From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: wish for Linux MD mirrored raid types Date: Fri, 06 May 2011 09:51:45 +0200 Message-ID: References: <20110506071752.GA22063@www2.open-std.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20110506071752.GA22063@www2.open-std.org> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 06/05/2011 09:17, Keld J=F8rn Simonsen wrote: > Hi List > > based on the recent discussion, that showed lacking knowledge > on Linux MD RAID10 features, I have some thoughts: > > It is really hard to disseminate information on "new" features > in MD RAID. RAID10 has been in the kernel since 2.6.9 - from 2004. > I have tried to give info on RAID10 at a number of web pages, > and still many people, even on our linux-raid list are not aware > of it. > > Also many people are confused about Linux MD raid10 and RAID1+0. > > So I think we shopuld rather name things in another way. > > I would like linux MD raid10 functionality to be part of the Linux MD > RAID1 module, and be called raid1. This is in accordance with the > use of the RAID1 term as standadized by SNIA. In fact the RAID10-offs= et > layout is an implementation of a SNIA RAID specification. The RAID10-= near > layout is an implementation of a simple RAID layout. And the RAID10-f= ar > layout is just another layout far a mirrored RAID. So all these type= s > could just be defined as different RAID1 layouts. > > I would then also wish for RAID10-far to be the default RAID1 > layout. There is general agreement on this list that RAID10-far > is the best mirrored layout for most purposes. In the interest of giv= ing > the best performance to the Linux RAID users, we should make > the defaults the best practise - users tend to choose defaults, > especially often they do not have much knowledge. > > We could still keep the RAID10 code for backwards compatibility, > or even let this new naming just be calls to the raid10 code > from the raid1 module. > I mostly agree with you, but for a few points: You say the various raid10 layouts match SNIA RAID1 specifications and=20 layouts - does that also apply if you have more than two disks? And=20 what about weird things like raid10 over three disks? There are times when it is important that a standard raid1 element is=20 also accessible as a normal disk (with metadata 0.90). Examples includ= e=20 booting and sometimes data recovery or transferring the disk to another= =20 machine. There are things you can do with raid1 that you cannot do with raid10 a= t=20 the moment, such as re-shaping and re-sizing. It wouldn't make sense t= o=20 classify raid10 as a type of raid1 until it also has this capability. It is also not clear how adding an extra drive to a raid10,far layout=20 should work. If you add an extra drive to a raid1 set, you get a=20 three-way mirror. If you add an extra drive to a raid10,far drive,=20 should it directly mirror one of the existing drives? Should it reshap= e=20 to a raid10,far3 arrangement? Should it turn into a raid10,far2 and=20 re-balance across the disks? Any of these might be valid choices. I support your "campaign for raid10,far awareness", but I'm not sure=20 that making it the default for raid1 is appropriate at the moment. Onc= e=20 raid10 re-shapes and re-sizes are fully supported, and once all main=20 distros have moved to a new enough version of grub (which supports=20 booting from raid10, and different metadata versions), then there will=20 be few reasons for anyone to choose "standard" raid1 rather than raid10= ,far. What would make a bigger difference is to get better raid support into=20 the distro's installers. Most of these, when faced with two disks, wil= l=20 just ask you which drive you want to use - if the support raid at all=20 during installation, it is accessed through "advanced" and "manual=20 partition" screens. And getting grub onto both disks is very much a=20 post-install manual operation, for those that know that it needs to be=20 done. Getting distro installers to set up raid10,far by default would=20 be a much bigger step. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html