SSD maximal performance : native sector size, partition alignment

User Support Hardware Issues
30

As far as I understand, the reason why 4K dm-crypt breaks some VM volumes on LVM Thin but not on Btrfs is a combination of two things.

  1. LVM Thin uses the same logical sector size as the underlying (dm-crypt) block device. And then a 4K LVM Thin block device in dom0 results in a 4K xen-blkfront block device in the VM, because Xen automatically passes through the logical sector size.

    Whereas file-reflink layouts like Btrfs use loop devices, which are currently always configured (by /etc/xen/scripts/block from Xen upstream) with 512 byte logical sectors - again passed through to the VM.

  2. The “root” xvda and “volatile” xvdc volumes don’t properly work with 4K sectors because they are disk images containing a GPT/MBR partition table, which can only specify sizes and locations in sector units:

    • The VM initramfs script formatting “volatile” on every VM start currently assumes that a sector is 512 bytes, which should be straightforward to fix (WIP)

    • It’s going to be more difficult to somehow make the “root” volume sector-size agnostic…

    (The “private” xvdb and “kernel” xvdd volumes seem to work fine if /etc/xen/scripts/block is patched to configure them with 4K sectors. They’re just ext4/ext3 filesystem images without a partition table.)

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