Re: consistency detect

[Brad Campbell <brad@wasp.net.au>]

Michael Tokarev wrote:

>> In short, it does not deal with it at all. RAID will deal with a disk 
>> failure, it has no guarantees about consistency on power failures, 
>> hard lockups or other catastrophic events.
> 
> 
> This is incorrect.  In-kernel raid code keeps track of arrays and
> underlying disk state during write operations.  On clean shutdown,
> when everything has been written, raid superblocks on all disks
> gets updated to indicate this.  In case of unclean shutdown, raid
> code will reconstruct older copies of data using most recent ones
> (ie, from a disk which has most recent "events" value in superblock).
> The same is done for all other raid levels (4, 5, 6), but onot for
> raid0 for obvious reasons (as there's no R in raid0 per se).


When does the "events" value in the superblock actually get updated? I understood it only got 
updated on an event, ie raid start, raid stop, disk add/remove/fail.

I realise the system does an auto rebuild when started after an unclean shutdown, the question 
really is how does it know which disk is the freshest in a raid-1? In a raid-4,5,6 it's pretty 
obvious as there is really only one copy of the data, but then does the code actually ensure that 
the data gets written before the updated parity? or does it just flush the lot to disk in what it 
thinks is the most optimum fashion?

The In-kernel data becomes pretty moot when the kernel has just blasted a couple of large blocks out 
to a couple of disks and the plug has been pulled. It's going to be pretty indeterminate as to which 
disk has the most accurate image of what was actually sent to it. Thus my comment that there is 
really no way of accurately dealing with a catastrophic failure, and RAID is not there to do that 
anyway.

I guess if you had a hardware RAID card that had a battery backed up RAM you have a much better 
chance but then you really have a mini-ups :p)

Brad