Re: Harware vs Kernel RAID (was Re: External SATA enclosures: SiI3124 and CentOS 5?)

[Christopher Chan <christopher.chan@xxxxxxxxxxxxxxx>]

nate wrote:
> Chan Chung Hang Christopher wrote:
>   
>> Complete bollocks. The bottleneck is not the drives themselves as
>> whether it is SATA/PATA disk drive performance has not changed much
>> which is why 15k RPM disks are still king. The bottleneck is the bus be
>> it PCI-X or PCIe 16x/8x/4x or at least the latencies involved due to bus
>> traffic.
>>     
>
> In most cases the bottleneck is the drives themselves, there is
> only so many I/O requests per second a drive can handle. Most workloads
> are random, rather than sequential, so the amount of data you can
> pull from a particular drive can be very low depending on what
> your workload is.
>   
Which is true whether you are running hardware or software raid 0/1/1+0. 
However, when it comes to software raid, given enough disks, the 
bottleneck moves from the disk to the bus especially for raid5/6.
>
> Fortunately the large caches(12GB per controller, mirrored with
> another controller) on the array buffer the higher response
> times on the disks resulting in host response times of
> around 20 milliseconds for reads, and 0-5 milliseconds for
> writes, which by most measures is excellent.
>
>   

Haha, yeah, if you have such large scale setups, nobody would compare 
software raid.
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