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SubjectRe: MD raid speed going down....

On Fri, 13 Mar 1998, Robert O'Kane wrote:

> The only thing which changes is the kernel.
>
> Asus-P2T4 with IBM 6x86MX - PR233+
> 64Mb RAM
> Libc-5.4.44
> Linux-threads-0.71
> (2x) Symbios-810A scsi cards with (4x)Fujitsu 4Gb drives
> Useing the 'old' 53c7,8xx driver.
> (Its MUCH faster than the 'new' ncr driver in the 89-pre5
> kernel. 89-final and above both are the same speed).

I have observed that lower latency in queuing commands to devices may
have the adverse effect to stress the devices in such a way that they will
disconnect the BUS more often, and so will wast BUS bandwitch. In such a
situation, the overall performances may be worse using the fastest thing.

On the other hand, the kernel and the devices perform caching and read
ahead in order to lower effects of latencies on overall performances.

So, my opinion is that a significant difference using 2 drivers on the
same SCSI system with same features enabled is not easy to explain, and
that deducing definitely that the faster driver is the one that gives
better overall performance is loose.

Yes, I think that a slower driver can sometimes give better overall
performances.
I suggest you to try something like 2xSymbios-875 boards using each 3 good
Ultra Wide SCSI disks, and let me know how the ncr53c8xx driver performs.


> Raid-tools-0.42-pre3
>
> Anyone have an Idea? I could start hand patching stuff
> but I would like to have a 'good' starting point!
>
> Thanks,
>

Gerard.


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