[luau] INFO: Heavy duty storage needs

[R. Scott Belford]

I have searched for such a concise and practical explanation of why SCSI 
is accepted as the better storage solution for high-intensive seeks.  
Now I understand.  Thanks for explaining this so clearly.

On Thursday, May 2, 2002, at 12:46 PM, MonMotha wrote:

> 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
>
>
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