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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>
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| .. | ||
| cache-policies.rst | ||
| cache.rst | ||
| delay.rst | ||
| dm-clone.rst | ||
| dm-crypt.rst | ||
| dm-dust.rst | ||
| dm-ebs.rst | ||
| dm-flakey.rst | ||
| dm-ima.rst | ||
| dm-init.rst | ||
| dm-integrity.rst | ||
| dm-io.rst | ||
| dm-log.rst | ||
| dm-pcache.rst | ||
| dm-queue-length.rst | ||
| dm-raid.rst | ||
| dm-service-time.rst | ||
| dm-uevent.rst | ||
| dm-zoned.rst | ||
| era.rst | ||
| index.rst | ||
| kcopyd.rst | ||
| linear.rst | ||
| log-writes.rst | ||
| persistent-data.rst | ||
| snapshot.rst | ||
| statistics.rst | ||
| striped.rst | ||
| switch.rst | ||
| thin-provisioning.rst | ||
| unstriped.rst | ||
| vdo-design.rst | ||
| vdo.rst | ||
| verity.rst | ||
| writecache.rst | ||
| zero.rst | ||