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

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Message-ID: <358b8933-952e-0e4b-e708-c40ca98ae7d7@huaweicloud.com>
Date: Mon, 3 Nov 2025 21:09:04 +0800
From: Li Nan <linan666@...weicloud.com>
To: Xiao Ni <xni@...hat.com>, linan666@...weicloud.com
Cc: corbet@....net, song@...nel.org, yukuai@...as.com, hare@...e.de,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-raid@...r.kernel.org, yangerkun@...wei.com, yi.zhang@...wei.com
Subject: Re: [PATCH v8 4/4] md: allow configuring logical block size



在 2025/11/3 11:11, Xiao Ni 写道:
> On Thu, Oct 30, 2025 at 2:36 PM <linan666@...weicloud.com> wrote:
>>
>> From: Li Nan <linan122@...wei.com>
>>
>> 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@...wei.com >
> 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@...hat.com>
> 

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