Re: raid1 read balance with write mostly

[Tommy Apel Hansen <tommyapeldk@gmail.com>]

On Sun, 2013-02-03 at 04:07 +0600, Roman Mamedov wrote:
> On Sat, 02 Feb 2013 15:52:12 -0600
> Stan Hoeppner <stan@hardwarefreak.com> wrote:
> 
> > 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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