Doug,

EVMS integrates all of this stuff together into one cohesive peice of 
technology.

But I agree, LVM should be modified to support RAID 1 and RAID 5, or MD 
should be modified to support volume management.  Since RAID 1 and RAID 
5 are easier to implement, LVM is probably the best place to put all 
this stuff.

Doug Ledford wrote:

On Thu, Nov 21, 2002 at 11:34:24AM -0800, Joel Becker wrote:
 

quoted

On Wed, Nov 20, 2002 at 08:46:25PM -0500, Doug Ledford wrote:
   

quoted

I haven't yet played with the new dm code, but if it's like I expect it to 
be, then I predict that in a few years, or maybe much less, md and dm will 
be two parts of the same whole.  The purpose of md is to map from a single 
     

Most LVMs support mirroring as an essential function.  They
don't usually support RAID5, leaving that to hardware.
I certainly don't want to have to deal with two disparate
systems to get my code up and running.  I don't want to be limited in my
mirroring options at the block device level.
DM supports mirroring.  It's a simple 1:2 map.  Imagine this LVM
volume layout, where volume 1 is data and mirrored, and volume 2 is some
scratch space crossing both disks.

[Disk 1]	[Disk 2]
  [volume 1]	  [volume 1 copy]
         [       volume 2              ]

If DM handles the mirroring, this works great.  Disk 1 and disk
2 are handled either as the whole disk (sd[ab]) or one big partition on
each disk (sd[ab]1), with DM handling the sizing and layout, even
dynamically.
If MD is handling this, then the disks have to be partitioned.
sd[ab]1 contain the portions of md0, and sd[ab]2 are managed by DM.  I
can't resize the partitions on the fly, I can't break the mirror to add
space to volume 2 quickly, etc.
   

Not at all.  That was the point of me entire email, that the LVM code 
should handle these types of shuffles of space and simply use md modules 
as the underlying mapper technology.  Then, you go to one place to both 
specify how things are laid out and what mapping is used in those laid out 
spaces.  Basically, I'm saying how I think things *should* be, and you're 
telling me how they *are*.  I know this, and I'm saying how things *are* 
is wrong.  There *should* be no md superblocks, there should only be dm 
superblocks on LVM physical devices and those DM superblocks should 
include the data needed to fire up the proper md module on the proper 
physical extents based upon what mapper technology is specified in the 
DM superblock and what layout is specified in the DM superblock.  In my 
opinion, the existence of both an MD and DM driver is wrong because they 
are inherently two sides of the same coin, logical device mapping support, 
with one being better at putting physical disks into intelligent arrays 
and one being better at mapping different logical volumes onto one or more 
physical volume groups.