[nSLUG] RAID and partitioning

Bill Davidson billdavidson at eastlink.ca
Wed May 9 16:22:31 ADT 2007


Hi:

On Tue, 2007-05-08 at 18:38 -0300, D G Teed wrote:
>         raw disks -> MD -> LVM -> filesystem.
> 
> Right, and now we can drop one part and get the same features
> (at least for my purposes):
> 
> raw disks -> MD -> filesystem
> 
> No need of additional device configuration, modules, etc. 

Alternatively:

raw disks -> LVM -> filesystem.

"Ah," you say, "but I want the redundancy that md offers."  Yes, but see
config DM_MIRROR in Linux 2.6:

         Allow volume managers to mirror logical volumes, also
         needed for live data migration tools such as 'pvmove'.

And recent versions of lvcreate have the option "-m" which creates a
mirrored volume.

"OK," I hear you say, "but RAID offers performance improvements by
striping across disks and so speeding up reads."  Yes, but so does LVM!

In fact, in some ways it seems like LVM and MD are converging into a
single system.  And on the down side, I understand that the mirror
support in LVM is still pretty strange, actually requiring a third
physical device for storing a log -- if you had three disks you would
probably want to use RAID5 so you get to use 2/3 of that space.


-- 
Bill Davidson <billdavidson at eastlink.ca>



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