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Re: [dm-devel] RAID5 support ?



On Thu, Nov 03, 2005 at 11:39:43AM +0000, Alan Hourihane wrote:
> On Thu, 2005-11-03 at 22:20 +1100, Neil Brown wrote:
> > On Thursday November 3, alanh fairlite demon co uk wrote:
> > > 
> > > I mean "Load the kernel image from an md device" which is what it means
> > > to me too.
> > > 
> > > So, you go on to say that booting from raid5 almost certainly requires
> > > bios support - so I'm back to needing the onboard Sil 3114 BIOS to
> > > handle that and then get md/dmraid to sort the rest out - which gets
> > > back to my original question on when device mapper would get RAID5
> > > support so that dmraid can handle the SIL RAID5 metadata.
> > 
> > "device mapper" and "dmraid" are of course different things.  
> > One is a kernel subsystem, the other is a configuration tool.
> > For dmraid to be able to handle SIL RAID5, it is not necessary for
> > "device mapper" to get RAID5 support, only for dmraid to have
> > appropriate access to some sort of raid5 support in the Linux kernel.
> > 
> > I would like md/raid5 (which is quite separate from device mapper) to
> > be able to provide the required level of support so that if the dmraid
> > developers so chose, they could use it rather than needing a second
> > raid5 implementation in the kernel.  Hence my original question, asking
> >  "how md/raid5 can be made to be sufficient" for the needs for dmraid?
> 
> So what exactly is Heinz working on if it isn't support for RAID5 in
> device-mapper ?

It is a RAID4 and RAID5 device-mapper target.

> 
> > I suspect all it needs is to be able to interface with a user-space
> > metadata handler, and possibly some extra configuration options to
> > match the expected functionality of the SiL.
> > The former I hope be possible by 2.6.16.  The later I'm interested in
> > advice on.
> 
> I'll defer to Heinz to make that choice - he's a lot closer to the code.

The device-mapper kernel subsystem aims to avoid multiple drivers for
block device virtualization being implemented seperately in the Linux
kernel by adding device-mapper targets as we go.

Implementing a target for RAID4 and RAID5 is just the long term approach
to get more mappings supported by device-mapper in order to minimize code
duplication.

> 
> Alan.
> 
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-- 

Regards,
Heinz    -- The LVM Guy --

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Heinz Mauelshagen                                 Red Hat GmbH
Consulting Development Engineer                   Am Sonnenhang 11
Cluster and Storage Development                   56242 Marienrachdorf
                                                  Germany
Mauelshagen RedHat com                            +49 2626 141200
                                                       FAX 924446
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