scsi bus volume questions

[bongo x]

Hey,

Ok, I'm trying to figure some of this out. I have searched the boards but I can't seem to find exactly what I'm looking for.

1. How does the volume of information affect the scsi bus, as in: if I am recording 24 tracks to a drive on a single channel UW scsi card what happens if I'm recording (or playing back) 24 tracks to/from another drive. Does the scsi bus on the card get maxed out and at approx. what point? (assuming each drive would do the 24 tracks by itself).

2. If it was 32 tracks on drive1 and 16 on drive2 would they be better both on the scsi card or drive1 on the card and drive2 on the internal scsi bus? Is the disadvantage of being on the narrow 10m internal bus made up for by being on a seperate bus?

3. What about having the boot/app drive on the scsi card? Does this affect the performance of the audio drives? I'm talking about a wide drive and the accessing of the drive not just physical being there.

4. What kind of track count can you get from a single channel UW SCSI card assuming you have enough drives?

5. Why do people talk about putting cd-roms on one side of a dual channel scsi card? Wouldn't they be better on the int or ext bus? Is this only people with ata boot drives? why not get a cheaper ide cdrom or burner? I was under the impression that no cd's came close to the speed of an built in scsi bus.

Basically I'm trying to figure out how to arrange these drives and whether the bottleneck comes from the speed of the bus or number of busses or both and to what extent. If I was thinking I'm sure the questions could be asked in a logical way that makes more sense. If you figure out what I'm talking about, please let me know.

Thanks for any help you might have.



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Billy
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Let me get this straight, anything less than 96/24 ***** , but MP3 sounds great!



[This message has been edited by bongo x (edited February 13, 2000).]

[bongo x]

The thing that ***** about this forum is that no one answers anything. I guess that's why people stick stuff in the Mac troubleshooting forum!



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Billy
__________________________________________________ ______
Let me get this straight, anything less than 96/24 sucks, but MP3 sounds great!

[Steve Rosenthal]

Billy,

1. How does the volume of information affect the scsi bus, as in: if I am recording 24 tracks to a drive on a single channel UW scsi card what happens if I'm recording (or playing back) 24 tracks to/from another drive. Does the scsi bus on the card get maxed out and at approx. what point? (assuming each drive would do the 24 tracks by itself).

Assuming your talking about 24 bit audio (48k sample rate for this example), a total of 48 tracks would amount to a sustained data throughput rate of around 6.8MB/sec. That's not going to max out your SCSI bus. What's more likely to happen is your drives will run into their access time limits (seek, rotation, command overhead) depending on the number of edits/crossfades before you saturate the SCSI bus.

2. If it was 32 tracks on drive1 and 16 on drive2 would they be better both on the scsi card or drive1 on the card and drive2 on the internal scsi bus?

Both on the SCSI card. Though throughput is not as important for Pro Tools as access time, if we're assuming wide drives, it's better to keep them operating as such by placing them on the wide SCSI card. This doubles the burst rates right away, just by virtue of the fact that the drives are operating at their full 16 bit width. Plus, it eliminates the need for adaptive cabling to step the drive down the narrow bus.


Is the disadvantage of being on the narrow 10m internal bus made up for by being on a seperate bus?

It's kind of a wash.

3. What about having the boot/app drive on the scsi card? Does this affect the performance of the audio drives? I'm talking about a wide drive and the accessing of the drive not just physical being there.

Well, as far as Pro Tools is concerned, the boot drive doesn't do any disk access during a record or playback pass, so it wouldn't have any effect on the audio drives. If you were doing a save-on-the-fly, that would add some additional load to the bus for the duration of the save. Depending on what kind of load the audio drives were under, this might cause the record or playback operation to fail.

4. What kind of track count can you get from a single channel UW SCSI card assuming you have enough drives?

64 voices. However, there's a point of diminishing returns when you add more than 3 drives to a single SCSI bus channel, you begin to see the cumulative effect of disconnect/reconnect overhead as the drives all negotiate for their turn on the bus.[/i]

5. Why do people talk about putting cd-roms on one side of a dual channel scsi card?

I'm not sure what the average person's rationale it, but the best reason I can think of it that it limits cabling hassles by keeping your CD burner (which is almost inevitably a narrow device) and any other narrow devices seperate from your wide devices.

Wouldn't they be better on the int or ext bus?

Yes, assuming they are narrow devices and assuming you have a Mac with built-in SCSI.

Is this only people with ata boot drives? why not get a cheaper ide cdrom or burner?

If you're using MLCD, it'll only recognize SCSI CD burners. I can't speak for Toast or Jam or any other CD burning application.

I was under the impression that no cd's came close to the speed of an built in scsi bus.

That's fairly accurate.

Basically I'm trying to figure out how to arrange these drives and whether the bottleneck comes from the speed of the bus or number of busses or both and to what extent.

Typically, the bottleneck is the drives, not the bus.



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--Steve Rosenthal, Digidesign ETS

[Steve Rosenthal]

The thing that ***** about this forum is that no one answers anything. I guess that's why people stick stuff in the Mac troubleshooting forum!

I volunteer my time on the DUC so I can't answer everything all the time. If it's a quickie, I'll fire off a response as soon as I can. If it requires more detail (like your questions), I have to wait until I've got some extra time.




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--Steve Rosenthal, Digidesign ETS

[bongo x]

Hey Steve,

Thanks a lot for your help. I really appreciate it. I've really been having a hard time getting specifics on this whole "scsi acceleration as it pertains to PT" thing.

I wasn't trying to be a jackass before.(now I just try and see what the digi board filters out!)

"It's a test, designed to provoke an emotional response"

You're a wealth of information and I thank you again.



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Billy
__________________________________________________ ______
Let me get this straight, anything less than 96/24 sucks, but MP3 sounds great!

[Steve Rosenthal]

Billy,

Glad to help. Don't hesitate to ask more questions!

--Steve Rosenthal, Digidesign ETS

[JV]

thanks steve for the time and the information