90/8 is not the right calculation. True, a byte is 8 bits but that does not account for the TCP/IP overhead. Let's start with an 8 Kbyte block. It can't be transmitted over TCP/IpP on a 100BaseT link. The maximum is only 1500 bytes per packet (rough numbers). So the 8Kb block must be sent as 6 packets. Each packet needs TCP/IP headers that include IP addresses (source and destination) along with other details. So that 8bit byte is looking more like 10bits with all the overhead. But the data doesn't travel in one direction. The packets must be disassembled, transmitted, acknowledged, retransmitted if necessary, and reassembled into the original data record. That's where server overhead (driver time) gets involved.
Secondly, you'll never get 90% throughput on your 100Mbit link. After you browse Rick Jones' NetPerf web pages, you'll understand more about real network performance.
http://www.netperf.org/ is the place to go for LAN performance. With some TCP?IP tuning, you might improve on the 50% number but not by much.
Finally, as mentioned before, you'll probably get shoe-shine performance out of your tape drive and that will seriously affect throughput as well as prematurely wear out your tape drive. You pay a very stiff penalty for not keeping the drive busy. In the old days of reel-to-reel tapes, the drive actually started, recorded and stopped for every record (really old stuff). These drives would literally buzz as the capstan rollers and brakes did their work. Later, the drives started buffering a few Kbytes and allowed the tape to avoid the mechanical start/stop cycles and performance soared.
Then in the 80's, the concept of a streaming tape drive became a reality. No more start/stop. Instead, a small motor would slowly spin the tape up to full speed. Once at speed, data would be recorded continuously. The idea is that the computer would keep sending data fast enough to keep the tape moving. But if the computer could not keep the data coming fast enough, the tape drive would run out of data (an underrun condition) and it would have to stop, backup, take a running start again, figure out where it was on the tape, find the end of the last record and then start recording again at just the right spot.
Streamers were inexpensive nad performed well but were terrible if the data stream wasn't fast enough, and for an entire tape, you could resposition several thousand times thus causing recording head wear equivalent to several hundred tape passes without repositioning.
Now, virtually all tape drives are streamers. The difference is that they have a ENORMOUS appetite for data. The LTO-2 drive is no exception. True, these drives have bigger data buffers (many megs) but unless the data stream is slightly faster than the tape drive (measured over many seconds), the tape will still be repositioned once the buffers are exhausted. This is a case where a slower tape drive is actually a better choice. You match the average data rate of the tape drive with the data rate of your backup stream.
So the bottom line is that you need a very fast link between computers. GigE is a possibility, just computer-to-computer if necessary, or aggregate multiple LAN cards if both ends support it. Otherwise, you may need to look at a local tape drive to keep the expected backup speeds and not destroy your drive in 6 months.
Bill Hassell, sysadmin
