Re: Roadmap for md/raid ???

On Wednesday January 14, davidsen@tmr.com wrote:

Neil Brown wrote:

quoted

Not really a roadmap, more a few tourist attractions that you might
see on the way if you stick around (and if I stick around)...

  

Thanks for sharing, although that last comment is a little worrisome.

No one in irreplaceable... but I'm not planning on going anywhere
just now :-)

quoted

Comments welcome.

  

Here's one, is this in some sense a prioritized list? If so I might 
comment on the order, and I'm sure others would feel even more strongly 
than I. ;-)

No, the list isn't prioritised.  I'll do things as the mood takes me.

quoted

- raid5->raid6 conversion.
   This is also a fairly commonly asked for feature.
   The first step would be to define a raid6 layout where the Q block
   was not rotated around the devices but was always on the last
   device.  Then we could change a raid5 to a singly-degraded raid6
   without moving any data.

   The next step would be to implement in-place restriping. 
   This involves 
      - freezing a section of the array (all IO blocks)
      - copying the data out to a safe backup
      - copying it back in with the new layout
      - updating the metadata to indicate that the restripe has
        progressed.
      - repeat.

  

It would seem very safe, something like
 1 - call the chunk on the new drive the available space
 2 - determine what needs to be in the available space
 3 - if data, copy the data chunk to the available chunk, mark the old 
location avail, repeat step 2
 4 - Q goes in the available chunk, calculate it and the stripe is done
I don't see the move to a safe backup if you move one chunk at a time 
until you are ready for Q, unless there are moves I'm missing. You 
always have a free space to move one chunk, when all data is in the 
right place and the P value is in place (does it move?), then Q is 
calculated and saved. In other words, no out of stripe storage needed.

The difficulty is "How do you handle a crash?"
If you crash and restart in the middle of a reshape, you need to know
where all the data is.
To follow your scheme, you would need to update the metadata for
every block that is moved.

The value of having a large avail space is that you only update the
metadata only every time the space fills up.



Thanks for your thoughts.

NeilBrown