LVM and device mapper: Logical Volume device sector size

This seems to be a problem regarding the sector size of the mapped device for a logical volume between different machines.

More specifically, I'd like to know if and how the sector size of a mapped device corresponding to a logical volume can be configured.

Here is a description of the problem, comparing two machines.

Machine 1

I have an entire disk image in a logical volume mytestlv in a volume group named MyVolumeGroup.

This disk image has its own partition table (completely independent of the actual disk it's stored on).

For example, fdisk /dev/mapper/MyVolumeGroup-mytestlv shows this:

Disk /dev/mapper/MyVolumeGroup-mytestlv: 30 GiB, 32212254720 bytes, 62914560 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 4096 bytes I/O size (minimum/optimal): 4096 bytes / 4096 bytes Disklabel type: gpt Disk identifier: ... Device Start End Sectors Size Type /dev/mapper/MyVolumeGroup-mytestlv-part1 2048 4095 2048 1M BIOS boot /dev/mapper/MyVolumeGroup-mytestlv-part2 4096 4198399 4194304 2G Linux swap /dev/mapper/MyVolumeGroup-mytestlv-part3 4198400 62914526 58716127 28G Linux filesystem 

If I need access to the data on a partition of that disk image, I can use kpartx and mount the partition.

kpartx -a /dev/mapper/MyVolumeGroup-mytestlv creates these devices files, which can be used to mount a partition within that disk image, for example:

/dev/mapper/MyVolumeGroup-mytestlvl /dev/mapper/MyVolumeGroup-mytestlv2 /dev/mapper/MyVolumeGroup-mytestlv3 

Machine 2

This was now copied onto a different machine (content is exactly the same, checksums of both entire /dev/mapper/MyVolumeGroup-mytestlv are identical).

Using the configured block size, fdisk /dev/mapper/MyVolumeGroup-mytestlv shows this:

/dev/mapper/MyVolumeGroup-mytestlv: 30 GiB, 32212254720 bytes, 7864320 sectors Units: sectors of 1 * 4096 = 4096 bytes Sector size (logical/physical): 4096 bytes / 4096 bytes I/O size (minimum/optimal): 4096 bytes / 4096 bytes Disklabel type: dos Disk identifier: 0x00000000 Device Boot Start End Sectors Size Id Type /dev/mapper/MyVolumeGroup-mytestlv-part1 1 7864319 7864319 30G ee GPT 

Differences

The logical sector size is 512 on Machine 1, but 4096 on Machine 2.

On Machine 1, blockdev --getss /dev/mapper/MyVolumeGroup-mytestlv returns 512.

On Machine 2, blockdev --getss /dev/mapper/MyVolumeGroup-mytestlv returns 4096.

Workaround

On Machine 2, forcing the sector size to 512 with fdisk helps it see the partition table correctly.

fdisk --sector-size 512 /dev/mapper/MyVolumeGroup-mytestlv 

Disk /dev/mapper/MyVolumeGroup-mytestlv: 30 GiB, 32212254720 bytes, 62914560 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 4096 bytes I/O size (minimum/optimal): 4096 bytes / 4096 bytes Disklabel type: gpt Disk identifier: ... Device Start End Sectors Size Type /dev/mapper/MyVolumeGroup-mytestlv-part1 2048 4095 2048 1M BIOS boot /dev/mapper/MyVolumeGroup-mytestlv-part2 4096 4198399 4194304 2G Linux swap /dev/mapper/MyVolumeGroup-mytestlv-part3 4198400 62914526 58716127 28G Linux filesystem 

Unfortunately, other tools such as sfdisk or kpartx don't seem to have those options.

If I create a loopback device explicitly from that LV, then that loopback device has a sector size of 512, and everything works (in fact, versions of losetup that are not too old have an explicit sector size option). After losetup --show -f /dev/mapper/MyVolumeGroup-mytestlv:

• blockdev --getss /dev/loop0 returns 512
• kpartx -a /dev/loop0 creates /dev/mapper/loop0p{1,2,3}

Where is that sector size configured?

I'd like to be able to use fdisk, sfdisk, kpartx without relying on an additional losetup.

On Machine 2, cat /sys/block/dm-2/queue/hw_sector_size returns 4096 (/dev/mapper/MyVolumeGroup-mytestlv is actually a soft-link to /dev/dm-2). I've tried to change this using echo 512 > /sys/block/dm-2/queue/hw_sector_size, but this doesn't seem to be possible.

• Is there a way to tell LVM that I'd like the sector size for the device corresponding to /dev/mapper/MyVolumeGroup-mytestlv to be 512?

• Is that something that affects all the logical volumes in the group or on the system?

I can't see any options with lvcreate or lvdisplay related to this.