On Wednesday January 14, davidsen@xxxxxxx wrote: > Neil Brown wrote: > > 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 :-) > > > 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. > > - 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 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
