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Re: Hardware vs Software Raid

From: david(at)lang(dot)hm
To: "Peter T(dot) Breuer" <ptb(at)inv(dot)it(dot)uc3m(dot)es>
Cc: Merlin Moncure <mmoncure(at)gmail(dot)com>, Matthew Wakeling <matthew(at)flymine(dot)org>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Hardware vs Software Raid
Date: 2008-06-26 20:17:28
Message-ID: alpine.DEB.1.10.0806261316170.13412@asgard (view raw or flat)
Thread:
Lists: pgsql-performance
On Thu, 26 Jun 2008, Peter T. Breuer wrote:

> "Also sprach Merlin Moncure:"
>> The linux software raid algorithms are highly optimized, and run on a
>
> I can confidently tell you that that's balderdash both as a Linux author
> and as a software RAID linux author (check the attributions in the
> kernel source, or look up something like "Raiding the Noosphere" on
> google).
>
>> presumably (much faster) cpu than what the controller supports.
>> However, there is still some extra oomph you can get out of letting
>> the raid controller do what the software raid can't...namely delay
>> sync for a time.
>
> There are several design problems left in software raid in the linux kernel.
> One of them is the need for extra memory to dispatch requests with and
> as (i.e. buffer heads and buffers, both). bhs should be OK since the
> small cache per device won't be exceeded while the raid driver itself
> serialises requests, which is essentially the case (it does not do any
> buffering, queuing, whatever .. and tries hard to avoid doing so). The
> need for extra buffers for the data is a problem. On different
> platforms different aspects of that problem are important (would you
> believe that on ARM mere copying takes so much cpu time that one wants
> to avoid it at all costs, whereas on intel it's a forgettable trivium).
>
> I also wouldn't aboslutely swear that request ordering is maintained
> under ordinary circumstances.

which flavor of linux raid are you talking about (the two main families I 
am aware of are the md and dm ones)

David Lang

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