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1d57628ff9
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>
68 lines
1.8 KiB
C
68 lines
1.8 KiB
C
/* SPDX-License-Identifier: GPL-2.0-or-later */
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#ifndef _DM_PCACHE_H
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#define _DM_PCACHE_H
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#include <linux/device-mapper.h>
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#include "../dm-core.h"
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#define CACHE_DEV_TO_PCACHE(cache_dev) (container_of(cache_dev, struct dm_pcache, cache_dev))
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#define BACKING_DEV_TO_PCACHE(backing_dev) (container_of(backing_dev, struct dm_pcache, backing_dev))
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#define CACHE_TO_PCACHE(cache) (container_of(cache, struct dm_pcache, cache))
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#define PCACHE_STATE_RUNNING 1
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#define PCACHE_STATE_STOPPING 2
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struct pcache_cache_dev;
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struct pcache_backing_dev;
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struct pcache_cache;
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struct pcache_cache_options;
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struct dm_pcache {
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struct dm_target *ti;
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struct pcache_cache_dev cache_dev;
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struct pcache_backing_dev backing_dev;
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struct pcache_cache cache;
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struct pcache_cache_options opts;
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spinlock_t defered_req_list_lock;
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struct list_head defered_req_list;
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struct workqueue_struct *task_wq;
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struct work_struct defered_req_work;
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atomic_t state;
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atomic_t inflight_reqs;
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wait_queue_head_t inflight_wq;
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};
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static inline bool pcache_is_stopping(struct dm_pcache *pcache)
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{
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return (atomic_read(&pcache->state) == PCACHE_STATE_STOPPING);
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}
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#define pcache_dev_err(pcache, fmt, ...) \
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pcache_err("%s " fmt, pcache->ti->table->md->name, ##__VA_ARGS__)
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#define pcache_dev_info(pcache, fmt, ...) \
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pcache_info("%s " fmt, pcache->ti->table->md->name, ##__VA_ARGS__)
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#define pcache_dev_debug(pcache, fmt, ...) \
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pcache_debug("%s " fmt, pcache->ti->table->md->name, ##__VA_ARGS__)
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struct pcache_request {
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struct dm_pcache *pcache;
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struct bio *bio;
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u64 off;
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u32 data_len;
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struct kref ref;
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int ret;
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struct list_head list_node;
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};
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void pcache_req_get(struct pcache_request *pcache_req);
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void pcache_req_put(struct pcache_request *pcache_req, int ret);
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void pcache_defer_reqs_kick(struct dm_pcache *pcache);
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#endif /* _DM_PCACHE_H */
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