On Tue, 18 Nov 2025, Benjamin Marzinski wrote:
> On Tue, Nov 18, 2025 at 01:00:36PM +0100, Mikulas Patocka wrote:
> >
> >
> > On Tue, 18 Nov 2025, Uladzislau Rezki wrote:
> >
> > > Hello, Mikulas!
> > >
> > > > 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?
> > > >
> > > It is 512. Whereas a physical is bigger, i.e. my device can not perform
> > > I/O by 512 granularity.
> >
> > And what is physical block size? Is it 8192?
> >
> > > As for virtual machine, i just simulated the problem so people can set
> > > it up and check. The commit message describes how it can be reproduced.
> > >
> > > The dm-ebs target which i setup does ebs to ubs conversion, so the NVME
> > > driver gets BIOs are in size and aligned to ubs size. The ubs size
> > > corresponds to the underlying physical device I/O size.
> > >
> > > So your patch does not work if logical < physical. Therefore it does
> > > not help my project.
> >
> > Logical block size is the granularity at which the device can accept I/O.
> > Physical block size is the block size on the medium.
> >
> > If logical < physical, then the device performs read-modify-write cycle
> > when writing blocks that are not aligned at physical block size.
> >
> > So, your setup is broken, because it advertises logical block size 512,
> > but it is not able to perform I/O at this granularity.
>
> This emulated nvme device is broken, but the question still remains,
> "should dm-ebs enfore writing at the alignment that was specified in its
> table line?" If you don't specify a ubs in the table line, it defaults
> to the logical block size. So, if you do specify a ubs, it stands to
> reason that you want IO at that alignment, instead of the logical-block
> size (perhaps because your device is broken, and advertises the wrong
> logical block size).
>
> So, I think that Uladzislau's patch makes sense, in addition to your
> own.
>
> -Ben
OK - I accepted Uladzislau's patch. As logical block size and physical
block size seem to be unreliable, it's better to set the size in dm-ebs.
Mikulas
