Re: Some very basic questions

[Avi Kivity <avi@redhat.com>]

Ric Wheeler wrote:
> 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.
>

Spare disks have the advantage of a fully linear access pattern 
(ignoring normal working load).  Spare capacity has the advantage of 
utilizing all devices (if you have a hundred-disk fs, all surviving 
disks participate in the rebuild; whereas with spare disks you are 
limited to the surviving raidset members.

Spare capacity also has the advantage that you don't need to rebuild 
free space.
> 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).

This seems to be orthogonal to the sparing method used; and in both 
cases the answer is to tolerate dual failures.  File-based redundancy 
has the advantage here of allowing triple mirroring for metadata and 
frequently written files, and double parity raid for large files.

> I suspect that what will be used in mission critical deployments will 
> be more conservative than what is used in less critical path systems

That's true, unfortunately.  But with time people will trust the newer, 
more efficient methods.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.