Date:	Sun, 17 Jun 2007 13:14:10 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Neil Brown <neilb@...e.de>
CC:	david@...g.hm, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org
Subject: Re: limits on raid

Neil Brown wrote:
> On Thursday June 14, david@...g.hm wrote:
>   
>> On Fri, 15 Jun 2007, Neil Brown wrote:
>>
>>     
>>> On Thursday June 14, david@...g.hm wrote:
>>>       
>>>> what is the limit for the number of devices that can be in a single array?
>>>>
>>>> I'm trying to build a 45x750G array and want to experiment with the
>>>> different configurations. I'm trying to start with raid6, but mdadm is
>>>> complaining about an invalid number of drives
>>>>
>>>> David Lang
>>>>         
>>> "man mdadm"  search for "limits".  (forgive typos).
>>>       
>> thanks.
>>
>> why does it still default to the old format after so many new versions? 
>> (by the way, the documetnation said 28 devices, but I couldn't get it to 
>> accept more then 27)
>>     
>
> Dunno - maybe I can't count...
>
>   
>> it's now churning away 'rebuilding' the brand new array.
>>
>> a few questions/thoughts.
>>
>> why does it need to do a rebuild when makeing a new array? couldn't it 
>> just zero all the drives instead? (or better still just record most of the 
>> space as 'unused' and initialize it as it starts useing it?)
>>     
>
> Yes, it could zero all the drives first.  But that would take the same
> length of time (unless p/q generation was very very slow), and you
> wouldn't be able to start writing data until it had finished.
> You can "dd" /dev/zero onto all drives and then create the array with
> --assume-clean if you want to.  You could even write a shell script to
> do it for you.
>
> Yes, you could record which space is used vs unused, but I really
> don't think the complexity is worth it.
>
>   
How about a simple solution which would get an array on line and still 
be safe? All it would take is a flag which forced reconstruct writes for 
RAID-5. You could set it with an option, or automatically if someone 
puts --assume-clean with --create, leave it in the superblock until the 
first "repair" runs to completion. And for repair you could make some 
assumptions about bad parity not being caused by error but just unwritten.

Thought 2: I think the unwritten bit is easier than you think, you only 
need it on parity blocks for RAID5, not on data blocks. When a write is 
done, if the bit is set do a reconstruct, write the parity block, and 
clear the bit. Keeping a bit per data block is madness, and appears to 
be unnecessary as well.
>> while I consider zfs to be ~80% hype, one advantage it could have (but I 
>> don't know if it has) is that since the filesystem an raid are integrated 
>> into one layer they can optimize the case where files are being written 
>> onto unallocated space and instead of reading blocks from disk to 
>> calculate the parity they could just put zeros in the unallocated space, 
>> potentially speeding up the system by reducing the amount of disk I/O.
>>     
>
> Certainly.  But the raid doesn't need to be tightly integrated
> into the filesystem to achieve this.  The filesystem need only know
> the geometry of the RAID and when it comes to write, it tries to write
> full stripes at a time.  If that means writing some extra blocks full
> of zeros, it can try to do that.  This would require a little bit
> better communication between filesystem and raid, but not much.  If
> anyone has a filesystem that they want to be able to talk to raid
> better, they need only ask...
>  
>   
>> is there any way that linux would be able to do this sort of thing? or is 
>> it impossible due to the layering preventing the nessasary knowledge from 
>> being in the right place?
>>     
>
> Linux can do anything we want it to.  Interfaces can be changed.  All
> it takes is a fairly well defined requirement, and the will to make it
> happen (and some technical expertise, and lots of time .... and
> coffee?).
>   
Well, I gave you two thoughts, one which would be slow until a repair 
but sounds easy to do, and one which is slightly harder but works better 
and minimizes performance impact.

-- 
bill davidsen <davidsen@....com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979

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