On Fri, May 06, 2011 at 09:51:45AM +0200, David Brown wrote: > On 06/05/2011 09:17, Keld Jørn 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-offset > >layout is an implementation of a SNIA RAID specification. The RAID10-near > >layout is an implementation of a simple RAID layout. And the RAID10-far > >layout is just another layout far a mirrored RAID. So all these types > >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 giving > >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 > layouts - does that also apply if you have more than two disks? And > what about weird things like raid10 over three disks? I don't know. But anyway, you can see it as an extension of SNIA standard RAID1 layouts. And then if the SNIA standard does not cover it, the standard should be enhanced. I have sent specs on the far layout to SNIA twice, but witout any response. > There are times when it is important that a standard raid1 element is > also accessible as a normal disk (with metadata 0.90). Examples include > booting and sometimes data recovery or transferring the disk to another > machine. yes, and then we should document how this is done. Also boot loaders are becoming more intelligent, so one common case where the raid also need to be seen as a normal disk is going away. > There are things you can do with raid1 that you cannot do with raid10 at > the moment, such as re-shaping and re-sizing. It wouldn't make sense to > classify raid10 as a type of raid1 until it also has this capability. Well, theoretically Linux MD RAID10 is a mirrored RAID type - that is a RAID1 in standard parlance. You could document that some RAID1 layouts cannot be re-shaped or re-sized. I generally like to be backward compatible, and I think this can be esily done just by naming the different layouts of RAID1, for example: classic, near, far, offset, directly corresponding to the functionalities and possibly kernel modules that we have today. > It is also not clear how adding an extra drive to a raid10,far layout > should work. If you add an extra drive to a raid1 set, you get a > three-way mirror. You need to be clear on how you add disks - if you want more copies, or more space with the same number of copies. I think the default would be to add more space. It is quite uncommon to want more than 2 copies, while a need for more space happens all the time. > If you add an extra drive to a raid10,far drive, > should it directly mirror one of the existing drives? Should it reshape > to a raid10,far3 arrangement? Should it turn into a raid10,far2 and > re-balance across the disks? Any of these might be valid choices. Agree that any of these are valid. And thus all of them should be doable. But implementation will most likely be piece wise. I see no problem with architecture there. You can always say that some features are implemented, and some are not. That is at least the current situation, and this is a very normal situation. > I support your "campaign for raid10,far awareness", but I'm not sure > that making it the default for raid1 is appropriate at the moment. Once > raid10 re-shapes and re-sizes are fully supported, and once all main > distros have moved to a new enough version of grub (which supports > booting from raid10, and different metadata versions), then there will > be few reasons for anyone to choose "standard" raid1 rather than raid10,far. Everything takes time. But we can set a goal, and then plan for reaching the goal. I am sure if we do it right, we can get there in a time less than what we have had since the introduction of the raid10 driver in 2004. > just ask you which drive you want to use - if the support raid at all > during installation, it is accessed through "advanced" and "manual > partition" screens. And getting grub onto both disks is very much a > post-install manual operation, for those that know that it needs to be > done. Getting distro installers to set up raid10,far by default would > be a much bigger step. I would like it to be done in distro installers, with the help of grub et al. And work is underway in some distros to do that. Best regards keld -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
