linux-ext4 - ext4/xfs: about switching underlying 512B sector devices to 4K ones

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Message-ID: <CANR1yOpz9o9VcAiqo18aVO5ssmuSy18RxnMKR=Dz884Rj8_trg@mail.gmail.com>
Date:   Wed, 29 Jul 2020 19:38:33 +0900
From:   Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:     linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: ext4/xfs: about switching underlying 512B sector devices to 4K ones

I have a question: is it possible to make existing ext4/xfs filesystems
formatted on 512B sector devices run as is on 4k sector devices?

Problem:

We are maintaining some legacy servers whose data is stored on
ext4/xfs filesystems formatted on lvm2 raid1 devices.

These raid1 devices consist of a few iSCSI devices, so the
remote storage servers running as iSCSI targets are the actual
data storage.

  /dev/md127 --  /dev/sda  --(iSCSI)-- remote storage server
                 /dev/sdb  --(iSCSI)-- remote storage server

A problem happened when we tried to add a new storage server with
4k sector disks as an iSCSI target. After lvm2 added that iSCSI
device and started syncing the blocks from existing 512B sector
storage servers to the new 4k sector ones, we got
"Bad block number requested" messages, and soon after that,
the new device was removed from the lvm2 raid1 device.

  /dev/md127 --  /dev/sda  --(iSCSI)-- remote storage server(512)
                 /dev/sdb  --(iSCSI)-- remote storage server(512)
              *  /dev/sdc  --(iSCSI)-- remote storage server(4k)

  The combined raid1 device had been recognized as a 4k device
  as described in this article:
    https://access.redhat.com/articles/3911611

It seemed like 512B unaligned requests from the xfs filesystem
were sent to the raid1 device, and mirrored requests caused
the problem on the newly added 4k sector storage.

The xfs was formatted with its sector_size_options set to the
default (512).
See https://www.man7.org/linux/man-pages/man8/mkfs.xfs.8.html

In the case of ext4, the device continued to run, but I was not
sure if there could be any problems.

Question:

Is it possible to change the underlying storage to 4k sector ones
as written above without copying the data on the ext4/xfs
filesystems to outside of the raid1 device?

ext4: I am not seeing any apparent errors after adding the 4k
  device. Is this an expected behavior?

xfs: is it possible to change the filesystem sector size?

  I read this explanation and thought if I could change the
  journal related metadata, it might be possible.
  https://www.spinics.net/lists/linux-xfs/msg14495.html

Thanks,
  Takuya