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Thread: Specify me a PC

  1. #21
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    SCSI Ultra 640 was first introduced in 2003 and has a maximum sustained throughput of 640MB/s (hence the name) which is over twice that of SATA2's 300MB/s maximum burst rate. It uses either standard 68 pin high density connectors, or the slightly newer 80 pin connectors, and supports up to 16 devices on the chain, with each needing its own ID (or LUN - Logical Unit Number), and the last device in the chain needs to be terminated so the signal is reflected back down the cable correctly.

    So basically it works in the same way as all SCSI versions and the cables are the same as have been used since Ultra Wide SCSI and then Ulta2 Wide SCSI were first introduced back in the mid 90's.

    As for your question about the speed of the interface depending on the number of devices. SCSI is shared bandwidth, much like IDE, so the more devices on the chain, the less bandwidth each device will have. In contrast SATA is device independent so all devices get the full bandwidth (in the case of SATA2 all devices would have full access to 300MB/s each). Therefore you can instantly see the initial advantage of SATA over SCSI once you start to have more physical devices attached to a system.

    But you do also need to consider other things and this shows reasons why SCSI is often still chosen over SATA. SATA drives still mostly tend to spin at 7200RPM, whereas SCSI drivers are mostly 10K or 15K, so their seek times are much faster. The other is the physical actual data rate of the drive. Most SATA drives still only have a data rate around 50-60MB/s maximum, so even though they are on a 300MB/s SATA connection they wont be using it all. In contrast SCSI drivers are commonly around the 89MB/s range, with some recent ones supporting 150MB/s! Therefore although SCSI shares bandwidth, if you had 4 drives connected to one interface you would still be able to maintain the full 89MB/s x4 bandwidth (356MB/s total) and even the full bandwidth of 4 150MB/s drives, and when you compare this to 4 devices connected via SATA, each on their own independent 300MB/s bandwith, the SCSI will still be faster as each SATA device will probably only be managing around 60MB/s max, for a total of 240MB/s across all 4 drives combined. So you can see the advantages of SCSI over SATA still.

    Obviously if you were to start having more tha 4 devices on a SCSI chain it would start to eat into the total shared bandwidth, especially if you were using HDs with 150MB/s data rate. And then SATA might start to be a better solution. Faster SATA drives are starting to appear with faster spin rates and faster data rates closer to those of SCSI drives so SCSI is starting to look less appealing.

    Also with SATA having independent bandwidth per channel, you can utilise something called a port splitter. This allows you to connect 4 physical SATA devices to a single SATA channel on the controller and utilise the 300MB/s bandwidth per channel, shared between 4 devices which is more than enough bandwidth to access 4 average SATA HDs. You could therefore add 4 port splitters to a 4 channel SATA card and have 16 physical SATA HDs connected, all accessible at full bandwidth. You couldn't do this with SCSI.

    One thing worth noting is the Ultra 640 is probably going to be the last of the original Parallel SCSI type. Serial SCSI and iSCSI are now taking over, as well as most manufacturers now including SATA instead of SCSI on their motherboards. Serial SCSI has the big advantage of supporting hot swapping and faster data rates. Whereas iSCSI is SCSI over TCP/IP which is idea for use with Fibre Optic cabling.
    Last edited by Harrison; 15th April 2008 at 17:16.

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  2. #22
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    *brushes up on 'write it out in your own words' skills*

    That was interesting to know. Got any recomendations of SCSI hard disks and controllers?

    Would this 640MB/s maximum be affected if it was on a PCI card? I'm not sure how fast PCI and it's newer whatsits are.

    Also, I got a little confused when reading about this on other sites. Does this increased speed affect the maximum cable length?

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    Cable lengths are about the same. There is a bottleneck with a standard PCI bus because it can only support 133MB/s shared across the whole PCI bus, so a SCSI card connected on a standard PCI bus would need to share bandwidth with any other PCI cards such as network and sound.

    In more professional systems there are alternative faster system bus types that SCSI is connected to which doesn't have such limitations. Even more so if the SCSI controller is integrated into the motherboard.

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    What could be used instead of PCI?

    How fast is PCI Express then? Would that be suitable for using an Ultra 320 controller on?
    Last edited by Stephen Coates; 15th April 2008 at 21:27. Reason: Double posting - posts have been automerged

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    Servers until recently have used PCI-X slots. This type of slot was developed purely for servers in the 90's and uses a 64bit bus running at either 66MHz or 133MHz which gives a maximum bandwidth of 527MB/s and 1064 MB/s respectively. So much faster than standard PCI slots in a home computer. Systems with these are a lot more expensive and is part of the reason that pro workstations and server do cost much more than a standard PC.

    PCI-X slots use a parallel interface that is directly compatible with standard 5V PCI devices so you can still use normal PCI cards in them as well as faster PCI-X specific cards. And likewise most PCI-X cards will work in a slower 32bit PCI slot.

    The 133MHz PCI-X slot is the most common found in servers giving 1064MB/s.

    But there are two faster standards developed in 2003 under the updated PCI-X 2.0 standard. This adds 266MHz and 533MHz variants to the standards, giving roughly 2.15GB/s and 4.3GB/s respectively. Now this is fast! Although these types of slot and supporting cards are very rare and extremely expensive.

    And PCI-X slots are slowly being replaced with PCI-Express (PCI-e) slots and cards. PCI-e slots are very different. PCI-X slots are based on normal PCI slots and the PCI-X cards are backwardly compatible with standard PCI slots and PCI-X slots use a parallel interface.

    In contrast PCI-e is a serial interface and is not compatible with any other interface type so you couldn't use a PCI card in a PCI-e slot. And PCI-e is much faster. The slowest x1 offers 250 MB/s in both directions, and currently the fastest slot is now up to x32 offering 8 GB/s bandwidth which is twice that of the rarest fastest PCI-X slots available.

    PCI-e also has many other advantages over PCI and PCI-X. Due to the older PCI standards being parallel interfaces the trace routing has to be much more complex and able to deliver all signals at the same time across the interface. It also suffers from a lot of noise from adjacent slots which can cause interference. In contrast PCI-e's serial interface is much simpler with much less traces needed and it is also full duplex bidirectional rather then the half duplex bidirectional of PCI-X and PCI.

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    Ahhhh. PCI-X. When I said PCI-X I actually meant PCI Extreme.

    I got the X and E mixed up. Why did they have to make it so comfusion?

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    That has always been the problem in the computer world. Different companies working on slightly different versions with the same end goals in mind and all with similar names. At least now we seem to finally be converging into a single standard for each thing. PCI-E slots for all expansion cards, SATA for drives and USB2 for external devices. So much easier, and I still think it is quite amusing that we have gone full circle from serial interfaces to parallel and now back to serial.

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    I'm still getting the Extreme and the Express mixed up.

    So, just to summarise,

    PCI-X is PCI-Extreme and is the one which is compatible with PCI
    PCI-E is PCI-Express and is the other one.

    Assuming that I havn't got them the wrong way round again, I think that the X should stand for Express and E should stand for Extreme.

  9. #29
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    Yes you have them round the right way now.

    And I agree, still confusing for most people.

    Technically PCI-X is the extreme version of PCI because it has much more bandwidth due to the cards being much longer with a lot more connections in the slot/card design to add the extra needed parallel traces.

    And I personally think they should have named PCI-E something completely different without using the PCI name at all because unlike the previous slot types it isn't compatible with them and therefore isn't in the same family. It's a completely new serial slot design and using PCI in the name just adds more confusion. Although as PCI stands for Peripheral Component Interconnect you can see the logic for keeping it too.

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    That makes sense. If PCI doesn't stand for something specific then it would make sense to keep using it if PCI-E is also a 'Peripheral Component Interconnect'.

    Kindof like PC. When people refer to IBM compatible PCs as just a PC. Well, it is a personal computer, but so is a Macintosh.

    I decided to go for dual quad core Xeon 5300s at 3GHz. No idea how good that would be, but I assume it would be more than fast enough. I also opted for a nVidia Quadro FX 1700. Again I don't know how good it is but it would probably be suitable. Will certainly be expensive though. I think the graphics card is priced at around 200 and dual xeon motherboards also seem to be around that as far as I can tell.

    For hard drives I went for Seagate Cheetah 15k.5 ones. They use Ultra320, have a maximum speed of about 120MB/s spin at 15000RPM. I went for the 146GB option, but there were sized like 300GB and 74GB.

    Why is it that SCSI HDs always tend to come in odd capacities, like 74GB and 36GB, when IDE ones just tend to be 40GB, 80GB etc?

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