Ultra Wide SCSI and Ultra IDE

Ultra Wide SCSI and Ultra IDE

Post by Tae-Wan Ki » Fri, 15 May 1998 04:00:00



Hello,

I am considering to buy a hard drive.

I'd like to know the difference between
Untra Wide SCSI and Ultra IDE.

I know only that SCSI is expensive than IDE.

My motherboard is ASUS P2B and CPU is
Intel Pentium II 400 MHz.

If SCSI is faster than IDE, how much is
SCSI faster in general?

What other advantage of SCSI over IDE?

Someone said that SCSI is very useful only if
the whole system is designed for SCSI.
Is this true?  
I only buy Ultra Wide SCSI
hard dive and NCR controller.

Thank you.

Best regards,

Tae-wan Kim

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by meut » Fri, 15 May 1998 04:00:00


SCSI is faster, but I choose Ultra IDE, because I didn't think the
trouble associated with SCSI, and the cost were worth it. IF you have a
good Ultra DMA33 Controller, and a Good Hard drive to match, the SCSI
isn't worth it (unless of course you are going to have a RAID :)).
Realize though that this will require recompiling your Linux kernel to
support UATA.
Glenn


> Hello,

> I am considering to buy a hard drive.

> I'd like to know the difference between
> Untra Wide SCSI and Ultra IDE.

> I know only that SCSI is expensive than IDE.

. . .

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by William Burr » Sun, 17 May 1998 04:00:00


On Thu, 14 May 1998 11:56:50 -0400,


>I am considering to buy a hard drive.

>My motherboard is ASUS P2B and CPU is
>Intel Pentium II 400 MHz.

>If SCSI is faster than IDE, how much is
>SCSI faster in general?

SCSI tends to be faster than IDE for a large number of file requests.
A single user system doing very little from the disk is unlikely to
benefit greatly from SCSI.

Quote:>What other advantage of SCSI over IDE?

The advantage is that SCSI can take a large demand on the disk and
still perform well.  Unlike IDE, SCSI is fully bus-mastering so that
the CPU does little or no work during file transfers from the disk.
This can be quite important on a file server.  Even my supposedly
bus-mastering IDE system, the computer will lock up for short periods
of time when heavily accessing the disk (esp. when swapping).

Quote:>Someone said that SCSI is very useful only if
>the whole system is designed for SCSI.
>Is this true?  
>I only buy Ultra Wide SCSI
>hard dive and NCR controller.

That will be sufficient.

--
--
William Burrow, VE9WIL (Adv, 5wpm)  --  New Brunswick, Canada
Copyright 1997 William Burrow  

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by Tom Payn » Fri, 22 May 1998 04:00:00



: The advantage is that SCSI can take a large demand on the disk and
: still perform well.  Unlike IDE, SCSI is fully bus-mastering so that
: the CPU does little or no work during file transfers from the disk.

That sounds like a disadvantage for SCSI.  Isn't it be better if the
CPU can get work done while a transfer is in progress?

Tom Payne

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by Aaron M. Ren » Fri, 22 May 1998 04:00:00



>: The advantage is that SCSI can take a large demand on the disk and
>: still perform well.  Unlike IDE, SCSI is fully bus-mastering so that
>: the CPU does little or no work during file transfers from the disk.

>That sounds like a disadvantage for SCSI.  Isn't it be better if the
>CPU can get work done while a transfer is in progress?

You misunderstood.  During an UW-SCSI data transfer, the CPU does very
little work related to the transfer itself, thus is free to do other things.

--
*****************************************************
* Aaron M. Renn                                     *

* Homepage: <URL:http://www.urbanophile.com/arenn/> *
*****************************************************

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by Joe Tsa » Fri, 22 May 1998 04:00:00




> : The advantage is that SCSI can take a large demand on the disk and
> : still perform well.  Unlike IDE, SCSI is fully bus-mastering so that
> : the CPU does little or no work during file transfers from the disk.

> That sounds like a disadvantage for SCSI.  Isn't it be better if the
> CPU can get work done while a transfer is in progress?

> Tom Payne

the cpu can do _other_ work during disk access; it's not involved in the
file transfer.

joe

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by Nathan R Heag » Fri, 22 May 1998 04:00:00


The IDE your motherboard has (which is the same as mine, btw) has a speed of
33MB/Sec
SCSI Wide Ultra has a speed of 40MB/Sec.

SCSI also has the advantages of chaining many (7 external, 7 internal)
devices together. Also, once SCSI is set up, you (likely) won't need extra
drivers for the drives you use. My external SCSI Zip drive is seen by both
Linux and Windows without any extra drivers.
--
Nathan R Heagy
-- -- --
Building Quality Web Sites
http://heagy.com


>I am considering to buy a hard drive.

>I'd like to know the difference between
>Untra Wide SCSI and Ultra IDE.

>I know only that SCSI is expensive than IDE.

>My motherboard is ASUS P2B and CPU is
>Intel Pentium II 400 MHz.

>If SCSI is faster than IDE, how much is
>SCSI faster in general?

>What other advantage of SCSI over IDE?

>Someone said that SCSI is very useful only if
>the whole system is designed for SCSI.
>Is this true?
>I only buy Ultra Wide SCSI
>hard dive and NCR controller.

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by Jacek Pliszk » Fri, 22 May 1998 04:00:00



> The IDE your motherboard has (which is the same as mine, btw) has a speed of
> 33MB/Sec
> SCSI Wide Ultra has a speed of 40MB/Sec.

It is maximum possible speed. In reality it is much slower.
UDMA drives might be as fast as SCSI in transfer of big files
when there is no other activity on the systems.
Although when you have many small files and bigger load on your
system their performance deteriorates much faster than SCSI.

My personal experience is that if you are single user
and use a single program at the time - you may use IDE,
but if you have multple users/programs at the same time
- you definitely should use SCSI. Also for 'bandwidth critical'
hardware as CD-R SCSI is advised.

I haven't had a lot of experience with that so somebody
please correct me if I am wrong.

Regards,

Jacek Pliszka


 
 
 

Ultra Wide SCSI and Ultra IDE

Post by Tom Payn » Sun, 24 May 1998 04:00:00



: >

: >
: > : The advantage is that SCSI can take a large demand on the disk and
: > : still perform well.  Unlike IDE, SCSI is fully bus-mastering so that
: > : the CPU does little or no work during file transfers from the disk.
: >
: > That sounds like a disadvantage for SCSI.  Isn't it be better if the
: > CPU can get work done while a transfer is in progress?
: >
: > Tom Payne

: the cpu can do _other_ work during disk access; it's not involved in the
: file transfer.

What transfer-related demands do SCSI and the UltraDMA place on the
CPU?  An interrupt per sector?  An interrupt per multi-sector request?
How much of burden are those likely to be at say a 10ms latency per
request?

Tom Payne

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by Tom Payn » Tue, 26 May 1998 04:00:00





: : >

: : >
: : > : The advantage is that SCSI can take a large demand on the disk and
: : > : still perform well.  Unlike IDE, SCSI is fully bus-mastering so that
: : > : the CPU does little or no work during file transfers from the disk.
: : >
: : > That sounds like a disadvantage for SCSI.  Isn't it be better if the
: : > CPU can get work done while a transfer is in progress?
: : >
: : > Tom Payne

: : the cpu can do _other_ work during disk access; it's not involved in the
: : file transfer.

: What transfer-related demands do SCSI and the UltraDMA place on the
: CPU?  An interrupt per sector?  An interrupt per multi-sector request?
: How much of burden are those likely to be at say a 10ms latency per
: request?

I'd like to elaborate a bit on these questions.  

There are three places where it seems that SCSI might have an advantage.

CPU blockage:  It might be that during a transfer UDMA blocks the
  memory bus so that the CPU cannot fetch instructions and access data.

Per-byte overhead:  It might be that UDMA requires more per-byte
  computation for such things as computing checksums.

Per-sector overhead:  It might be that UDMA controllers interrupt
  and need to be rearmed after each sector.  There are two possibilities.
  * the controller can rearm during the intersector gap, in which case
    the overhead is small, since the read head spends a very small
    percent of its time between sectors.  
  * the controller cannot rearm during the intersector gap, in which
    case there is a serious problem, since sectors cannot be read
    sequentially at full disk transfer speed.  The best that the
    controller can do is read every other sector.

I'm very curious as to exactly where SCSI gets its performance
advantages.

Tom Payne

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by graywo » Thu, 28 May 1998 04:00:00


:
: I'm very curious as to exactly where SCSI gets its performance
: advantages.
:

Well Tom,

In my experience, the biggest advantage to SCSI is on multi disk
installations.  SCSI can do overlapping read/writes, so several
drives can be operating independently.  I seldom see this mentioned in
the newsgroups.  But it is why I have several small (500m) drives in
my old 486 server.  Using three older 500m drives, i get about three
times the disk through put as I did with one more modern 1.2 gig IDE.

I have /, /var, and /local drives and quite often Linux uses all
three simutaneously.  It can not do that with an IDE drive.  I
don't know if Linux can make two accesses on a pentium type system with
two separate controllers on the board, but my 486 can make several
overlapping accesses on the 4 harddrives and 1 cdrom hooked up to the
old, old buslogic EISA card.
--
Graywolf
(Tom Rittenhouse)
---------------------------------------------------------------

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by felicity+s.. » Thu, 28 May 1998 04:00:00



Quote:> SCSI is faster, but I choose Ultra IDE, because I didn't think the
> trouble associated with SCSI, and the cost were worth it. IF you have a

... trouble associated with SCSI?  Recompile the kernel, plug in the
controller and drive, reboot.  It's as easy as IDE, plus the step for the
controller.

--
To reply via mail, please remove the obvious from the email address.

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by Jeffrey S. Dutk » Sun, 31 May 1998 04:00:00



> There are three places where it seems that SCSI might
> have an advantage.

> CPU blockage:  It might be that during a transfer UDMA
> blocks the memory bus so that the CPU cannot fetch
> instructions and access data.

> Per-byte overhead:  It might be that UDMA requires more
> per-byte computation for such things as computing checksums.

> Per-sector overhead:  It might be that UDMA controllers
> interrupt and need to be rearmed after each sector.
> There are two possibilities.
>   * the controller can rearm during the intersector gap,
>     in which case the overhead is small, since the read
>     head spends a very small percent of its time between
>     sectors.
>   * the controller cannot rearm during the intersector gap,
>     in which case there is a serious problem, since sectors
>     cannot be read sequentially at full disk transfer speed.
>     The best that the controller can do is read every other
>     sector.

> I'm very curious as to exactly where SCSI gets its performance
> advantages.

The first two (blocking the memory bus and per-byte overhead)
are fairly unlikely. Some blockage of the memory bus will
occure, but it will be momentary. The CPU should be able to
make due with what it has in cache. Also, the SCSI system
would need to transfer the same data blocks over the memory
bus as would the UDMA system, so this won't be a loss or a win
for either one. As for the per-byte overhead, I would expect
that the UDMA system has all of that taken care of by the
drive controller electronics, the SCSI system certainly would.

As for the per-sector overhead, this is much more likely to be
a loss for UDMA. SCSI systems don't know anything about sectors
(or other disk geometry), they just request blocks from a device
and let the controller on the device take care of the details.
If UDMA has to worry about what the drive is doing between sectors
then it has a lot more work for the CPU than SCSI.

The main advantage for SCSI is, I think, the ability to have
multiple outstanding requests to multiple devices simultaneously.
I would bet that the advantage of SCSI over UDMA EIDE is fairly
small for a system with only a fixed disk and CD-ROM drive.

The ability to have multiple outstanding requests allows the
system to hide much of the disk access latency by issueing
further requests while the first request is still in process.
If you have several fixed disks, in you system then you can
stripe them and get much higher bandwidth than any single disk
can deliver.

In a *nix style system, where there is a lot of disk activity
due to virtual memory and logging daemons (not to mention the
disk activity inherent in your application) you can see a big
benefit from SCSI. Other systems are less likely to see much
benefit in SCSI over UDMA EIDE. Certain applications are also
likely to see a lot of benefit: CD mastering, digital video,
anything that need a LOT of disk bandwidth.

- Jeff Dutky

 
 
 

Ultra Wide SCSI and Ultra IDE

Post by Tom Payn » Tue, 02 Jun 1998 04:00:00



: > There are three places where it seems that SCSI might
: > have an advantage.
: >
: > CPU blockage:  It might be that during a transfer UDMA
: > blocks the memory bus so that the CPU cannot fetch
: > instructions and access data.
: >
: > Per-byte overhead:  It might be that UDMA requires more
: > per-byte computation for such things as computing checksums.
: >
: > Per-sector overhead:  It might be that UDMA controllers
: > interrupt and need to be rearmed after each sector.
: > There are two possibilities.
: >   * the controller can rearm during the intersector gap,
: >     in which case the overhead is small, since the read
: >     head spends a very small percent of its time between
: >     sectors.
: >   * the controller cannot rearm during the intersector gap,
: >     in which case there is a serious problem, since sectors
: >     cannot be read sequentially at full disk transfer speed.
: >     The best that the controller can do is read every other
: >     sector.
: >
: > I'm very curious as to exactly where SCSI gets its performance
: > advantages.

[...]
: The main advantage for SCSI is, I think, the ability to have
: multiple outstanding requests to multiple devices simultaneously.
: I would bet that the advantage of SCSI over UDMA EIDE is fairly
: small for a system with only a fixed disk and CD-ROM drive.

: The ability to have multiple outstanding requests allows the
: system to hide much of the disk access latency by issueing
: further requests while the first request is still in process.
: If you have several fixed disks, in you system then you can
: stripe them and get much higher bandwidth than any single disk
: can deliver.

So, up to the overall bandwidth limit the SCSI bus, multiple disks can
interleave their data transfers on a byte-by-byte basis, and that is
not possible on a UDMA controller.  Of course, with very fast disks,
whose speed exceeds half the bus bandwidth, this would be no advantage
since any concurrent transfer would exceed the bus's bandwidth.

Will UDMA allow one disk to seek while another is transferring?

Tom Payne