On Tue, 2007-10-16 at 17:57 -0400, Mike Accetta wrote:

Was the disk driver generating any low level errors or otherwise
indicating that it might be retrying operations on the bad drive at
the time (i.e. console diagnostics)?  As Neil mentioned later, the md layer
is at the mercy of the low level disk driver.  We've observed abysmal
RAID1 recovery times on failing SATA disks because all the time is
being spent in the driver retrying operations which will never succeed.
Also, read errors don't tend to fail the array so when the bad disk is
again accessed for some subsequent read the whole hopeless retry process
begins anew.

The console was full of errors like:

end_request: I/O error, dev sdb, sector 42644555

I don't know what generates those messages.

As I asked before but never got an answer, is there a way to do timeouts
within the md code so that we are not at the mercy of the lower layer
drivers?

I posted a patch about 6 weeks ago which attempts to improve this situation
for RAID1 by telling the driver not to retry on failures and giving some
weight to read errors for failing the array.  Hopefully, Neil is still
mulling it over and it or something similar will eventually make it into
the main line kernel as a solution for this problem.
--
Mike Accetta

Thanks,

Alberto