Re: wish for Linux MD mirrored raid types

[David Brown <david@westcontrol.com>]

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?

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.

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.

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.  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.


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.


What would make a bigger difference is to get better raid support into 
the distro's installers.  Most of these, when faced with two disks, will 
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.


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