> From: Dan Williams <dan.j.willi...@intel.com> > Sent: Tuesday, February 5, 2019 9:12 AM > On Tue, Feb 5, 2019 at 8:53 AM Dexuan Cui <de...@microsoft.com> wrote: > > > > > From: Dan Williams <dan.j.willi...@intel.com> > > > Sent: Sunday, February 3, 2019 11:14 AM > > > > ... > > > > As I understand, the essence of the issue is: Hyper-V emulates the > > > > label mechanism (i.e. it supports _LSI and LSR), but doesn't do it > > > > right (i.e. it doesn't support _LSW). > > > > > > > > To manage the namespaces, Linux can choose to use label or not. > > > > > > > > If _LSI/_LSR are supported, Linux assumes _LSW is supported as well > > > > and chooses to use label (i.e. the label mode), but since Hyper-V > > > > doesn't > > > > support _LSW, Linux fails to change the namespace configuration. > > > > > > No, that's not quite right. The reason Linux does not see the fsdax > > > mode configuration is due to the missing "address abstraction GUID" in > > > the label produced the default Hyper-V configuration. > > Hi Dan, > > Do you mean NVDIMM_DAX_GUID? > > Correct. FYI: in create_namespace_pmem(), label0->abstraction_guid is guid_null in the case of Hyper-V NVDIMM. It looks Hyper-V does't use the guid.
> > > In label-less mode there is no "address abstraction GUID" to validate so > > > it falls back to just using the info-block directly. > > In the case of not using label storage area, as I understand the info-block > > resides in regular data storage area. Can you please tell me where exactly > > the info-block is and how its location is decided? > > And I suppose the info-block contains the NVDIMM_DAX_GUID? > > The info-block always lives in the data-storage area, even with > labels. It's placed at namespace base address + 4K. > > When labels are present the label gives the namespace a uuid and the > info-block "parent uuid" field must match that value. Without labels > the "parent uuid" field is unused / filled with zero's. Also with v1.2 > labels the address abstraction uuid must match the info-block type. Thanks for elaborating on this! It looks the label mechanism is for advanced usages, e.g., a PMEM namespace can consist of multiple NVDIMMs, or a namespace only uses part of the capacity of a NVDIMM, or a NVDIMM contains both PMEM and Block-Mode namespaces. For a simple usage, e.g. if a NVDIMM only contains one PMEM namespace which consumes all the storage space of the NVDIMM, it looks the namespace can be normally used with the help of the info-block, and we don't really need the label. IMO this is the case of Hyper-V, i.e. we don't use the label at all, since _LSW is not implemented. > > I'm asking because I found I lose ~500MBytes of the 32GBytes Hyper-V > > NVDIMM device, when the namespace is in fsdax mode. When it's in > > raw mode, I'm able to use all of the 32GB space. > > Yes. A portion of the capacity is reserved for page structures. Got it. It looks the size of the info-block (and the related padding?) is 2MB, and "the overhead is 64-bytes per 4K (16GB per 1TB) on x86", so the total overhead in my 32GB case is: 2MB + 32GB/(4096/64) = 514MB. Thanks for sharing the link! Now I realized we can use the -M parameter to not store the page metadata in the NVDIMM: -M; --map= A pmem namespace in “fsdax” or “devdax” mode requires allocation of per-page metadata. The allocation can be drawn from either: “mem”: typical system memory “dev”: persistent memory reserved from the namespace Thanks, --Dexuan
