Date: Thu, 26 Jun 2008 13:17:28 -0700 (PDT)
From: david@lang.hm
To: "Peter T. Breuer" <ptb@inv.it.uc3m.es>
cc: Merlin Moncure <mmoncure@gmail.com>,
	Matthew Wakeling <matthew@flymine.org>, pgsql-performance@postgresql.org
Subject: Re: Hardware vs Software Raid
In-Reply-To: <200806260503.m5Q53cA24979@inv.it.uc3m.es>
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References: <200806260503.m5Q53cA24979@inv.it.uc3m.es>
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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