Find Allocation Unit Size

Find Allocation Unit Size

[Marc Smith]

Hi,

I'm testing some NVMe drives, and playing with creating multiple
namespaces (via the nvme CLI tool). I've noticed with some values,
there is rounding that is occurring (or so I assume). I'm using a 4096
block size, and with values like 200 GiB or 1000 GiB I do not see the
rounding occur (checking value in bytes against unvmcap after
creation).

But other values do seem to round... I suspect this is aligning to the
allocation unit size, but I'm not sure where to find that value for my
NVMe drives? Hoping someone could point me in the right direction!

Thanks in advance.


--Marc

Find Allocation Unit Size

[Keith Busch]

On Fri, Dec 14, 2018@01:39:42AM -0500, Marc Smith wrote:
> Hi,
> 
> I'm testing some NVMe drives, and playing with creating multiple
> namespaces (via the nvme CLI tool). I've noticed with some values,
> there is rounding that is occurring (or so I assume). I'm using a 4096
> block size, and with values like 200 GiB or 1000 GiB I do not see the
> rounding occur (checking value in bytes against unvmcap after
> creation).
> 
> But other values do seem to round... I suspect this is aligning to the
> allocation unit size, but I'm not sure where to find that value for my
> NVMe drives? Hoping someone could point me in the right direction!
> 
> Thanks in advance.

The spec says a device may have a minimal size granularity for creating
namespaces, but currently doesn't provide a way to discover what that is.`

Find Allocation Unit Size

[Marc Smith]

On Fri, Dec 14, 2018@9:44 AM Keith Busch <keith.busch@intel.com> wrote:
>
> On Fri, Dec 14, 2018@01:39:42AM -0500, Marc Smith wrote:
> > Hi,
> >
> > I'm testing some NVMe drives, and playing with creating multiple
> > namespaces (via the nvme CLI tool). I've noticed with some values,
> > there is rounding that is occurring (or so I assume). I'm using a 4096
> > block size, and with values like 200 GiB or 1000 GiB I do not see the
> > rounding occur (checking value in bytes against unvmcap after
> > creation).
> >
> > But other values do seem to round... I suspect this is aligning to the
> > allocation unit size, but I'm not sure where to find that value for my
> > NVMe drives? Hoping someone could point me in the right direction!
> >
> > Thanks in advance.
>
> The spec says a device may have a minimal size granularity for creating
> namespaces, but currently doesn't provide a way to discover what that is.`

Interesting... do you believe that is something that the device
manufacturer can (or will) provide? Or maybe something in the device
spec sheet (going to check that now...)?

Thanks for the quick response.

--Marc