Re: Distributed spares

[Bill Davidsen <davidsen@tmr.com>]

Neil Brown wrote:
> On Monday October 13, davidsen@tmr.com wrote:
>   
>> Over a year ago I mentioned RAID-5e, a RAID-5 with the spare(s) 
>> distributed over multiple drives. This has come up again, so I thought 
>> I'd just mention why, and what advantages it offers.
>>
>> By spreading the spare over multiple drives the head motion of normal 
>> access is spread over one (or several) more drives. This reduces seeks, 
>> improves performance, etc. The benefit reduces as the number of drives 
>> in the array gets larger, obviously with four drives using only three 
>> for normal operation is slower than four, etc. And by using all the 
>> drives all the time, the chance of a spare being undetected after going 
>> bad is reduced.
>>
>> This becomes important as array drive counts shrink. Lower cost for 
>> drives ($100/TB!), and attempts to drop power use by using fewer drives, 
>> result in an overall drop in drive count, important in serious applications.
>>
>> All that said, I would really like to bring this up one more time, even 
>> if the answer is "no interest."
>>     
>
> How are your coding skills?
>
> The tricky bit is encoding the new state.
> We can not longer tell the difference between "optimal" and "degraded"
> based on the number of in-sync devices.  We also need some state flag
> to say that the "distributed spare" has been constructed. 
> Maybe that could be encoded in the "layout".
>
> We would also need to allow a "recovery" pass to happen without having
> actually added any spares, or having any non-insync devices.  That
> probably means passing the decision "is a recovery pending" down into
> the personality rather than making it in common code.  Maybe have some
> field in the mddev structure which the personality sets if a recovery
> is worth trying.  Or maybe just try it anyway after any significant
> change and if the personality finds nothing can be done it aborts.
>
>   
My coding skills are fine, here, but I have to do a lot of planning 
before even considering this.
Here's why:
  Say you have a five drive RAID-5e, and you are running happily. A 
drive fails! Now you can rebuild on the spare drive, but the spare drive 
must be created on the parts from the remaining functional drives, so it 
can't be done pre-failure, the allocation has to be defined after you 
see what you have left. Does that sound ugly and complex? Does to me, 
too. So I'm thinking about this, and doing some reading, but it's not 
quite as simple as I thought.
> I'm happy to advise on, review, and eventually accept patches.
>   

Actually what I think I would do is want to build a test bed in software 
before trying this in the kernel, then run the kernel part in a virtual 
machine. I have another idea, which has about 75% of the benefit with 
10% of the complexity. Since it sounds too good to be true it probably 
is, I'll get back to you after I think about the simpler solution, I 
distrust free lunch algorithms.
> NeilBrown
-- 

Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark