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SubjectRe: Performance Characteristics of All Linux RAIDs (mdadm/bonnie++)


On Wed, 28 May 2008, Chris Snook wrote:

> Justin Piszcz wrote:
>>
>>
>> On Wed, 28 May 2008, Chris Snook wrote:
>>
>>> Justin Piszcz wrote:
>>>> Hardware:
>>>>
>>> Given that one of the greatest benefits of NCQ/TCQ is with parity RAID,
>>> I'd be fascinated to see how enabling NCQ changes your results. Of
>>> course, you'd want to use a single SATA controller with a known good NCQ
>>> implementation, and hard drives known to not do stupid things like disable
>>> readahead when NCQ is enabled.
>> Only/usually on multi-threaded jobs/tasks, yes?
>
> Generally, yes, but there's caching and readahead at various layers in
> software that can expose the benefit on certain single-threaded workloads as
> well.
>
>> Also, I turn off NCQ on all of my hosts that has it enabled by default
>> because
>> there are many bugs that occur when NCQ is on, they are working on it in
>> the
>> libata layer but IMO it is not safe at all for running SATA disks w/NCQ as
>> with it on I have seen drives drop out of the array (with it off, no
>> problems).
>>
>
> Are you using SATA drives with RAID-optimized firmware? Most SATA
> manufacturers have variants of their drives for a few dollars more that have
> firmware that provides bounded latency for error recovery operations, for
> precisely this reason.
I see--however, as I understood it there were bugs utilizing NCQ in libata?

But FYI--
In this test, they were regular SATA drives, not special raid-ones (RE2,etc).

Thanks for the info!

Justin.



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