From mboxrd@z Thu Jan 1 00:00:00 1970 From: Neil Brown Subject: Re: raid5: two writing algorithms Date: Fri, 8 Feb 2008 12:51:39 +1100 Message-ID: <18347.46507.422011.243347@notabene.brown> References: <20080207161337.GA6044@rap.rap.dk> <18347.26939.358283.172299@notabene.brown> <20080208011041.GD12985@rap.rap.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Return-path: Sender: linux-raid-owner@vger.kernel.org To: Keld =?iso-8859-1?Q?J=F8rn?= Simonsen Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Friday February 8, keld@dkuug.dk wrote: > On Fri, Feb 08, 2008 at 07:25:31AM +1100, Neil Brown wrote: > > On Thursday February 7, keld@dkuug.dk wrote: > > > > So I hereby give the idea for inspiration to kernel hackers. > > > > and I hereby invite you to read the code ;-) > > I did some reading. Is there somewhere a description of it, especially > the raid code, or are the comments and the code the best documentation? No. If a description was written (and various people have tried to describe various parts) it would be out of date within a few months :-( Look for "READ_MODIFY_WRITE" and "RECONSTRUCT_WRITE" .... no. That only applied to raid6 code now.. Look instead for the 'rcw' and 'rmw' counters, and then at 'handle_write_operations5' which does different things based on the 'rcw' variable. It used to be a lot clearer before we implemented xor-offload. The xor-offload stuff is good, but it does make the code more complex. > > Do you say that this is already implemented? Yes. > > I am sorry if you think I am mailing too much on the list. You aren't. > But I happen to think it is fun. Good. > And I do try to give something back. We'll look forward to that. > > > Code reading is a good first step to being a > > > > > > Yoyr kernel hacker wannabe > > ^^^^^^^^^^^^^ > > > > NeilBrown > > Well, I do have a hack in mind, on the raid10,f2. > I need to investigate some more, and possibly test out > what really happens. But maybe the code already does what I want it to. > You are possibly the one that knows the code best, so maybe you can tell > me if raid10,f2 always does its reading in the first part of the disks? Yes, I know the code best. No, raid10,f2 doesn't always use the first part of the disk. Getting it to do that would be a fairly small change in 'read_balance' in md/raid10.c. I'm not at all convinced that the read balancing code in raid10 (or raid1) really does the best thing. So any improvements - backed up with broad testing - would be most welcome. NeilBrown