On Thu, May 21, 2009 at 7:48 AM, Neil Brown [off-list ref] wrote:

On Tuesday May 19, dan.j.williams@intel.com wrote:

quoted

On Sat, May 2, 2009 at 2:46 PM, raz ben yehuda [off-list ref] wrote:

quoted

Neil Hello
The bellow is the raid0 grow code.I have decided to fix raid0 and not
perform the transformation raid0-raid4-raid0 due to two reasons:
1. raid0 zones. this patch support any zone transformations.
2. Undesired dependency of raid0 over raid4 re-striping code.

Hi Raz,

Can you explain a bit more about why the raid4 approach is
undesirable?  I think making reshape only available to raid0 arrays
where all the members are the same size is a reasonable constraint.
We then get the nice benefit of reusing the raid5 reshape
infrastructure.  In other words I am not convinced that the benefits
of reimplementing reshape in raid0 outweigh the costs.

I've been thinking about this too... Is it something we really want to
do?

My thoughts include:

 - I don't like special cases - it would be nice to support reshape on
  all arrays, even RAID0 with different sizes devices.
 - Anyone who does this with a raid0 made of simple drives is asking
  for trouble.  But a RAID0 over a bunch of RAID5 or RAID1 might make
  sense.
 - Maybe we should support different sized drives in RAID4.  As long
  as the parity drive is as big as the largest data drive it could be
  made to work.  Similarly hot spares would need to be big, but you
  could have 2 hot spares and take the smallest one that is big
  enough.
  If a drive in the RAID4+ (or is it is the thing called NORAID?)
  failed and was replaced with a bigger drive, it would be cool to be
  able to incorporate that extra space into the array.

  If we did all that, then the 0->4->0 conversion could make use of
  the same code.
 - Surely RAID0 is (like LVM) just a legacy idea until we get sensible
  file systems that actually understand multiple devices and do all
  this stuff for you are a more sensible level - so why are we
  busting a gut(*) to make RAID0 work well??  Answer is of course
  that no-one has made a sensible file system yet. (well... maybe zfs
  or btrfs, not sure)
 - If you read the DDF spec carefully, you find there is a secondary
  raid level which stripes over heterogeneous arrays a different way.
  You divide every primary array up into N chunks, so the chunk sizes are
  different on different arrays.  Then you make a secondary array by
  striping over those chunks.
  So e.g. you might have a 4Gig RAID5 and a 1GIG RAID1.  So the
  stripes array on top of these could take 4Meg from the RAID5, then
  1Meg from the RAID1, then another 4 from the RAID5 etc.
  So we want to support that?  And would we want to reshape such a
  thing??

So: lots of thoughts, some pointing in different directions.
But I'm not against reshape code appearing in RAID0 providing it is
well designed, maintainable, reliable, and doesn't slow down normal
RAID0 processing.  I suspect we can get there.

NeilBrown

This may be in a FAQ / wiki somewhere, but is the long range plan of
mdraid to stick to the formal "raid levels" or to implement "raid
equivalent levels".

I am a big fan of the raid equivalent concept and would love to see
mdraid moving in that direction.

==> What I mean by raid equivalent levels

More and more arrays allow the user to simply say "give me a 100 GB
logical volume with Raid 5 equivalent protection.  The array then
looks at the drives it has available and puts together the necessary
pieces.  As drives are added, removed it moves the data around under
its own control, but maintains the raid equivalent protection.

Especially when working with dozens of drives and lots of logical
volumes it makes life much easier.  Admittedly it may come at a cost
of not being able to specify raid levels with the specificity that
mdraid currently allows.

==>

The reason I ask if this is the goal is that doing so may factor into
decisions about how reshaping is implemented.

Greg
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