quoted

quoted

On Tue, 25 Dec 2007 19:08:15 +0000,
pg_lxra@lxra.for.sabi.co.UK (Peter Grandi) said:

[ ... ]

quoted

It's the raid10,f2 *read* performance in degraded mode that is
strange - I get almost exactly 50% of the non-degraded mode
read performance. Why is that?

[ ... ] the mirror blocks have to be read from the inner
cylinders of the next disk, which are usually a lot slower
than the outer ones. [ ... ]

Just to be complete there is of course the other issue that
affect sustained writes too, which is extra seeks. If disk B
fails the situation becomes:

    DISK
   A X C D

   1 X 3 4
   . . . .
   . . . .
   . . . .
   -------
   4 X 2 3       
   . . . .
   . . . .
   . . . .

Not only must block 2 be read from an inner cylinder, but to
read block 3 there must be a seek to an outer cylinder on the
same disk. Which is the same well known issue when doing
sustained writes with RAID10 'f2'.