Re: Triple parity and beyond

[David Brown <david.brown@hesbynett.no>]

On 25/11/13 03:14, Russell Coker wrote:
> On Mon, 25 Nov 2013, Stan Hoeppner <stan@hardwarefreak.com> wrote:
>>> If that is the problem then the solution would be to just enable
>>> read-ahead. Don't we already have that in both the OS and the disk
>>> hardware?  The hard- drive read-ahead buffer should at least cover the
>>> case where a seek completes but the desired sector isn't under the
>>> heads.
>>
>> I'm not sure if read-ahead would solve such a problem, if indeed this is
>> a possible problem.  AFAIK the RAID5/6 drivers process stripes serially,
>> not asynchronously, so I'd think the rebuild may still stall for ms at a
>> time in such a situation.
> 
> For a RAID block device (such as Linux software RAID) read-ahead should work 
> well.  For a RAID type configuration managed by the filesystem where you might 
> have different RAID levels in the same filesystem it might not be possible.
> 
> It would be a nice feature to have RAID-0 for unimportant files and RAID-1 or 
> RAID-6 for important files on the same filesystem.  But that type of thing 
> would really complicate RAID rebuild.
> 

I think btrfs is planning to have such features - different files can
have different raid types.  It certainly supports different raid levels
for metadata and file data.  But it is definitely a feature you want on
the filesystem level, rather than the raid block device level.