Re: XP RAID vs md

[Vladimir Milovanovic <vlad@webmail.co.za>]

Well Daniel what is your take on this? I am very interested in this 
since I am working on a project which has to use RAID, and performance 
is paramount (after reliability of course). I need to make a decision, 
and I would like very much to stick with md, since it is what I know and 
am used to. But if one can get huge performance margins using XP, then I 
am going to have to consider that.

Cheers,

Vlad.


Illtud Daniel wrote:

>[posted & mailed]
>
>Lars Marowsky-Bree wrote:
>
>  
>
>>>Wow... I hope that one of the maintainers will comment on this, I didn't
>>>even know that XP had a sw RAID implementation. Up to 100% more on read
>>>and 60% more on write is quite a significant margin. Is there anything
>>>in favour of the md driver if this is true?
>>>      
>>>
>>I've not really checked these numbers yet, so take the following with a grain
>>of salt.
>>    
>>
>
>Have a look at http://www.hpl.hp.com/techreports/2002/HPL-2002-352.pdf
>- that'll tell you were the figures are from - the 100% is worst-case.
>
>  
>
>>However, with RAID1 for example, I got approximately twice the read speed and
>>95% of the write speed (compared to just using a single disk).
>>    
>>
>
>One thing I forgot to mention in my post is that this is all (AFAICS)
>RAID0.
>
>  
>
>>I have a really hard time imagining a 100% read boost; that would simply
>>exceed disk bandwidth, and 60% writes - how should that work?
>>    
>>
>
>The md driver seriously underperforms at certain request sizes, and
>is generally underperforming. The average boost is less, but still about
>40-60% read and 30-40% write. XP's RAID0 is better than JBOD above a
>certain request size, which is pretty good.
>
>  
>
>>I'm not claiming md is perfect or the fastest imaginable solution, but it is
>>rather close to theoretical disk bandwidth. A two digit percentage performance
>>improvement just can't be done.
>>    
>>
>
>Well, that's why I asked in my original post for people to look at
>the report. Is isn't that long and it's pretty clear (although short
>on config details).
>
>--
>Illtud Daniel                                 illtud.daniel@llgc.org.uk
>Uwch Ddadansoddwr Systemau                       Senior Systems Analyst
>Llyfrgell Genedlaethol Cymru                  National Library of Wales
>Yn siarad drosof fy hun, nid LlGC   -  Speaking personally, not for NLW
>-
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