Re: [PATCH v8 4/4] md: allow configuring logical block size

[Li Nan]

在 2025/11/3 11:11, Xiao Ni 写道:
> On Thu, Oct 30, 2025 at 2:36 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 disk at runtime could
> > unexpectedly increase RAID's LBS, risking corruption of existing
> > partitions. This can be reproduced by:
> >
> > ```
> >    # LBS of sd[de] is 512 bytes, sdf is 4096 bytes.
> >    mdadm -CRq /dev/md0 -l1 -n3 /dev/sd[de] missing --assume-clean
> >
> >    # LBS is 512
> >    cat /sys/block/md0/queue/logical_block_size
> >
> >    # create partition md0p1
> >    parted -s /dev/md0 mklabel gpt mkpart primary 1MiB 100%
> >    lsblk | grep md0p1
> >
> >    # LBS becomes 4096 after adding sdf
> >    mdadm --add -q /dev/md0 /dev/sdf
> >    cat /sys/block/md0/queue/logical_block_size
> >
> >    # partition lost
> >    partprobe /dev/md0
> >    lsblk | grep md0p1
> > ```
> >
> > Simply restricting larger-LBS disks is inflexible. In some scenarios,
> > only disks with 512 bytes LBS are available currently, but later, disks
> > with 4KB LBS may be added to the array.
> >
> > Making LBS configurable is the best way to solve this scenario.
> > After this patch, the raid will:
> >    - store 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.
> >
> > Only 1.x metadata supports configurable LBS. 0.90 metadata inits all
> > fields to default values at auto-detect. Supporting 0.90 would require
> > more extensive changes and no such use case has been observed.
> >
> > Note that many RAID paths rely on PAGE_SIZE alignment, including for
> > metadata I/O. A larger LBS than PAGE_SIZE will result in metadata
> > read/write failures. So this config should be prevented.
> >
> > Signed-off-by: Li Nan <linan122@xxxxxxxxxx >
> Hi Li Nan

Hi Xiao,

Thanks for your review.

> The problem can't be fixed if there is no user space (mdadm) patch, right?

Yeah, mdadm should update same time. And Guanghao will send a mdadm patch
later.

> The patch Looks good to me.
> Reviewed-by: Xiao Ni <xni@xxxxxxxxxx>

Sorry for the trouble. I sent the v9 with some changes to the
Documentation. Could you please review v9 patch when you have time?

> .


--
Thanks,
Nan