On 25/11/13 03:14, Russell Coker wrote:

On Mon, 25 Nov 2013, Stan Hoeppner [off-list ref] wrote:

quoted

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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.