On Mon, Nov 25, 2002 at 12:29:11AM +0100, Jan-Benedict Glaw wrote: > On Mon, 2002-11-25 10:17:44 +1100, Donovan Baarda <[EMAIL PROTECTED]> > wrote in message <[EMAIL PROTECTED]>: > > On Sun, Nov 24, 2002 at 08:45:04PM +0100, Emilio Brambilla wrote: > > > hello, > > > On Sun, 24 Nov 2002, Russell Coker wrote: [...] > Command queuing is quite new to ide, and only IBM drives support it up > to now, but others are to follow...
Ahh, perhaps only the spec supported it, and no actual hardware :-) > > In any case, command queuing makes a big difference when you have lots of > > slow drives sharing a mega-bandwidth buss. IDE has only two drives, so it's > > That's not really right. Command Queuing allows to to tell the drive you > want to have, say, 10 sectors scattered across the whole drive. If you > give 10 synchronous commands, you'll see 10 seeks. Issuing them as > queued commands will fetch them _all_ within _one_ seek, if there's good > firmware on the drive. Only the drive itself does know the optimal order > of fetching them, the OS only knows some semantics... I'm pretty sure most device drivers for both IDE and SCSI do some degree of command-reordering before issuing the commands down the buss. I wonder how much real-world benefit can be gained from drive-level command re-ordering, and how many SCSI drives actualy bother to implement it well :-) > > not as relevant. I believe most benchmarking shows only marginal peformance > > hit for two IDE's on the same bus (this might be because IDE does have a > > form of command queuing, or it could just be because it doesn't make much > > difference). I know SCSI shows nearly no hit for two drives on one bus, but > > Or it is because the benchmark doesn't ask _both_ drive to send their > very maximum of data... I'm pretty sure any benchmarks done on this would have been hammering both drives at once... that would be the point, wouldn't it? > > when you compare 8 SCSI's on one bus with 8 IDE's on 4 buses, I bet they > > turn out about the same. > > > If you have 6 or less devices, IDE is just as good as SCSI, and bucketloads > > cheaper. > > Only true if you don't want to see your devices to send at their maximum > speed _all the time_. The point is, 4 IDE buses will probably match 1 SCSI bus for sustained transfer rates....4x133 =533MB/sec... more than 1x the fastest SCSI. Throw in the IDE crappy performance, and you get.... about the same. -- ---------------------------------------------------------------------- ABO: finger [EMAIL PROTECTED] for more info, including pgp key ---------------------------------------------------------------------- -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
