Received: from localhost (unknown [200.46.204.184]) by postgresql.org (Postfix) with ESMTP id 07DA82E037A for ; Wed, 26 Dec 2007 18:24:27 -0400 (AST) Received: from postgresql.org ([200.46.204.71]) by localhost (mx1.hub.org [200.46.204.184]) (amavisd-maia, port 10024) with ESMTP id 98657-05-2 for ; Wed, 26 Dec 2007 18:24:17 -0400 (AST) Received: from ironport2-out.pppoe.ca (ironport2-out.pppoe.ca [206.248.154.182]) by postgresql.org (Postfix) with ESMTP id 3C9C32E00BD for ; Wed, 26 Dec 2007 18:24:22 -0400 (AST) X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: Ao8CABRjckfO+Igp/2dsb2JhbACCb6Zq X-IronPort-AV: E=Sophos;i="4.24,209,1196658000"; d="scan'208,217";a="11989712" Received: from smtp.pppoe.ca ([65.39.196.238]) by ironport2-out.pppoe.ca with ESMTP; 26 Dec 2007 17:24:21 -0500 Received: from [206.248.136.41] ([206.248.136.41]) by smtp.pppoe.ca (Internet Mail Server v1.0) with ESMTP id FIY75720; Wed, 26 Dec 2007 17:24:20 -0500 Message-ID: <4772D3DA.10302@mark.mielke.cc> Date: Wed, 26 Dec 2007 17:21:14 -0500 From: Mark Mielke User-Agent: Thunderbird 2.0.0.9 (X11/20071115) MIME-Version: 1.0 To: Bill Moran CC: david@lang.hm, Florian Weimer , Fernando Hevia , 'pgsql-performance' Subject: Re: With 4 disks should I go for RAID 5 or RAID 10 References: <0f8201c847f6$08e15090$8f01010a@iptel.com.ar> <87zlvxnpkl.fsf@mid.deneb.enyo.de> <4772CA58.5050406@mark.mielke.cc> <4772CE34.2040608@mark.mielke.cc> <20071226171105.889f6146.wmoran@collaborativefusion.com> In-Reply-To: <20071226171105.889f6146.wmoran@collaborativefusion.com> Content-Type: multipart/alternative; boundary="------------060301010805000101030407" X-Virus-Scanned: Maia Mailguard 1.0.1 X-Archive-Number: 200712/390 X-Sequence-Number: 28206 This is a multi-part message in MIME format. --------------060301010805000101030407 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Bill Moran wrote: > >> What do you mean "heard of"? Which raid system do you know of that reads >> all drives for RAID 1? >> > > I'm fairly sure that FreeBSD's GEOM does. Of course, it couldn't be doing > consistency checking at that point. > According to this: http://www.freebsd.org/cgi/man.cgi?query=gmirror&apropos=0&sektion=8&manpath=FreeBSD+6-current&format=html There is a -b (balance) option that seems pretty clear that it does not read from all drives if it does not have to: Create a mirror. The order of components is important, because a component's priority is based on its position (starting from 0). The component with the biggest priority is used by the prefer balance algorithm and is also used as a master component when resynchronization is needed, e.g. after a power failure when the device was open for writing. Additional options include: *-b* /balance/ Specifies balance algorithm to use, one of: *load* Read from the component with the lowest load. *prefer* Read from the component with the biggest priority. *round-robin* Use round-robin algorithm when choosing component to read. *split* Split read requests, which are big- ger than or equal to slice size on N pieces, where N is the number of active components. This is the default balance algorithm. Cheers, mark -- Mark Mielke --------------060301010805000101030407 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Bill Moran wrote: 

What do you mean "heard of"? Which raid system do you know of that reads 
all drives for RAID 1?
    

I'm fairly sure that FreeBSD's GEOM does.  Of course, it couldn't be doing
consistency checking at that point.
According to this:

http://www.freebsd.org/cgi/man.cgi?query=gmirror&apropos=0&sektion=8&manpath=FreeBSD+6-current&format=html

There is a -b (balance) option that seems pretty clear that it does not read from all drives if it does not have to:

    Create a mirror. 
                 The order of components is important,
                 because a component's priority is based on its position
                 (starting from 0).  The component with the biggest priority
                 is used by the prefer balance algorithm and is also used as a
                 master component when resynchronization is needed, e.g. after
                 a power failure when the device was open for writing.
    Additional options include:

                 -b balance  Specifies balance algorithm to use, one of:

                             load         Read from the component with the
                                          lowest load.

                             prefer       Read from the component with the
                                          biggest priority.

                             round-robin  Use round-robin algorithm when
                                          choosing component to read.

                             split        Split read requests, which are big-
                                          ger than or equal to slice size on N
                                          pieces, where N is the number of
                                          active components.  This is the
                                          default balance algorithm.


Cheers,
mark

-- 
Mark Mielke <mark@mielke.cc>
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