Re: [RFC PATCH 00/11] pcache: Persistent Memory Cache for Block Devices

[Dan Williams]

Dongsheng Yang wrote:
> Hi All,
> 
>     This patchset introduces a new Linux block layer module called
> **pcache**, which uses persistent memory (pmem) as a cache for block
> devices.
> 
> Originally, this functionality was implemented as `cbd_cache` within the
> CBD (CXL Block Device). However, after thorough consideration,
> it became clear that the cache design was not limited to CBD's pmem
> device or infrastructure. Instead, it is broadly applicable to **any**
> persistent memory device that supports DAX. Therefore, I have split
> pcache out of cbd and refactored it into a standalone module.
> 
> Although Intel's Optane product line has been discontinued, the Storage
> Class Memory (SCM) field continues to evolve. For instance, Numemory
> recently launched their Optane successor product, the NM101 SCM:
> https://www.techpowerup.com/332914/numemory-releases-optane-successor-nm101-storage-class-memory
> 
> ### About pcache
> 
> +-------------------------------+------------------------------+------------------------------+------------------------------+
> | Feature                       | pcache                       | bcache                       | dm-writecache                |
> +-------------------------------+------------------------------+------------------------------+------------------------------+
> | pmem access method            | DAX                          | bio                          | DAX                          |
> +-------------------------------+------------------------------+------------------------------+------------------------------+
> | Write Latency (4K randwrite)  | ~7us                         | ~20us                        | ~7us                         |
> +-------------------------------+------------------------------+------------------------------+------------------------------+
> | Concurrency                   | Multi-tree per backend,      | Shared global index tree,    | single indexing tree and     |
> |                               | fully utilizing pmem         |                              | global wc_lock               |
> +-------------------------------+------------------------------+------------------------------+------------------------------+
> | IOPS (4K randwrite 32 numjobs)| 2107K                        | 352K                         | 283K                         |
> +-------------------------------+------------------------------+------------------------------+------------------------------+
> | Read Cache Support            | YES                          | YES                          | NO                           |
> +-------------------------------+------------------------------+------------------------------+------------------------------+
> | Deployment Flexibility        | No reformat needed           | Requires formatting backend  | Depends on dm framework,     |
> |                               |                              | devices                      | less intuitive to deploy     |
> +-------------------------------+------------------------------+------------------------------+------------------------------+
> | Writeback Model               | log-structure; preserves     | no guarantee between         | no guarantee writeback       |
> |                               | backing crash-consistency;   | flush order and app IO order;| ordering                     |
> |                               | important for checkpoint     | may lose ordering in backing |                              |
> +-------------------------------+------------------------------+------------------------------+------------------------------+
> | Data Integrity                | CRC on both metadata and     | CRC on metadata only         | No CRC                       |
> |                               | data (data crc is optional)  |                              |                              |
> +-------------------------------+------------------------------+------------------------------+------------------------------+

Thanks for making the comparison chart. The immediate question this
raises is why not add "multi-tree per backend", "log structured
writeback", "readcache", and "CRC" support to dm-writecache?
device-mapper is everywhere, has a long track record, and enhancing it
immediately engages a community of folks in this space.

Then reviewers can spend the time purely on the enhancements and not
reviewing a new block device-management stacking ABI.