Re: Sector size changes creating filesystem problems

[Roman Mamedov <rm@romanrm.net>]

On Wed, 16 Jul 2025 15:30:20 +0100
Filipe Maia <filipe.c.maia@gmail.com> wrote:

> Hi,
> 
> When a 4Kn disk is added to an mdadm array with sector size 512, its
> sector size changes to 4096 to accommodate the new disk.
> 
> Here's an example:
> 
> ```
> truncate -s 1G /tmp/loop512a
> truncate -s 1G /tmp/loop512b
> truncate -s 1G /tmp/loop512c
> truncate -s 1G /tmp/loop4Ka
> losetup --sector-size 512  --direct-io=on /dev/loop0  /tmp/loop512a
> losetup --sector-size 512  --direct-io=on /dev/loop1  /tmp/loop512b
> losetup --sector-size 512  --direct-io=on /dev/loop2  /tmp/loop512c
> losetup --sector-size 4096  --direct-io=on /dev/loop3  /tmp/loop4Ka
> mdadm --create /dev/md2 --level=5 --raid-devices=3 /dev/loop[0-2]
> # blockdev returns 512
> blockdev --getss /dev/md2
> mdadm /dev/md2 -a /dev/loop3
> mdadm /dev/md2 -f /dev/loop2
> # blockdev still returns 512
> blockdev --getss /dev/md2
> mdadm -S /dev/md2
> mdadm -A /dev/md2 /dev/loop0 /dev/loop1 /dev/loop3
> # blockdev now returns 4096
> blockdev --getss /dev/md2
> ```
> 
> This breaks filesystems like XFS, with new mounts failing with:
> `mount: /mnt: mount(2) system call failed: Function not implemented.`

If you dd the XFS image from an old 512b disk onto a newly bought large
4K-sector HDD, would it also stop mounting on the new disk in the same way?

Perhaps something to be improved on the XFS side?

-- 
With respect,
Roman