On Sun, 2013-02-03 at 04:07 +0600, Roman Mamedov wrote:

On Sat, 02 Feb 2013 15:52:12 -0600
Stan Hoeppner [off-list ref] wrote:

quoted

So under heavy read load, in this case we'd probably want the slow 3TB drive
to contribute to the workload.

No, it should behave as documented.

       -W, --write-mostly
              subsequent devices listed in a --build, --create, or --add  com‐
              mand will be flagged as 'write-mostly'.  This is valid for RAID1
              only and means that the 'md'  driver  will  avoid  reading  from
              these devices if at all possible.

                             ^^^^^^^^^^^^^^^^^^^
I would assume that if the kernel thinks the SSD is starved for io (that
being the %util column in iostat) it would consider sending io to the
HDD. At least that would be what I would expect to happen, so if the SSD
reports wrong utilization numbers back or anything else involved in the
io processing the the behavior is as expected.

 

if at all possible = if there is any other mirror still alive, do not read from
this one.

BTW I also run an SSD+HDD array, and after a brief testing today I did notice
some reads from the HDD on reading from the array showing up in iostat. So I
can confirm the bug.

/Tommy

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