Re: Suggestion for hot-replace

[joystick <joystick@shiftmail.org>]

On 11/25/12 07:37, H. Peter Anvin wrote:
> I was looking at the hot-replace (want_replacement) feature, and I had 
> a thought: it would be nice to have this in a form which *didn't* fail 
> the incumbent drive after the operation is over, and instead turned it 
> into a spare.  This would make it much easier and safer to 
> periodically rotate and test any hot spares in the system.  The main 
> problem with hot spares is that you don't actually know if they work 
> properly until there is a failover...
>
>     -hpa
>

Sorry I don't agree.

Firstly, it causes confusion. If you want a replacement in 90% of cases 
it means that the current drive is defective. If you put the replaced 
drive into the spare pool instead of kicking it out then you have to 
remember (by serial number?) which one it was to actually remove it from 
the system. If you forget to note it down, then you are in serious 
troubles, because if that "spare" then gets caught in another (or the 
same) array needing a recovery, you will have a high probability of 
exotic and unexpected multiple failures situations.

Also, if you are uncertain of the health of your spares, risking your 
array by throwing one into the array is definitely unwise. There are 
other tecniques to test a spare that don't involve risking you array on 
it: you can remove one spare from the spare pool (best if you have 2+ 
spares but can also be done with 1), read/write all of it various times 
as a validation, then re-add it back to the spares pool. Even just 
reading it from beginning to end with dd could be enough and for this 
you don't even have to remove it from the spare pool. And this doesn't 
degrade the array performances, while your suggestion would.

Thirdly, if you really want that (imho unwise) behaviour, it's easy to 
implement from userspace without asing the MD developers to do so: 
monitor the replacement process, as soon as you see it terminating and 
you see the target drive in Failed status, remove and re-add it back as 
a spare. That's it.