Re: Dumb question: Which is "better" SCSI or IDE disks?

Harvey Fishman (
Thu, 3 Dec 1998 23:38:40 -0500 (EST)

On Thu, 3 Dec 1998, Russell Leighton wrote:

> What are good rules of "thumb" for
> choosing IDE vs SCSI when building
> a Linux system?
> Both are fast these days...I always
> thought that IDE==cheap, but loaded
> the CPU, so you didn't want that in
> a server or high performance machine
> that might be busy with other things
> besides waiting for the current disk
> that fair? just basically
> wrong?

Devices using an ATA (IDE) interface are not capable of overlapping tasks
and must fully complete each one before even thinking about the next. This
is fine for DOS and Win 3x or Win 9x as their I/O design is limited to this
type of operation. That is why the ATA interface was designed this way.

But Linux, other Unix-like O/Ss, OS/2, NT, &c. are all capable of
asynchronous block I/O, where the kernel can send many tasks to the BIOS
and device drivers at once and wait for the first to be completed while it
moves on to other tasks that do not require I/O. With the synchronous I/O
of an ATA interfaced device, the device may complete its task as quickly as
with a more capable device (such as SCSI), but the O/S has been slowed
while waiting. In general, you will get less performance from a capable
O/S using ATA interfaced devices than with SCSI. Of course, the ATA
devices can cost a lot less, so it is a situation where you must determine
how much performance you are willing to pay for.

If you had two drives with identical HDAs (head-disk-assemblies) but one
with an ATA interface and the other with SCSI, with DOS or Win 3x/9x the
ATA device would probably give slightly better preformance because of a LOT
less overhead in the interface. But with a more sophisticated O/S that is
actually doing disk-intensive multi-tasking, the SCSI system will probably
blow the ATA system away.


Harvey Fishman | | A little heresy is good for the soul.
718-258-7276 |

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