From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: [PATCH 1/4] md: Factor out RAID6 algorithms into lib/ Date: Fri, 17 Jul 2009 11:40:34 -0400 Message-ID: <4A609B72.2010901@zytor.com> References: <1247494302.19180.268.camel@macbook.infradead.org> <4A5F6590.9000006@zytor.com> <4A608913.1060808@redhat.com> <4A6096A0.5050501@zytor.com> <4A609A52.7070506@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4A609A52.7070506@redhat.com> Sender: linux-btrfs-owner@vger.kernel.org To: Ric Wheeler Cc: Dan Williams , David Woodhouse , chris.mason@oracle.com, linux-btrfs@vger.kernel.org, neilb@suse.de, linux-raid@vger.kernel.org List-Id: linux-raid.ids Ric Wheeler wrote: >> >> I have seen the papers; I'm not sure it really makes that much >> difference. One of the things that bugs me about these papers is that he >> compares to *his* implementation of my optimizations, but not to my >> code. In real life implementations, on commodity hardware, we're limited >> by memory and disk performance, not by CPU utilization. >> > > Fair enough - I thought that his coverage of the other open source > friendly encodings beyond RAID6 was actually quite interesting. > > If you have specifics that you found unconvincing in his work, I am > pretty sure that he would be delighted to hear from you first hand. > James seemed to me to be very reasonable and very much a pro-Linux > academic, so I would love to be able to get him and his grad students > aligned in a useful way for us :-) > The main flaw, as I said, is in the phrase "as implemented by the Jerasure library". He's comparing his own implementations of various algorithms, not optimized implementations. The bottom line is pretty much this: the cost of changing the encoding would appear to outweigh the benefit. I'm not trying to claim the Linux RAID-6 implementation is optimal, but it is simple and appears to be fast enough that the math isn't the bottleneck. -hpa