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

[Justin Piszcz <jpiszcz@lucidpixels.com>]

On Thu, 29 May 2008, Kasper Sandberg wrote:

> On Wed, 2008-05-28 at 15:27 -0400, Justin Piszcz wrote:
>>
>> 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?
> You wouldnt happen to have some more information about this? i havent
> personally had problems yet, but i havent used it for very long - but
> since it comes activated by DEFAULT, i would assume it to be relatively
> stable?
Not off-hand, check LKML and my email address from early this year or last 
year and/or the ide-list.

Justin.