Re: [LSF/MM/BPF TOPIC] Large block for I/O

Luis Chamberlain
2024-03-07T07:29:41Z

Then it does seem we have everything we need already, and no changes
are needed. Because these drives *do* support the logical block size
advertised, it's just not optimal.

On Thu, Mar 07, 2024 at 04:31:48PM +1100, Dave Chinner wrote:
> Sure, doing 512 byte aligned/sized IO to a 4kB sector sizer device
> is not optimal. IO will to the file will be completely serialised
> because they are sub-fs-block DIO writes, but it does work because
> the underlying device allows it. Nobody wanting a performant
> application will want to do this,

It's a good time to ask though if there may be users who want to opt-in to
promote this sort of situation so that the logical block size is lifted
to prevent any IOs. For NVMe drives it could be where the atomic >= Indirection
Unit (IU). This is applicable even today on a 4k IU drive with 4k atomic
support. Who would want this? Since any IO issued to a drive which is
smaller than the IU implicates a RMW, it means your if you restrict the
drive to only IOs matching the IU could in theory improve endurance.

> but there are cases where this case fulfils important functional requirements.

Sure.

> e.g. fs tools and loop devices that use direct IO to access file
> based filesystem images that have 512 byte sector size will just
> work on such a fs and storage setup, even though the host filesystem
> isn't configured to use 512 byte sector alignment directly
> itself....

It would seem like quite a bit of things. This is useful, thanks.

Luis