I think you expect too much from ISA/VL SCSI.
I can say that because mkarcher and I have done extensive tests with the Adaptec 2842 VL.
Compared to PIO IDE, you surely will benefit from its DMA transfers, but sadly that controller does not benefit from the faster VL bus.

If you think about using a PCI controller, you should consider using an Adaptec 2940 UW or a NCR/Symbios/LSI 875.
The ISA 1542 is a good enhancement for an ISA 386 SX/DX system.
The EISA 2742 W also shows good performance on any EISA 386 / 486 system.

SCSI was in many ways more professional, had fewer structural constraints and more advanced components.
In a statistical sense, you'll find more really fast stuff in SCSI as in IDE, from that era.
E.g., there were immensely expensive and elaborate cached controllers around.

Which, afaik, was the main reason for the "VIP" boards - PCI mobos with legacy VLB slot:
Re: Need help with FIC 486-VIP-IO and AMD 486 120mhz
Apart from rather rare graphics cards that validated the same, it was mostly for very expensive SCSI controllers that people would carry a VLB device onto a later PCI board.

So, while not all SCSI controller-drive-combinations will be faster than any IDE one, many will be - especially considering CPU loads during operation if the machine does something else than just lugging data around.

Btw., much later, around 1999, I remember my first 4x CD-writer to be a bit less expensive than IDE alternatives. I was suprised and my only explanation is that such rather elite stuff was actually produced in larger quantities for SCSI back then. I also got a ROM drive along with that - which was a bit more expensive in SCSI. But I never had problems with spoilt blanks due to interrupted writing.

in short - it goes like this:
good isa ide < good isa scsi < good vlb scsi < good vlb eide < pci p/s ata

in the scope of home personal computers and single user workstations:

until early 90ies scsi held the performance high ground.
once good eide components hit the market things changed.

in other words - for 386 and before scsi is in general better. for vlb 486 and later hardware designs - not so much.

no idea how things were for file servers and such.

I use an RaSCSI (Pi-attached SCSI target emulator) with SCSI card to feed my 486 the OS drive image and to mount ISOs as CDs in DOS. So, that's one benefit of going SCSI.

I was thinking of adding a SCSI card to the DX4-100 I've been building the last few weeks but couldn't think of what to actually use it for aside from filling a slot. I've got plenty of SCSI hardware but don't really need more or faster local storage. An RaSCSI for mounting ISOs might be just the thing though - especially if it can handle multi-mode CD images (data+audio track).

being my recent computer with vlb (in theory it would also have PCI if I found the IBM ISA + PCI riser) from what I understand it is not worth the expense that would be considerable between controllers, hard drives and cd-roms more than 250 €. If I find one at a very low price I will get it in the future just to play with it but not because it is necessary. Did I get it right?

In the early days of CD-Rs SCSI was better. My first IDE CD-R was terrible at eating CD-Rs. There are pluses and minuses.

Depends what you want, Raw speed? probably CF on IDE, unless you can do SATA+SSD using modern tech. Period correct? SCSI drives came in faster models. If you have an IDE and SCSI drive that are equals, just the interface is different, then the difference will be minimal. Internal-data-transfer-rate is the key metric. How fast can you pull data off the platters. One big difference is PIO IDE vs SCSI. The CPU load will be much higher in the IDE system.

SCSI really starts to outperform IDE when you get into multiple devices. It supports disconnection. The drive gets a command, disconnects from the bus, then does it's work, reconnects and ships the data. While it is disconnected it doesn't hold up the bus. IDE can't do this. In a typical one spindle system you wont notice much difference, but with multiple drives IDE bogs-down. If you really want the worst-case example, write out a tape on the IDE bus, while accessing a drive on the same channel.

If you want the best performance that is period correct, then SCSI is the way to go. But only really a big difference if you bolt in lots of stuff to the computer.

mR_Slug wrote on 2022-03-27, 21:22:

In the early days of CD-Rs SCSI was better. My first IDE CD-R was terrible at eating CD-Rs. There are pluses and minuses. […]
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In the early days of CD-Rs SCSI was better. My first IDE CD-R was terrible at eating CD-Rs. There are pluses and minuses.

Depends what you want, Raw speed? probably CF on IDE, unless you can do SATA+SSD using modern tech. Period correct? SCSI drives came in faster models. If you have an IDE and SCSI drive that are equals, just the interface is different, then the difference will be minimal. Internal-data-transfer-rate is the key metric. How fast can you pull data off the platters. One big difference is PIO IDE vs SCSI. The CPU load will be much higher in the IDE system.

SCSI really starts to outperform IDE when you get into multiple devices. It supports disconnection. The drive gets a command, disconnects from the bus, then does it's work, reconnects and ships the data. While it is disconnected it doesn't hold up the bus. IDE can't do this. In a typical one spindle system you wont notice much difference, but with multiple drives IDE bogs-down. If you really want the worst-case example, write out a tape on the IDE bus, while accessing a drive on the same channel.

If you want the best performance that is period correct, then SCSI is the way to go. But only really a big difference if you bolt in lots of stuff to the computer.

only hard disk and cd-rom (rarely used). Ther are some problem on a 40Mhz bus with the vlb scsi controller?

One thing scsi will let you do is circumvent bios hard drive size limitations.

chrismeyer6 wrote on 2022-03-27, 22:31:

One thing scsi will let you do is circumvent bios hard drive size limitations.

To a certain extent.

To pass the 8 GB limit on NCR/Symbios/LSI-Logic 810, look here:
INT13 Extensions to SDMS 3.0 & 4.0 - SCSI by NCR / Symbios Logic / LSI Logic

To pass the 8 GB limit on Adaptec 2742 series (EISA) and Adaptec 2842 VL, look here:
INT13 Extensions to Adaptec 274x EISA & 284x VLB

AlessandroB wrote on 2022-03-27, 21:32:

only hard disk and cd-rom (rarely used). Ther are some problem on a 40Mhz bus with the vlb scsi controller?

I've never ran any vlb SCSI controller. Infoworld or PC mag did a review of the adaptec and another iIRC. They match the findings of whoever said they were disappointing. You'd have to look up the specs of the specific controller for 40MHz.

DX4 era systems was about the time IDE CD-ROMs became common. CD-Rs would have very likely been SCSI at that time.

SCSI was always more expensive, so in the '94-'95 era would likely have been found on PCI Pentium systems. Previously high-end 486 systems would have EISA and done SCSI via that bus. I don't think SCSI on VLB was common at all.

But, depends what you want to build. I like SCSI systems, manly because I have loads of spare HDDs. I'd like a VLB-SCSI system just because it's odd. It's difficult to recommend it on performance, unless you are on a very strict era specific build, and want the fastest HDD available at that time.

I don't have a huge amount of experience with VLB, but it certainly seems that its practical uses outside of graphics adapters were few and far between