CONFIG_BCACHE is tristate, so dm-pcache can also be built-in.
Switch the Makefile to use obj-$(CONFIG_DM_PCACHE) so the target can be
linked into vmlinux instead of always being a loadable module.
Also rename cache_flush() to pcache_cache_flush() to avoid a global
symbol clash with sunrpc/cache.c's cache_flush().
Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Change the 'ret' variable from u32 to int to store negative error codes or
zero returned by cache_kset_close().
Storing the negative error codes in unsigned type, doesn't cause an issue
at runtime but it's ugly. Additionally, assigning negative error codes to
unsigned type may trigger a GCC warning when the -Wsign-conversion flag
is enabled.
No effect on runtime.
Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
This patch introduces dm-pcache, a new DM target that places a DAX-
capable persistent-memory device in front of any slower block device and
uses it as a high-throughput, low-latency cache.
Design highlights
-----------------
- DAX data path – data is copied directly between DRAM and the pmem
mapping, bypassing the block layer’s overhead.
- Segmented, crash-consistent layout
- all layout metadata are dual-replicated CRC-protected.
- atomic kset flushes; key replay on mount guarantees cache integrity
even after power loss.
- Striped multi-tree index
- Multi‑tree indexing for high parallelism.
- overlap-resolution logic ensures non-intersecting cached extents.
- Background services
- write-back worker flushes dirty keys in order, preserving backing-device
crash consistency. This is important for checkpoint in cloud storage.
- garbage collector reclaims clean segments when utilisation exceeds a
tunable threshold.
- Data integrity – optional CRC32 on cached payload; metadata always protected.
Comparison with existing block-level caches
---------------------------------------------------------------------------------------------------------------------------------
| Feature | pcache (this patch) | bcache | dm-writecache |
|----------------------------------|---------------------------------|------------------------------|---------------------------|
| pmem access method | DAX | bio (block I/O) | DAX |
| Write latency (4 K rand-write) | ~5 µs | ~20 µs | ~5 µs |
| Concurrency | multi subtree index | global index tree | single tree + wc_lock |
| IOPS (4K randwrite, 32 numjobs) | 2.1 M | 352 K | 283 K |
| Read-cache support | YES | YES | NO |
| Deployment | no re-format of backend | backend devices must be | no re-format of backend |
| | | reformatted | |
| Write-back ordering | log-structured; | no ordering guarantee | no ordering guarantee |
| | preserves app-IO-order | | |
| Data integrity checks | metadata + data CRC(optional) | metadata CRC only | none |
---------------------------------------------------------------------------------------------------------------------------------
Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
