Comment by Sesse__

Comment by Sesse__ 5 days ago

> Do you mean WSS and mdadm/lvm will allow an automatic live rebalance and then reconfigure of the drive topo?

mdadm can convert RAID-5 to a larger or smaller RAID-5, RAID-6 to a larger or smaller RAID-6, RAID-5 to RAID-6 or the other way around, RAID-0 to a degraded RAID-5, and many other fairly reasonable operations, while the array is online, resistant to power loss and the likes.

I wrote the first version of this md code in 2005 (against kernel 2.6.13), and Neil Brown rewrote and mainlined it at some point in 2006. ZFS is… a bit late to the party.

ryao 5 days ago

Doing this with the on disk data in a merkle tree is much harder than doing it on more conventional forms of storage.

By the way, what does MD do when there is corrupt data on disk that makes it impossible to know what the correct reconstruction is during a reshape operation? ZFS will know what file was damaged and proceed with the undamaged parts. ZFS might even be able to repair the damaged data from ditto blocks. I don’t know what the MD behavior is, but its options for handling this are likely far more limited.

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Sesse__ 5 days ago

Well, then they made a design choice in their RAID implementation that made fairly reasonable things hard.
I don't know what md does if the parity doesn't match up, no. (I've never ever had that happen, in more than 25 years of pretty heavy md use on various disks.)

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- ryao 5 days ago
  
  I am not sure if reshaping is a reasonable thing. It is not so reasonable in other fields. In architecture, if you build a bridge and then want more lanes, you usually build a new bridge, rather than reshape the bridge. The idea of reshaping a bridge while cars are using it would sound insane there, yet that is what people want from storage stacks.
  Reshaping traditional storage stacks does not consider all of the ways things can go wrong. Handling all of them well is hard, if not impossible to do in traditional RAID. There is a long history of hardware analogs to MD RAID killing parity arrays when they encounter silent corruption that makes it impossible to know what is supposed to be stored there. There is also the case where things are corrupted such that there is a valid reconstruction, but the reconstruction produces something wrong silently.
  Reshaping certainly is easier to do with MD RAID, but the feature has the trade off that edge cases are not handled well. For most people, I imagine that risk is fine until it bites them. Then it is not fine anymore. ZFS made an effort to handle all of the edge cases so that they do not bite people and doing that took time.
  
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  Sesse__ 5 days ago
  
  > I am not sure if reshaping is a reasonable thing.
  Yet people are celebrating when ZFS adds it. Was it all for nothing?
  
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- amluto 5 days ago
  
  I’ve experienced bit rot on md. It was not fun, and the tooling was of approximately no help recovering.
  
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