Re: [PATCH v4 2/2] md: allow configuring logical_block_size

[Li Nan]

在 2025/9/15 16:50, Xiao Ni 写道:
> On Mon, Sep 15, 2025 at 10:15 AM Li Nan <linan666@xxxxxxxxxxxxxxx> wrote:
> > 在 2025/9/15 8:33, Xiao Ni 写道:
> > > Hi Nan
> > >
> > > On Thu, Sep 11, 2025 at 3:41 PM <linan666@xxxxxxxxxxxxxxx> wrote:
> > > > From: Li Nan <linan122@xxxxxxxxxx>
> > > >
> > > > Previously, raid array used the maximum logical_block_size (LBS) of
> > > > all member disks. Adding a larger LBS during disk at runtime could
> > > > unexpectedly increase RAID's LBS, risking corruption of existing
> > > > partitions.
> > >
> > > Could you describe more about the problem? It's better to give some
> > > test steps that can be used to reproduce this problem.
> >
> > Thanks for your review. I will add reproducer in the next version.
>
> Thanks.
> > > > Simply restricting larger-LBS disks is inflexible. In some scenarios,
> > > > only disks with 512 LBS are available currently, but later, disks with
> > > > 4k LBS may be added to the array.
> > > >
> > > > Making LBS configurable is the best way to solve this scenario.
> > > > After this patch, the raid will:
> > > >     - stores LBS in disk metadata.
> > > >     - add a read-write sysfs 'mdX/logical_block_size'.
> > > >
> > > > Future mdadm should support setting LBS via metadata field during RAID
> > > > creation and the new sysfs. Though the kernel allows runtime LBS changes,
> > > > users should avoid modifying it after creating partitions or filesystems
> > > > to prevent compatibility issues.
> > >
> > > Because it only allows setting when creating an array. Can this be
> > > done automatically in kernel space?
> > >
> > > Best Regards
> > > Xiao
> >
> > The kernel defaults LBS to the max among all rdevs. When creating RAID
> > with mdadm, if mdadm doesn't set LBS explicitly, how does the kernel
> > learn the intended value?
> >
> > Gunaghao previously submitted a patch related to mdadm:
> > https://lore.kernel.org/all/3a9fa346-1041-400d-b954-2119c1ea001c@xxxxxxxxxx/
>
> Thanks for reminding me about this patch. First I still need to
> understand the problem. It may be a difficult thing for a user to


Thank you for your response.

Reproducer:
https://lore.kernel.org/all/3e26e8a3-e0c1-cd40-af79-06424fb2d54b@xxxxxxxxxxxxxxx/

I will add it to messge log in the next version.

> choose the logcial block size. They don't know why they need to
> consider this value, right? If we only need a default value, the
> kernel space should be the right place?

The scenario of our product is that they have a mix of 4k LBS and 512 LBS
disks. For most users, they do not need to modify the default values. This
is difficult to solve through default values.

> Regards
> Xiao
> > --
> > Thanks,
> > Nan
>
>
> .

--
Thanks,
Nan