Looking at the numbers I'd say in w2k the full U2W scsi bus bandwidth is available, though in 95 and NT4 it's limited to either wide SCSI2 speed or narrow SCSI3 speed. Both are probably a driver issue I'd think.

weedeewee wrote on 2021-08-08, 12:14:

Looking at the numbers I'd say in w2k the full U2W scsi bus bandwidth is available, though in 95 and NT4 it's limited to either wide SCSI2 speed or narrow SCSI3 speed. Both are probably a driver issue I'd think.

Sounds plausible, I'm using the latest Adaptec drivers and am sure I picked the correct one during install. I'll do some more testing, maybe i'll try some older drivers and maybe even UW instead of U2W drivers.

I just tried three different U2W driver versions (from oldest to newest) under NT4 and results are still the same, Reads are capped at 19MB/s.
I also tried to connect the drive to the UltraWide connector instead of Ultra2Wide and the results are the same.

Any settings available in the driver for the card or driver for the hard drive or inbetween that might allow you to set the speed?

weedeewee wrote on 2021-08-08, 12:58:

Any settings available in the driver for the card or driver for the hard drive or inbetween that might allow you to set the speed?

Just checked again, there are no settings available for the controller nor the disk when using the Control Panel - > SCSI Adapters -> Devices view and clicking properties on each.

When using Adaptec SCSI Explorer utility in NT4, it's showing that Read Cache is enabled, Write Cache is disabled. These are the only two configurable settings.

Problem solved, it turned out that I had "Initiate Wide Negotiation" option in the SCSI Bios disabled for few of the device IDs, one of them being the current drive I was using. Now in NT4 using ATTO I'm seeing 69MB/s Reads and 34MB/s writes. So this is a huge improvement.

Don't understand though how in Win2K I was able to get these speeds despite having the Wide negotiation disabled. Maybe Win2K has some advanced driver that overrides that setting.

It also would be nice to figure out how to get faster "writes". This is a 10K drive that I would think can sustain more than 34MB/s. Could this be a limit of the 2940U2W controller? Fast for reads but much slower for writes?

So far the fastest I/O devices I've used is the Promise SATA150 TX2 Plus controller. In this particular machine I really want to use SCSI though as it seems seems little bit more "correct" than SATA, and I'm also using SCSI CD-ROM, ZIP Drive, Tape Backup and don't have enough PCI slots for two controllers.

probably 95 and NT just use the scsi bios settings, while the w2k driver does its own settings/detection, similar to what the linux driver does.
also, try testing with the write cache enabled.