HamaZ

PCI64 SCSI HBAs on PCI32 bus

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Hello.

I heard (and read in several topics in this forum) that most 64-bit PCI SCSI *RAID* controllers (from Adaptec, LSI, Mylex, etc.) do not behave well at all when used on a regular 32b/33MHz PCI bus. According to my hearings/readings, they work, but performance is unnaturally slow (card's bandwidth tops at around 20-40 MB/s). AFAIK, PCI bus should put trough 100-110 MB/s in practice.

Is this generally true, or maybe those people had some incompatibilities etc.?

But this issue may not be applicable to non-RAID PCI64 SCSI HBAs, and that is my main question here. I intend to acquire Adaptec AHA-39160 (PCI64 dual-channel U160 non-RAID HBA) and to run it on a regular PCI 32/33 bus. Will the card's troughput match the actual troughput of my PCI bus, or will it drop dramatically (to how many MB/s)?

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Hello.

I heard (and read in several topics in this forum) that most 64-bit PCI SCSI *RAID* controllers (from Adaptec, LSI, Mylex, etc.) do not behave well at all when used on a regular 32b/33MHz PCI bus. According to my hearings/readings, they work, but performance is unnaturally slow (card's bandwidth tops at around 20-40 MB/s). AFAIK, PCI bus should put trough 100-110 MB/s in practice.

Is this generally true, or maybe those people had some incompatibilities etc.?

But this issue may not be applicable to non-RAID PCI64 SCSI HBAs, and that is my main question here. I intend to acquire Adaptec AHA-39160 (PCI64 dual-channel U160 non-RAID HBA) and to run it on a regular PCI 32/33 bus. Will the card's troughput match the actual troughput of my PCI bus, or will it drop dramatically (to how many MB/s)?

You have video or network cards or other cards that share that bandwidth on the bus yes? Also I agree much of what I read even back in the day with 8 x 3ware IDE drives people never peaked much above 100MiB/s. If you want speed, go PCI-e if at all possible.

Justin.

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Thank you for the reply. Other PCI cards are not of any concern, because they are rarely used at the same time as the disk subsystem, so it can be said that the SCSI card would have the PCI bus mostly free for itself.

So, the only concern is if the card will actually be able to take advantage of PCI 33/32 bus. My questions from the first post still remain active.

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Thank you for the reply. Other PCI cards are not of any concern, because they are rarely used at the same time as the disk subsystem, so it can be said that the SCSI card would have the PCI bus mostly free for itself.

So, the only concern is if the card will actually be able to take advantage of PCI 33/32 bus. My questions from the first post still remain active.

As far as the performance I would not expect stellar performance..

Remember, network I/O also goes over the same bus in older computers.

As far as the compatibility, you need to check the voltages, some 66 cards have weird voltages (5v) if I recall that may be incompatible with a 32bit, 33mhz PCI slot.

http://en.wikipedia.org/wiki/PCI-X

PCI-X is generally backward-compatible with most cards based on the PCI 2.x[1] or later standard, meaning that, a PCI-X card can be installed in a PCI slot, provided it has the correct voltage keying for the slot and (if inserting into a 32-bit slot) nothing obstructs the overhanging part of the edge connector. Originally the PCI bus was a 5-volt bus. Later, in PCI Revision 2.x, the PCI bus was a dual-voltage interconnect. In 3.0 this was changed to 3.3 volts only. The PCI-X bus is not compatible with the older 5-volt cards but newer 3.3-volt PCI cards will work in a PCI-X slot.[1] Apart from this, PCI and PCI-X cards can generally be intermixed on a PCI-X bus, but the speed will be limited to the speed of the slowest card. For example, a PCI 2.3 device running at 32 bits and 66 MHz on a PCI-X 133-MHz bus will limit the total throughput of the bus to 266 MB/s. To get around this limitation and the voltage compatibility issue, many motherboards have separate PCI-X channels that can be dedicated to different PCI hardware families if needed, allowing for better backward compatibility while maintaining higher total system bandwidth.

Edited by jpiszcz

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