Re: Random IO with md raid

[Goswin von Brederlow <goswin-v-b@web.de>]

Michael Evans <mjevans1983@gmail.com> writes:

> On Mon, Dec 14, 2009 at 1:06 AM, Matthieu Patou <mat@matws.net> wrote:
>> Hello Goswin,
>>> Software raid1 will write twice as much data. That means twice as much
>>> data goes over the system bus and into the controler cache.
>>> Effectively you have halve the cache size. Maybe that is all you see.
>>>
>>> MfG
>>>          Goswin
>>>
>> Your idea seems logical, and I took a few hours today to verify it and it's
>> the case as accessing the disk without software raid leads to almost the
>> same result as with hardware raid.
>>
>> Matthieu.
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>
> Thinking about it, the doubling of data going over the system bus part
> is correct.  You must still push the operation out to each device's
> buffer.  However for -most- modern systems the system-bus will not be
> the bottleneck for a reasonable number of drives.
>
> Further the later half which I'd skimmed over the first time is
> utterly incorrect.  Each drive would still only see the commands
> targeting that drive.  That should be virtually the same if not
> identical to the single-drive case.

The controler itself has cache. And that is shared between all drives.

> The two most likely bottlenecks are single-drive write speed, and any
> IO barriers that are used to ensure file system consistency in the
> event of sudden interruption.

MfG
        Goswin
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