On Sun, 21 May 2000, Ted Hilts wrote:
>
> Michael:
>
> SCSI is traditionally faster than IDE. But to run SCSI you will pay for
> a SCSI card and the hard drives and CD-ROMS are more expensive. You
> don't have the same limitations on SCSI as you do with IDE. On IDE you
> have 2 channels with a maximum of 2 Hard drives on each. Unlike SCSI
> operation if you require access of two hard drives on the same IDE
> channel then the operations are sequential whereas on SCSI they can
> occur in parallel. It's not quite that simple, it has to do with
> certain access features on the SCSI hard drives. The bottom line is
> that SCSI is faster. With SCSI you can utilize RAID drives (which again
> to keep it simple, is a way of combining multiple drives so they look
> like a single drive, and with the provision of a degree of protection in
> some raid set ups. You can therefore assempble a much bigger and faster
> drive using SCSI than you can can using IDE. So, if you had just 40Gb
> hard drives you could get on the standard IDE PC setup only 4x40Gb but
> on SCSI for a single channel you can get 7x40Gb and some cards come with
> more than one channel. I've been away from this for a while. But if I
> remember right, for a certain SCSI interface on product lines that will
> remain nameless in this discussion, I think there were a total of 14
> Hard Drives. I'm sure that someone will read this and tune us both into
> what the current capacities and limitation are. But this should get
> your started.
>
> I'm having a database server built for my network using the promise card
> and a RAID 10 setup which I'm not exactly sure what that means but I
> will have 80 gig consisting of 2 40gig drives configured as one RAID
> drive with another RAID image/shadow drive of the same arrangement with
> a total of 160gig. Then there will be 2 IDE drives UDMA66 protocol, one
> for the Linux operating sytem, and one for workspace for the sql
> operations and that sort of thing. The RAID technology employed here
> will ensure that if 1 drive dies (usually only one goes at a time) the
> system will remain up and running. So there's an example for you. It
> would probably be wise to have the two 10Mb IDE hard drives on separate
> IDE channels, with the CDROM as the slave on the first channel. I'd
> better check that out because I sure don't want sequenctial operation
> between the 2 IDE hard drives. In the past I've always put my IDE Hard
> drives on the first channel and the third on the second channel and
> after that the ATAPI IDE CD-ROM. Maybe that wasn't too bright. So I'm
> saying that if one has 2 IDE hard drives that maybe it would be better
> that they were on separate IDE channels. Maybe someone could tune us in
> on this one too.
>
>
> Have a nice day. You'll probably get a lot more input now, I'll be
> watching.
>
> Bye-thanks_TED
>
Let me clear up a couple of points. IDE-SCSI is accessing an IDE drive
using SCSI protocal. To the software, it looks like you are acceessing a
SCSI device. This is handy for devices that have software for the SCSI
versions, but not the IDE versions. The biggest use right now is for IDE
CD-ROM writters. You don't gain the SCSI speed and flexability, but you
can use the standard SCSI io-controls, and the driver translates them into
the ATAPI commands that the IDE device understands.
On the number of IDE drives you can have on a system, you can have up to 4
IDE interfaces, for a total of 8 IDE devices. (Your BIOS will probably
only know about the first 4 of them.) You may have problems finding PCI
interface cards that support the 3rd and 4th controller addresses, and you
usualy need one IRQ per interface. Sound cards with a built in IDE
interface will often support the 3rd or 4th IDE interface address. So do
some of the ISA controllers that used to come with CD-ROM drives. It is usualy
not a good to try and attach a hard drive to these cards.
As for the number of devices on a SCSI card, you can have 7 on a
"standard" SCSI I or II card, and with the Wide SCSI cards, you can have
up to 15 devices on one card, all sharing 1 IRQ. There may be cards that
will let you connect more devices, but I am not up on the cards that
support both SCSI 2 and SCSI 3 drives. Some of the hardware RAID setups
support many more drives, but the system sees them as one drive. You can
even get RAID arrays of IDE drives that connect up as one device on the
SCSI bus. (Way out of my price range...)
Hopefully this will clear things up, and not turn out to be ramblings from
lack of sleep!
Mikkel
--
Do not meddle in the affairs of dragons,
for you are crunchy and taste good with ketchup.
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