linux-kernel - Re: [PATCH v5 2/2] md: allow configuring logical block size

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Message-ID: <f54f4915-956b-895b-0e14-02c41b7b633e@huaweicloud.com>
Date: Thu, 25 Sep 2025 16:38:33 +0800
From: Li Nan <linan666@...weicloud.com>
To: Xiao Ni <xni@...hat.com>, Li Nan <linan666@...weicloud.com>
Cc: corbet@....net, song@...nel.org, yukuai3@...wei.com, hare@...e.de,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-raid@...r.kernel.org, martin.petersen@...cle.com,
 yangerkun@...wei.com, yi.zhang@...wei.com
Subject: Re: [PATCH v5 2/2] md: allow configuring logical block size



在 2025/9/23 22:06, Xiao Ni 写道:
> On Tue, Sep 23, 2025 at 9:37 PM Li Nan <linan666@...weicloud.com> wrote:
>>
>>
>>
>> 在 2025/9/23 19:36, Xiao Ni 写道:
>>> Hi Li Nan
>>>
>>> On Thu, Sep 18, 2025 at 8:08 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
>>>> ```
>>>
>>> Thanks for the reproducer. I can reproduce it myself.
>>>
>>>>
>>>> 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.
>>>
>>> If we add a disk with 4KB LBS and configure it to 4KB by the sysfs
>>> interface, how can we make the partition table readable and avoid the
>>> problem mentioned above?
>>>
>>
> 
> Hi
> 
>> Thanks for your review.
>>
>> The main cause of partition loss is LBS changes. Therefore, we should
>> specify a 4K LBS at creation time, instead of modifying LBS after the RAID
>> is already in use. For example:
>>
>> mdadm -C --logical-block-size=4096 ...
>>
>> In this way, even if all underlying disks are 512-byte, the RAID will be
>> created with a 4096 LBS. Adding 4096-byte disks later will not cause any
>> issues.
> 
> It can work. But it looks strange to me to set LBS to 4096 but all
> devices' LBS is 512 bytes. I don't reject it anyway :)
> 
In this scenario, there doesn't seem to be a better way. Do you have any
suggestions?

-- 
Thanks,
Nan