Re: Some very basic questions

[Ric Wheeler <rwheeler@redhat.com>]

Avi Kivity wrote:
> Chris Mason wrote:
>> One problem with the spare
>> capacity model is the general trend where drives from the same batch
>> that get hammered on in the same way tend to die at the same time.  Some
>> shops will sleep better knowing there's a hot spare and that's fine by
>> me.
>>   
>
> How does hot sparing help?  All your disks die except the spare.
>
> Of course, I've no objection to disk sparing as an additional option; 
> I just feel that capacity sparing is superior.
>
For any given set of disks, you "just" need to do the math to compute 
the utilized capacity, the expected rate of drive failure, the rebuild 
time and then see whether you can recover from your first failure before 
a 2nd disk dies.

In practice, this is not an academic question since drives do 
occasionally fail in batches (and drives from the same batch get stuffed 
into the same system).

I suspect that what will be used in mission critical deployments will be 
more conservative than what is used in less critical path systems, but 
this will be up to the end user to configure...

ric