[Gllug] A SATA Raid card worth having?

[Andy Farnsworth]

Daniel P. Berrange wrote:

>On Fri, Jan 27, 2006 at 01:40:41AM +0000, Andy Farnsworth wrote:
>  
>
>>Russell Howe wrote:
>>
>>    
>>
>>><snip>
>>>
>>>With SCSI, the bus speed is more important, since you can have perhaps
>>>15 devices on a single chain (perhaps more? I'm not too up to date with
>>>my SCSI knowledge). If you say your SCSI interface is 300Mbyte/s (U320,
>>>say?), that's 20Mbyte/s per device.
>>>
>>>
>>>      
>>>
>>This is why you use 15 drives on a SCSI chain :)  Each drive can easily 
>>supply the 20+ Mb/s throughput so you can keep your 320 Mb/s pipe fully 
>>saturated.  Actually, if you go with 15,000 RPM drives you should get 
>>almost double the throughput of 7200 drives, at least as long as you 
>>stay on a cylinder, once you cause the head to move, it is not 
>>appreciably faster on the 15,000 RPM drives, however latency is halved.  
>>This means that in most cases you can keep 320U SCSI saturated with 
>>about 4 drives on the channel.
>>    
>>
>
>Aren't you gonna hit the bandwidth limit on the connection from RAM
>over PCI bus to the SCSI controller at around 150 MB/s, making the
>theoretical 320 MB/s of 15 drivers on 1 SCSI chain unachievable ? 
>  
>
Lots of bandwidth available as most high end SCSI use PCI-X at least and 
are now starting in on PCIe.


Desc 	Bits 	Mhz 	Bandwidth (bits/sec) 	Bandwidth Bytes / Sec
PCI (normal) 	32 	33 	1,056,000,000 	132,000,000
PCI-X Partial 	64 	33 	2,112,000,000 	264,000,000
PCI-X Partial 	32 	66 	2,112,000,000 	264,000,000
PCI-X Full 	64 	66 	4,224,000,000 	528,000,000
PCIe 1x 	
	
	2,500,000,000 	312,500,000
PCIe 2x 	
	
	5000000000 	625,000,000
PCIe 4x 	
	
	10000000000 	1,250,000,000
PCIe 8x 	
	
	20000000000 	2,500,000,000
PCIe 16x 	
	
	40000000000 	5,000,000,000


Andy
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