RFC Request: NVMe backend support in dm-pcache

RFC Request: NVMe backend support in dm-pcache

[Giovanni Francesco]

Hello,

I would like to ask for consideration of adding NVMe disk support to the caching layer in `dm-pcache`. At present, `cache_dev` only supports `pmem`.

From my perspective, extending `pcache` to support NVMe devices (along with an LRU-based eviction policy) could make it a strong alternative to `bcache` in certain use cases. For example, `pcache` appears to work without requiring the backing storage devices to be reformatted, which I see as a practical advantage.

For my home server workloads, I experimented with `lvmcache`, but I found both writeback and read performance unsatisfactory compared to what I was hoping for. My test notes and benchmarks are available here for anyone interested:
https://github.com/TheLinuxGuy/ugreen-nas/tree/main/experiments-bench

Would adding NVMe support (and potentially configurable caching policies like LRU) be of interest to the project? I am happy to assist with testing or provide more detailed feedback from my benchmarks if that would be useful.

Thanks for your time and consideration.
Giovanni

Re: RFC Request: NVMe backend support in dm-pcache

[Dongsheng Yang]

Hi,

     Thanx for your information. Yes, pcache currently only allow pmem 
to be cache_dev.


(1) I would suggest you to do some testing on pmem firstly.

You can use memmap kernel option to emulate pmem device:

https://docs.pmem.io/persistent-memory/getting-started-guide/creating-development-environments/linux-environments/linux-memmap

Then you can try dm-pcache, to see is that what you want.

Of course, if you want to do some performance compare, remember to use 
pmem for bcache too, /dev/pmem0 is also working to be cache device for 
bcache.


(2) I would suggest you add more io pattern in you testing script:

iodepth=1, numjobs=1 for latency when workload is not heavy but it want 
ack from device ASAP. That's important in lots of latency sensitive 
application.

iodepth=1, numjobs=32 or 16. multiple application are running on a large 
disk.


(3) LRU is already on the TODO list 
in Documentation/admin-guide/device-mapper/dm-pcache.rst


(4) NVME backend is not impossible, but my current high-priority task is 
to stabilize dm-pcache and ensure it runs well in the pmem scenario. 
Then, consideration may be given to the nvme backend.


Thanx

Dongsheng

在 8/21/2025 3:15 AM, Giovanni Francesco 写道:
> Hello,
>
> I would like to ask for consideration of adding NVMe disk support to the caching layer in `dm-pcache`. At present, `cache_dev` only supports `pmem`.
>
>  From my perspective, extending `pcache` to support NVMe devices (along with an LRU-based eviction policy) could make it a strong alternative to `bcache` in certain use cases. For example, `pcache` appears to work without requiring the backing storage devices to be reformatted, which I see as a practical advantage.
>
> For my home server workloads, I experimented with `lvmcache`, but I found both writeback and read performance unsatisfactory compared to what I was hoping for. My test notes and benchmarks are available here for anyone interested:
> https://github.com/TheLinuxGuy/ugreen-nas/tree/main/experiments-bench
>
> Would adding NVMe support (and potentially configurable caching policies like LRU) be of interest to the project? I am happy to assist with testing or provide more detailed feedback from my benchmarks if that would be useful.
>
> Thanks for your time and consideration.
> Giovanni