[luau] INFO: Heavy duty storage needs

MonMotha monmotha at indy.rr.com
Thu May 2 15:46:30 PDT 2002


This is true.  In terms of raw I/O speed, IDE drives have caught up with 
SCSI.  The reason?  The bus is no longer the bottleneck.  Even the best 
of SCSI drives would have trouble saturating a 100MB/sec bus (though 
SCSI is 160).  If all you need is raw I/O speed (which is a good chunk 
of what your average single user desktop will be working with, 
especially if they're doing a lot of multimedia), IDE drives are a great 
choice (especialy combined with a software RAID solution, such as the 
Linux software RAID or some of the software "RAID" cards that are now 
out for the 'dozers).

SCSI shows it's strength in Tagged Command Queueing.  Unlike IDE 
devices, SCSI drives can work on more than one command at the same time, 
queueing them up in the best way possible.  For example, an IDE system 
where a person wants to simultaneously (quicker than a single seek on 
the drive) pull information from 10 places on the disk will have to do 
them in the order the system says.  With SCSI, the system can send all 
10 (or usually 9) requests in quick succession, and the drive will 
service them in the best way possible, minimizing redundant seeking, 
serving out of cache when possible (often even during a seek for the 
next command in the queue), etc.  In multiuser environments, this can 
give SCSI a HUGE advantage over IDE.  SCSI drives also tend to have 
lower seek times, often due to smaller platter sizes rotating at 
significantly faster speeds (15,000 RPM vs. 7200 or even 5400 RPM). 
This is also the reason SCSI drives tend to have smaller capacity as 
compared to IDE drives while still being expensive.  The same 
"technology" has to be used to pack all those bits into a small space, 
but the overall space is smaller (many SCSI drives could almost fit 
their platters into 2.5" laptop HDD enclosures, see fujisu specs on 
platter size).  These lower seek times also help performance in 
multiuser environments.

Also, though this is not as much of a deal with UDMA IDE, IDE uses the 
CPU for some of it's processing.  The same things are done on SCSI on 
the dedicated controller and onboard the drive itself.

Conclusion: Don't assume SCSI is better, but don't assume raw I/O 
bandwidth is the only measure of a hard drive.

--MonMotha

R. Scott Belford wrote:
> If you intend to record most or all of the traffic moving over your 
> network, you need to spend as much time thinking about your disk 
> subsystem as your processor and Ethernet card. Last year Sandstorm spent 
> several months comparing IDE drives with the UDMA100 interface to SCSI 
> LVD-160 drives. We also explored a variety of RAID systems. The 
> conclusion: today's IDE drives are significantly faster than SCSI drives 
> costing two or three times more per gigabyte stored.
> 
> This is not the result we were expecting, and it goes directly against 
> the conventional wisdom that says SCSI is inherently better than IDE. 
> Nevertheless, it does seem to be the ugly truth, at least for 
> straightforward read/write tests in a single-user environment. Although 
> we saw the highest performance with a hardware-based RAID 5 system 
> manufactured by _Advanced Computer & Network Corporation_, we saw nearly 
> the same performance with a RAID 5 system based on the _3Ware Escalade 
> 7000_ RAID controller.
> 
> from an article about network data capture at
> 
> http://www.oreillynet.com/lpt/a//network/2002/04/26/nettap.html





More information about the LUAU mailing list