Re: new bottleneck section in wiki

[Roger Heflin <rogerheflin@gmail.com>]

Keld Jørn Simonsen wrote:
> On Wed, Jul 02, 2008 at 01:08:04PM -0500, David Lethe wrote:
>>
>> And also the disk controllers, could these be bottlenecks? They typically
>> operate at 300 MB/s nominally, per disk channel, and presumably they
>> then have a connection to the southbridge that is capable of handling
>> this speed. So for a 4-disk SATA-II controller this would be at least
>> 1200 MB/s or about 10 gigabit. 
>>
>> best regards
>> keld
>> -------------------
>> It is much more complicated than just saying what the transfer rates are, especially in the world of blocking, arbitration, and unbalanced I/O.  
> 
> Yes, that is understood, but I am only listing some potential
> bottlenecs, of cause there may be more.
> 
>> Everything is a potential bottleneck.  As I am under NDA with most of the controller vendors, then I can not provide specifics, but suffice to say that certain cards with certain chipsets will max out at well under published speeds.  Heck, you could attach solid-state disks with random I/O access time in the nanosecond range and still only get 150MB/sec out of certain controllers, even on a PCIe X 16 bus. 
>>
>> BTW, there isn't a SATA-II controller in the planet that will deliver 1200 MB/sec with 4 disk drives. 
> 
> Yes, but I think this is normally due to that the max transfer speed per
> disk is in the ballpark of 80-120 MB/s - which is less than half the
> SATA-II max speed. And I think much of this slowdown comes from head movement,
> track-to-track, disk latency etc. I was of the impression, that when the
> transfer between the disk and the controller is going on, then the
> transfer speed would be not far from the 300 MB/s max speed, eg for
> 90 MB/s 1 TB disks that I bougth recently, or the faster 15000 RPM
> disks, which give something like 120 MB/s.

Actually enough sectors are only passing under the head at that 70-120MB/second 
rate, so when you add in the seeks and such things are a bit lower, but you 
aren't ever going to be able to exceed that given bit rate, and the bit rate 
changes from the inside of the disk to the outside of the disk (the outside has 
more sectors on it than the inside), in the disk data sheet's there is usually a 
range of bit rates listed showing this.

                             Roger

                                 Roger
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