Solved: In memory mode, the Optane memory is not persistent. How is the memory not persistent? I thought it was naturally persistent by virtue of its properti...

MWoga · ‎11-07-2019

Emeth_O_Intel · ‎11-09-2019

Hi,

Intel® persistent memory is an emerging technology where non-volatile media is placed onto a Dual In-Line Memory Module (DIMM) and installed on the memory bus. Persistent memory DIMMs exist alongside of traditional volatile memory DRAM DIMMs.

The key thing that differentiates persistent memory DIMMs from DRAM DIMMs is that the data stored on them can be retained when the system is shut down or loses power.

This allows them to be used as a form of permanent storage like Hard Disk Drives (HDDs) or Solid State Drives (SSDs), but with memory-like latencies.

Intel® Optane™ DC Persistent Memory (DCPMM) is based on the Intel® Optane™ Memory technology and provides the ability to keep more data, closer to the CPU for faster processing (that is, “warmer”). DCPMM is designed for use with Intel® Xeon® Scalable Platform processors code named Cascade Lake.

DCPMMs exist alongside DDR4 DRAM DIMMs to support a two-level memory architecture. The fastest tier is the DRAM-based “near memory” and the slower tier is DCPMM “far memory”. DCPMM can be configured for use in different modes: Memory Mode, App Direct Mode, or a combination of Memory Mode and App Direct Mode, which is referred to as “Mixed Mode”.

In Memory Mode, DCPMMs act as volatile system memory under the control of the operating system. Any DRAM in the platform will act as a cache working in conjunction with the DCPMMs.

In App Direct Mode, DCPMMs and DRAM DIMMs act as independent memory resources under direct load/store control of the application. This allows the DCPMM capacity to be used as byte-addressable persistent memory that is mapped into the system physical address space (SPA) and directly accessible by applications.

In Mixed Mode, a percentage of the DCPMM capacity is used in Memory Mode and the remainder in App Direct Mode.

App Direct Mode capacity can be used as Block over App Direct, in which case, the driver surfaces traditional block storage interface transparent to applications so they do not need to be modified. Block over App Direct is implemented with copy on write optimization via a block translation table to provide power-fail write atomicity.

Otherwise, applications can be modified to access App Direct capacity with direct load/store mechanisms using a persistent memory aware file system. This completely bypasses the kernel and provides the shortest code path to the persistent memory. To learn more about using and programming for persistent memory, refer to the http://pmem.io/.

On the other hand, could you please so kind and provide to me with more details of this implementation or project that you are trying to achieve?

I will be waiting for your response in order to proceed with the next step.

Emeth O.

Intel Customer Support Technician

A Contingent Worker at Intel

View solution in original post

Emeth_O_Intel · ‎11-07-2019

Hello,

Thank you for contacting Intel Optane DC Persistent Memory Community Support.

I would like to gather more details from your system; could you please so kind and share with me the following outputs:

A. ">ipmctl show -topology"

B. ">ipmctl show -firmware"

C. ">ipmctl show -dimm"

I will be waiting for your outcome in order to figure out the root cause of your issue.

Have a wonderful day,

Regards,

Emeth O.

Intel Customer Support Technician.

Under Contract to Intel Corporation.

MWoga · ‎11-07-2019

This is more of a general understanding than a problem. My understanding is that for Optane DIMMS (not SSDs), you can operate in two modes: memory or application mode. In application mode, the application uses DAX to directly attach to storage, bypassing the file system. In memory mode, the operating system treats the Optane DIMM as main memory. So, instead of using DRAM, it uses Optane. However, in memory mode, the Optane memory is not persistent. And, there is still DRAM used for caching. It is these last two points that confuse me. 1. How is the memory not persistent? I thought it was naturally persistent by virtue of its properties? e.g., if it is a GST material (not saying it is), then isn't it just persistent because of the nature of that material? 2. How does the DRAM caching work? Is that all done in hardware without the support of the OS?

Emeth_O_Intel · ‎11-09-2019