Sorry, this isn't an answer to your question, but what performance benefits did you expect going SCSI? Assuming you're using DOS or Win3.x with your DX2 and assuming you've got at least 16MB of memory I don't think a SCSI solution will yield any noticeable performance gains in games compared to a decent CF card on IDE or SD-IDE solution.
Correct me if I'm wrong please.

Yeah, I agree it's "inappropriate" to expect performance boost from SCSI... 🤣
But the disk performance I have seems "abnormal" and it is a way to maximaze disk capacity under DOS 6.22.

Have you checked the SCSI2SD Wiki?

http://www.codesrc.com/mediawiki/index.php/SCSI2SD

It seems that 1-2 MiB/s is about to be expected. You need the v6 hardware for better performance.

Again, I'm sorry I can't help you with your main issue, but maybe I can help you with an alternative if you don't succeed in getting it running snappy:
If you're already using a network card with empty EPROM socket in that machine you might be interested in XTIDE universal BIOS.
This helps you run large IDE storage solutions on old PCs. After installing this on your system you should be able to run four 2GB FAT16 partitions per drive under DOS 6.22 - just like with your SCSI solution.
I hope this is helpful in some way. 😀

Xeen wrote on 2020-02-27, 04:48:

However, when I check disk performance from Norton Sysinfo 8, they only produce around 996 Kilobytes/s and that is far below the original 512MB IDE DOM which can produce around 4045 Kilobytes/s.

What bus(es) you have in that 486?
I'm asking, as 4 MB/s is plain impossible on ISA IDE - what's possible is PIO mode 0, with theoretical limit of 3.3 MB/s.

root42 wrote on 2020-02-27, 08:19:

Have you checked the SCSI2SD Wiki?

http://www.codesrc.com/mediawiki/index.php/SCSI2SD

It seems that 1-2 MiB/s is about to be expected. You need the v6 hardware for better performance.

I use SCSI2SD V5 and hopefully "1.1MB/s on most older SCSI hosts" is not my case. 🙁

Doornkaat wrote on 2020-02-27, 08:37:

Again, I'm sorry I can't help you with your main issue, but maybe I can help you with an alternative if you don't succeed in get […]
Show full quote

Again, I'm sorry I can't help you with your main issue, but maybe I can help you with an alternative if you don't succeed in getting it running snappy:
If you're already using a network card with empty EPROM socket in that machine you might be interested in XTIDE universal BIOS.
This helps you run large IDE storage solutions on old PCs. After installing this on your system you should be able to run four 2GB FAT16 partitions per drive under DOS 6.22 - just like with your SCSI solution.
I hope this is helpful in some way. 😀

Thanks for the info, I will check what network card I have.

Grzyb wrote on 2020-02-27, 09:27:

What bus(es) you have in that 486?
I'm asking, as 4 MB/s is plain impossible on ISA IDE - what's possible is PIO mode 0, with theoretical limit of 3.3 MB/s.

It's a Gateway2000 486DX2-66v which DOM is directly connect to onboard IDE port.

Xeen wrote on 2020-02-27, 09:42:

It's a Gateway2000 486DX2-66v which DOM is directly connect to onboard IDE port.

So...
local bus IDE vs. ISA SCSI
Sorry, there's no way for SCSI to be the winner here.

1 MB/s indeed seems somewhat slow, but can't expect it to be faster than about 3 MB/s - Re: ISA Hard Drive controllers

Grzyb wrote on 2020-02-27, 09:57:

So... local bus IDE vs. ISA SCSI Sorry, there's no way for SCSI to be the winner here. […]
Show full quote

Xeen wrote on 2020-02-27, 09:42:

It's a Gateway2000 486DX2-66v which DOM is directly connect to onboard IDE port.

So...
local bus IDE vs. ISA SCSI
Sorry, there's no way for SCSI to be the winner here.

1 MB/s indeed seems somewhat slow, but can't expect it to be faster than about 3 MB/s - Re: ISA Hard Drive controllers

Then SCSI2SD is for storage, but how do I tweak it?

Another idea, is VLB SCSI card a game changer here?

Xeen wrote on 2020-02-27, 10:16:

Grzyb wrote on 2020-02-27, 09:57:

So... local bus IDE vs. ISA SCSI Sorry, there's no way for SCSI to be the winner here. […]
Show full quote

Xeen wrote on 2020-02-27, 09:42:

It's a Gateway2000 486DX2-66v which DOM is directly connect to onboard IDE port.

So...
local bus IDE vs. ISA SCSI
Sorry, there's no way for SCSI to be the winner here.

1 MB/s indeed seems somewhat slow, but can't expect it to be faster than about 3 MB/s - Re: ISA Hard Drive controllers

Then SCSI2SD is for storage, but how do I tweak it?

Another idea, is VLB SCSI card a game changer here?

Well, I just looked at the above link to the SCSI2SD Wiki and it says the V5 SCSI2SD can achieve

Up to 2.6MB/s read, 2.3MB/s write

(1.1MB/s on most older SCSI hosts)

So your original IDE DOM solution is probably going to still be faster than what your SCSI2SD will ever be. You'll need a V6 SCSI2SD (theoretically 20MB/s, realistically up to 10MB/s) plus VLB SCSI controller and honestly afaik with flash based drives as long as you're not shuffling around data while gaming you're not going to notice a difference in performance SCSI vs IDE.

Xeen wrote on 2020-02-27, 09:42:

I use SCSI2SD V5 and hopefully "1.1MB/s on most older SCSI hosts" is not my case. 🙁

I suspect that ISA SCSI hosts belong in the "older SCSI hosts" category here, so yes, that's exactly your case, everything is normal.
Replacing that Adaptec 1540CF with a VLB counterpart would surely help, but - as already mentioned by Doornkaat - it would still be slower than IDE.

Note that SCSI does data transfer without the CPU. So even if it is slower from the data throughput in your case (blame the SCSI2SD v5, the V6 is the fast(er) one; also a SD card is way slower than a DOM!), it may still be faster overall, since the CPU has more free time to do stuff while data is loaded.

Grzyb wrote on 2020-02-27, 11:49:

I suspect that ISA SCSI hosts belong in the "older SCSI hosts" category here, so yes, that's exactly your case, everything is normal.
Replacing that Adaptec 1540CF with a VLB counterpart would surely help, but - as already mentioned by Doornkaat - it would still be slower than IDE.

I believe it is the case, tried all card options including enabling BIOS shadowing and none of them can help.

derSammler wrote on 2020-02-27, 12:00:

Note that SCSI does data transfer without the CPU. So even if it is slower from the data throughput in your case (blame the SCSI2SD v5, the V6 is the fast(er) one; also a SD card is way slower than a DOM!), it may still be faster overall, since the CPU has more free time to do stuff while data is loaded.

It's not difficult to see SCSI2SD is slower than a DOM especially when switching drives and do "dir/w", but it's a surprise that SD card is way slower than a DOM. 😮

I replace the DOM with a 2GB CF card and performance is as good as DOM.

Which version should you buy ?
SCSI2SD V6 This board offers significantly better performance than the V5.x boards by adding support for synchronous SCSI transfers.
Easy access to virtual disk data over USB.

See SCSI2SD V6 Compatibility

SCSI2SD older version is your performance problem.

I use SCSI2SD v5, pretty old version.

For data backup and transfer purpose, it's doing a good job. I backup DOM to SCSI2SD (4 x 2GB drive or 8GB drive contents 4 partitions) then restore everything back on CF card, although speed is not good it's quite convenient.

Btw, is v5 SD card compatible with v6 device?

Now I have trouble to boot from SCSI2SD, controller can see SCSI drive but system freezes right after beep.

SCSI drive is working when I boot from IDE or floppy. FDISK and format works without problem, BIOS shadowing and CPU writ through cache are also enabled.

What do I miss? ((+_+))

I have both SCSI2SD 5.0 and 6.0 and different controllers.

Without much tuning I and average card I could get:

SCSI2SD 5.0 with 1542CP about 1000 kB/s. This is a limit of the SCSI2SD 5.0 performance and async transfers.
SCSI2SD 6.0 about 2138 kB/s which is pretty much ISA limited at this point.
SCSI2SD 6.0 on AHA-2842 (VL-Bus) or AHA-2940 (PCI) with a good card can get about 9000 kB/s. This seems to be narrow SCSI-2 limit.

So what you see is normal. I you want better sustained speed you need to upgrade to SCSI2SD device to something better.

Booting issue resolved, "FDISK /mbr" did the job and thanks for all the feedback, apparently I need a 6.0 and VLB controller card.

Offtopic remark: your isa ide throughputs are the fastest I have ever seen. I have a nearly 14MHz ISA clock and the fastest ide device I can get up to 3500kb/s. (DOM, HDDs all tried).
Wow!