Re: Roadmap for md/raid ???

[Bill Davidsen <davidsen@tmr.com>]

Neil Brown wrote:
> On Monday January 19, davidsen@tmr.com wrote:
>   
>> Neil Brown wrote:
>>     
>>> 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.
>>>
>>>   
>>>       
>> That would seem to be an issue doing it in a large task as well, if I 
>> understand what you want to do. After the data have been written to the 
>> spare space I believe you are going to write it back. If you update the 
>> metadata before the write and something happens you have old metadata 
>> but data in another format. Conversely, if you update the metadata 
>> first, and something happens, you have the metadata describing a change 
>> which didn't happen. So You would have to have a metadata reflecting the 
>> data not being in the array, but actually somewhere else, or you would 
>> have to keep the spare space inside the array, where room might not be 
>> available.
>>     
>
> I "freeze" a section of the array so that any writes block,
> I copy that data to somewhere safe, and mark that copy as being a
> valid copy of the moved data.
> Then I copy it back in the new layout and update the array metadata to
> show that the change-over point between old-layout and new-layout has
> changed.  Then I invalidate the copy.
> Then repeat.
>
> When restarting an array, mdadm checks the copy (which could be in a
> file on a different device) and if it is valid, it is copied on to
> the array and the metadata is updated.
>   

Sounds good...

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark