Why is SCSI so much faster with the write cache off (than ATA)?

[Scott Long]

Martin Cracauer wrote:
> I have observed it several times and I am missing one bit of knowledge
> here: 
> 
> why is SCSI so much faster when you turn off the write cache than
> P-ATA and SATA?
> 
> P-ATA and SATA crumble to about 1/10th of the speed (just writing one
> file with 8k blocks linear), whereas SCSI just loses 10-20%, for me.
> I have observed that 10 years ago with some 8 GB ATA IBM disk on the
> BX chipset versus some 4 GB Quantum Atlas, and now I see it again with
> Seagate 7200.7s and .8s versus a 10K Compaq labled 36 GB drive.
> 
> Personally I don't see why a linear write should be slow at all.
> Surely the computer delivers the data fast enough for sectors to be
> filled as they pass under the head.  Maybe the ATA disks lose one
> rotation per sector or per filesystem block written anyway? Then why
> don't SCSI disks lose the same way given they are not allowed to cache
> anything either?
> 
> Martin

The answer: Tagged Queueing

The drive can collect a bunch of I/O requests, sort them for optimal
layout, then stream their data in all at once.  This is not the same
thing as buffering through the write cache, but it has a similar 
performance benefit.

Scott