On Wed, Jan 9, 2019 at 7:39 PM james harvey <jamespharvey20@xxxxxxxxx> wrote: > Q2 - Is it correct that the blocks of size discard_granularity sent to > dm/LVM need to be aligned from the start of the volume, rather than > the start of the partition? ... This is probably what discard_alignment is for. # lvcreate --size 1G --chunksize 128MB --thin lvm/thinpoolntfs /dev/nvme0n1p4 # lvcreate --virtualsize 256M --thin lvm/thinpoolntfs --name ntfs # fdisk /dev/lvm/ntfs GPT New, Partition number 1, First sector 2048 (default was 32 sectors of 512 bytes?), +128M # kpartx -a /dev/lvm/ntfs # dmsetup ls | grep ntfs lvm-ntfs (254:16) lvm-ntfs1 (254:17) $ cat /sys/dev/block/254:16/discard_alignment 0 # makes sense, this is the volume itself $ cat /sys/dev/block/254:17/discard_alignment 133169152 That's equal to size of the volume (128 * 1024 * 1024) minus the start of the partition (2048 * 512 byte sector size). discard_alignment is described at https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-block as: "Devices that support discard functionality may internally allocate space in units that are bigger than the exported logical block size. The discard_alignment parameter indicates how many bytes the beginning of the device is offset from the internal allocation unit's natural alignment." So, is that value supposed to be how many bytes are LEFT in the first discard granularity block? Meaning, the fs should treat this many bytes as non-discardable? Instead of this value giving how many bytes INTO the first discard granularity block the partition starts? -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
