full SCSI speed.

Better than the Plus is the Macintosh SE. First, because it has better SCSI support than the Plus, up to two-and-a-half times faster. That translates to 5.1 Mbps,* using up to half of ethernet's 10Mbps bandwidth and about as much bandwidth as AppleTalk can use on a regular ethernet.

Second, with luck and patience you can find an ethernet card that fits the SE expansion slot. This can be significantly less expensive than SCSI or EtherWave adapters, making the SE a wonderful file server.

Picking Your Server

Scott recommends using AppleShare 3 and setting as large a disk cache as possible. AppleShare 3 will run on a Plus, SE, Mac II, and subsequent Macs. The biggest limitations are a maximum of 120 users and incompatibility with virtual memory and RAM Doubler - not a big drawback in the small office.

A Plus or SE with 4 MB of RAM leaves about 2 MB free. Set the maximum allowed disk cache (the disk cache is much faster than reading data from the SCSI port). The biggest drawbacks of the 68000-based Macs are limited RAM, which limits the size of the disk cache, and poor latency.

Another good low-cost solution is an LC or LC II with 10 MB RAM and an ethernet card. Despite their 16-bit data path, even these can match an ethernet network's bandwidth. With a VGA adapter, they can run without a monitor, keeping costs down. You can create a larger disk cache as you increase RAM.

A favorite choice is the SE/30: compact, fast enough, plus a built-in 9" screen. Best of all, if you can afford it, you can boost RAM to 128 MB and have a positively huge disk cache. The 16 MHz CPU is really a minimum for good server response.

Anything in the Mac II series can provide all the throughput 10Mbps ethernet can handle. For small files, they're great.

Other Factors

Then why did everyone switch to Quadra servers, and later to Power Macs? Didn't they do the math?

In most cases, no, we didn't do the math. We were surprised that AppleShare 4 wasn't any faster than AppleShare 3, but assumed that had to do with new features and the new 68040 CPU. So we upgraded.

What do you gain with a faster server?

In most cases the faster CPU means the computer has an easier time tracking multiple users (this is why Scott recommends this for small offices). And it looks through the file directory and disk cache more quickly, reducing latency (the time between your requesting a file and the server finding it).

You usually have the potential for adding more RAM, which can further reduce latency through the use of a larger disk cache.

But the benefit of 2 MBps (on the IIci), 3.4 MBps (Quadras), or faster SCSI is lost on a 10Mbps ethernet network. As for RAID, it's overkill on regular ethernet. The network itself remains a data bottleneck.

100 Mbps Ethernet

Of course, all this changes with 100 Mbps ethernet. Although it doesn't currently run ten times faster than 10Mbps ethernet on a Macintosh (more like 3-5 times faster), for moving lots of data it beats the slower ethernet hands down.

The ceiling on 100 Mbps ethernet seems to be 30-45 Mbps, which is about the top speed of SCSI on a Quadra. (Interestingly, it's also about the top speed for cable modems, explaining the 10Base-T port on the iMac.)

This is part of the reason graphic departments and others moving multimegabyte files love 100 Mbps ethernet. Four times faster means a lot when it takes several minutes to move your files.

UPDATE: Mac OS 8.5 has more native PowerPC code and uses OpenTransport 2, which overcomes some system bottlenecks and allow the Mac to take better advantage of 100 Mbps ethernet.

Other Protocols

LocalTalk. The other common networking scheme in the Mac world is LocalTalk, which runs at 230.4 kbps -- about 1/40 the speed of 10Base-T ethernet. Zip drives or floppy disks may be a faster way to move files, but even a lowly Plus is more than enough file server on LocalTalk.

FireWire. Some years ago Apple developed FireWire, a high speed serial protocol to replace SCSI. Offering speeds of up to 400 Mbps, it should become a viable alternative when computers start using it. Far faster then ethernet, FireWire will be another way to link computers together.

USB. Apple is just starting to use the universal serial bus (USB), a 12 Mbps serial protocol. You can look at it as the poor man's FireWire or souped up ADB. Although only one-third the speed of SCSI-1, USB is faster than 10Mbps ethernet, meaning a USB drive on a server would not be a liability. (How's that for thinking different?)

Conclusion

Unless you have a lot of network users making simultaneous use of your server or have a 100 Mbps ethernet network, a 68030-based Mac with AppleShare 3 may be your ideal server. The computers are very inexpensive ($200 tops); so are used ethernet cards. You can often buy the old RAM economically, allowing a large disk cache for improved latency.

Ideal small group servers are the SE/30 (compact, built-in monitor), IIsi, and IIci. There are advantages to faster, more powerful servers, but unless you're using 100 Mbps ethernet, you may no see a big improvement in throughput.

All this has convinced me to use my old Mac II as a home file server and get around to installing ethernet in the house.

Now comes the time to test the theory in Part 2.

* Maximum SCSI throughput from MacUser, February 1990, Buyer's Guide 7.