[linux-lvm] Re: raid 1 on a single disk

[ashwin chaugule]

read my earlier mails ... 
it is silly , but not as silly as trying to modify the kernel code for
just one specific disk : )




On Sat, 13 Nov 2004 13:59:19 +0100, Peter T. Breuer <ptb at lab.it.uc3m.es> wrote:
> ashwin chaugule <ashwin.chaugule at gmail.com> wrote:
> 
> (please do not top post - fixing. Please take note!)
> 
> 
> 
> > > > Is this correct if I want data to be mirrored across all 4 partitions
> > > > , using RAID 1 on a single disk ?
> > >
> > > So, you are talking about a mirror with 4 components, all of them
> > > partitions on the same disk?
> > >
> > > Well, that's silly, but nothing stops you doing it. Just maek a raidtab
> > > for the mirror and name the partitions as raid-disk components there.
> > > The RAID howto or faq should tell you all you need to know, as should
> > > the mananpage for the conf file or mdadm, or whatever ...
> 
> > ok so, i also do know, its performance is going to suck !
> 
> No, it'll be fine - merely a couple or more times slower at writing
> large streams. In ordinary use you may sometimes see more latency, but
> provided you aren't streaming or running synchronous writes, you
> shouldn't notice. What's silly is that there's no point in doing it -
> you get no protection against the disk disappearing, because all the
> mirror components are on the same disk.
> 
> It's like making 3 sets of spare housekeys, and then putting them all
> in the same keyholder as the original set, and walking around like that.
> 
> Silly, no?
> 
> > but i was under the impression that RAID 1 works on more that one disks only.
> 
> I don't understand you. While a mirror with only one component is
> trivial, it is a mirror.
> 
> > so you mean to say that, the linux RAID / md tools support raid 1 on
> > multiple partitiions of the same disk ?
> 
> Nobody cares where the mirror components are physically sited except
> you.  Why should any tool care?  Its job is to do what you say.  I don't
> understand why you should think that the tool would even know (well,
> there is a chance that it could look and check, but I don't recall any
> significant code in the driver dedicated to optimizations based on that).
> 
> 
> 
> Peter
> 
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> 


-- 
Ashwin Chaugule
Embedded Systems Engineer
Aftek Infosys ltd.
[Embedded Division]