Re: LVM over RAID, or plain disks? A:"Yes" = best of both worlds?

[Neil Brown <neilb@suse.de>]

On Tue, 30 Nov 2010 12:35:56 +0700 hansbkk@gmail.com wrote:

> On Tue, Nov 30, 2010 at 7:42 AM, Neil Brown <neilb@suse.de> wrote:
> > If you are comparing recovering after some sort of problem with
> > a RAID10 over 6 devices compared with  LVM over 2 2-device RAID1s, then the
> > former is certainly easier.  This is simply because there are less layers of
> > complexity where something could go wrong.
> >
> > In both cases, your data will be spread across multiple disks, and any one
> > disk or even any two disks would be of no use to you.
> 
> 
> Thanks Neil.
> 
> Still true with LVM on top of the 6-drive set in either case?

Yes.  LVM will spread the data around in a different way, but it is still not
possible to recovery anything reliably without all of the data.


> 
> 
> Scenario being
>  All the drives are together and OK (generic SATA2, cleanly
> disconnected) - but everything else is gone
>  Not practical to rebuild the whole set of hosts, just want to get at key data
>  Mount the disks on a new machine, boot from SystemRescueCD or Knoppix
>  and copy the key data off.

There should be no difficulty doing that in either case.  But there is more
room for things to go wrong if you use LVM+MD than if you just use MD.

So certainly use LVM if you need any of its features, but otherwise don't.

> 
> 
> And between RAID6 and RAID10?

I think this has already been answered.
RAID10 tends to be faster, but with 5 or more devices, RAID6 makes more space
available.
RAID6 can survive any 2 devices failing.  RAID10 over 6 devices can sometimes
survive 3 failures, and sometimes not survive 2.

NeilBrown



> 
> 
> > On Mon, 29 Nov 2010 23:00:19 +0700 hansbkk@gmail.com wrote:
> >
> >> 2010/11/29 Keld Jørn Simonsen <keld@keldix.com>:
> >> >> I can see how RAID6 is simpler than RAID10, but compared to RAID1?
> >> >
> >> > Hmm, does not compute by me. RAID1 and RAID10 are the same in complexity,
> >> > RAID10 is just a modern RAID1, and should actually have been called
> >> > RAID1.
> >>
> >> My understanding is that if I use RAID10 on a single pair of disks
> >> then that is literally the same as RAID1. These to me are very simple
> >> in that I can take either one of the pair and mount it on any normal
> >> machine and get at the data without doing anything special.
> >>
> >> However, if I have my six disks configured as a single RAID10 array, I
> >> believe this is no longer true - the data from (at least the larger
> >> of) the files has been distributed over all six disks, correct?
> >>
> >> Now compare putting LVM on top of this array, compared to three RAID1
> >> pairs on the one hand and a RAID6 array on the other (third) hand :)
> >>
> >> If I were trying to recover the data using the latest version of a
> >> LiveCD - say Fedora or Knoppix, which would be easier?
> >>
> >> I'm not trying to score any points, it's a genuine question.

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