Hi What is the logical_block_size of the underlying nvme device? - i.e. what's the content of this file /sys/block/nvme0n1/queue/logical_block_size in the virtual machine?
Mikulas On Mon, 17 Nov 2025, Uladzislau Rezki (Sony) wrote: > When performing a read-modify-write(RMW) operation, any modification > to a buffered block must cause the entire buffer to be marked dirty. > > Marking only a subrange as dirty is incorrect because the underlying > device block size(ubs) defines the minimum read/write granularity. A > lower device can perform I/O only on regions which are fully aligned > and sized to ubs. > > This change ensures that write-back operations always occur in full > ubs-sized chunks, matching the intended emulation semantics of the > EBS target. > > As for user space visible impact, submitting sub-ubs and misaligned > I/O for devices which are tuned to ubs sizes only, will reject such > requests, therefore it can lead to losing data. Example: > > 1) Create a 8K nvme device in qemu by adding > > -device > nvme,drive=drv0,serial=foo,logical_block_size=8192,physical_block_size=8192 > > 2) Setup dm-ebs to emulate 512B to 8K mapping > > urezki@pc638:~/bin$ cat dmsetup.sh > > lower=/dev/nvme0n1 > len=$(blockdev --getsz "$lower") > > echo "0 $len ebs $lower 0 1 16" | dmsetup create nvme-8k > urezki@pc638:~/bin$ > > offset 0, ebs=1 and ubs=16(in sectors). > > 3) Create an ext4 filesystem(default 4K block size) > > urezki@pc638:~/bin$ sudo mkfs.ext4 -F /dev/dm-0 > mke2fs 1.47.0 (5-Feb-2023) > Discarding device blocks: done > Creating filesystem with 2072576 4k blocks and 518144 inodes > Filesystem UUID: bd0b6ca6-0506-4e31-86da-8d22c9d50b63 > Superblock backups stored on blocks: > 32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632 > > Allocating group tables: done > Writing inode tables: done > Creating journal (16384 blocks): done > Writing superblocks and filesystem accounting information: mkfs.ext4: > Input/output error while writing out and closing file system > urezki@pc638:~/bin$ dmesg > > <snip> > [ 1618.875449] buffer_io_error: 1028 callbacks suppressed > [ 1618.875456] Buffer I/O error on dev dm-0, logical block 0, lost async page > write > [ 1618.875527] Buffer I/O error on dev dm-0, logical block 1, lost async page > write > [ 1618.875602] Buffer I/O error on dev dm-0, logical block 2, lost async page > write > [ 1618.875620] Buffer I/O error on dev dm-0, logical block 3, lost async page > write > [ 1618.875639] Buffer I/O error on dev dm-0, logical block 4, lost async page > write > [ 1618.894316] Buffer I/O error on dev dm-0, logical block 5, lost async page > write > [ 1618.894358] Buffer I/O error on dev dm-0, logical block 6, lost async page > write > [ 1618.894380] Buffer I/O error on dev dm-0, logical block 7, lost async page > write > [ 1618.894405] Buffer I/O error on dev dm-0, logical block 8, lost async page > write > [ 1618.894427] Buffer I/O error on dev dm-0, logical block 9, lost async page > write > <snip> > > Many I/O errors because the lower 8K device rejects sub-ubs/misaligned > requests. > > with a patch: > > urezki@pc638:~/bin$ sudo mkfs.ext4 -F /dev/dm-0 > mke2fs 1.47.0 (5-Feb-2023) > Discarding device blocks: done > Creating filesystem with 2072576 4k blocks and 518144 inodes > Filesystem UUID: 9b54f44f-ef55-4bd4-9e40-c8b775a616ac > Superblock backups stored on blocks: > 32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632 > > Allocating group tables: done > Writing inode tables: done > Creating journal (16384 blocks): done > Writing superblocks and filesystem accounting information: done > > urezki@pc638:~/bin$ sudo mount /dev/dm-0 /mnt/ > urezki@pc638:~/bin$ ls -al /mnt/ > total 24 > drwxr-xr-x 3 root root 4096 Oct 17 15:13 . > drwxr-xr-x 19 root root 4096 Jul 10 19:42 .. > drwx------ 2 root root 16384 Oct 17 15:13 lost+found > urezki@pc638:~/bin$ > > After this change: mkfs completes; mount succeeds. > > v1 -> v2: > - reflect a user space visible impact in the commit message. > > Signed-off-by: Uladzislau Rezki (Sony) <[email protected]> > --- > drivers/md/dm-ebs-target.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/md/dm-ebs-target.c b/drivers/md/dm-ebs-target.c > index 6abb31ca9662..b354e74a670e 100644 > --- a/drivers/md/dm-ebs-target.c > +++ b/drivers/md/dm-ebs-target.c > @@ -103,7 +103,7 @@ static int __ebs_rw_bvec(struct ebs_c *ec, enum req_op > op, struct bio_vec *bv, > } else { > flush_dcache_page(bv->bv_page); > memcpy(ba, pa, cur_len); > - dm_bufio_mark_partial_buffer_dirty(b, buf_off, > buf_off + cur_len); > + dm_bufio_mark_buffer_dirty(b); > } > > dm_bufio_release(b); > -- > 2.47.3 >
