Re: What do Events actually mean?

[David Greaves <david@dgreaves.com>]

Neil Brown wrote:
> On Saturday March 22, jpiszcz@lucidpixels.com wrote:
>>
>> On Sat, 22 Mar 2008, Peter Rabbitson wrote:
>>
>>> Robin Hill wrote:
>>>> On Fri Mar 21, 2008 at 07:01:43PM -0400, Bill Davidsen wrote:
>>>>
>>>>> Peter Rabbitson wrote:
>>>           UUID : b6a11a74:8b069a29:6e26228f:2ab99bd0 (local to host Arzamas)
>>>         Events : 0.183270
>>>
>>> As you can see it is pretty old, and does not have many events to speak of.
>> What do the 'Events' actually represent and what do they mean for RAID0,
>> RAID1, RAID5 etc?
> 
> An 'event' is one of:
>    switch from 'active' to 'clean'
>    switch from 'clean' to 'active'
>    device fails
>    device is added
>    spare replaces a failed device after a rebuild
> 
> I think that it all.
> 
> None of these are meaningful for RAID0, so the 'events' counter on
> RAID0 should be stable.
> 
> Unfortunately, the number looks like a decimal but isn't.
> It is a 64bit number.  We print out the top 32 bits, then the bottom
> 32 bits.  I don't remember why.  Maybe I'll 'fix' it.
> 
>> How are they calculated?
> 
>    events = events + 1;
> 
> 
> Feel free to merge this text into the wiki.

http://linux-raid.osdl.org/index.php?title=Event

I also added:

== What are they for? ==
When an array is assembled, all the disks should have the same number of events.
If they don't then something odd happened.

eg:
If one drive fails then the remaining drives have their event counter
incremented. When the array is re-assembled the failed drive has a different
event count and is not included in the assembly.


This lead me to ponder: How/when are events reset to equality?
I wrote:
  The event count on a drive is set to zero on creation and reset to the
majority on a resync or a forced assembly.

Is that right?

David