Make ZFS use the physical sector size when computing initial ashift

[Steven Hartland]

----- Original Message ----- 
From: "Justin T. Gibbs"
...
>>> > One issue I did spot in your patch is that you currently expose
>>> > zfs_max_auto_ashift as a sysctl but don't clamp its value which would
>>> > cause problems should a user configure values > 13.
>>> 
>>> I would expect the zio pipeline to simply insert an ashift aligned thunking
>>> buffer for these operations, but I haven't tried going past an ashift of 13 in
>>> my tests.  If it is an issue, it seems the restriction should be based on
>>> logical access size, not optimal access size.
>> 
>> Yes with your methodology you'll only see the issue if zfs_max_auto_ashift
>> and physical_ashift are both > 13, but this can be the case for example
>> on a RAID controller with large stripsize.
>
> I'm not sure I follow.  logical_ashift is available in our latest code, as is the
> physical_ashift.  But even without the logical_ashift, why doesn't the zio
> pipeline properly thunk zio_phys_read() access based on the configured ashift?

When I looked at it, which was a long time ago now so please excuse me if
I'm a little rusty on the details, zio_phys_read() was working more luck than
judgement as the offsets passed in where calculated from a valid start + increment
based on the size of a structure within vdev_label_offset() with no ashift
logic applied that I cound find.

The result was pools created with large ashift's where unstable when I
tested.

    Regards
    Steve

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