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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>
75 lines
1.9 KiB
C
75 lines
1.9 KiB
C
/* SPDX-License-Identifier: GPL-2.0-or-later */
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#ifndef _PCACHE_SEGMENT_H
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#define _PCACHE_SEGMENT_H
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#include <linux/bio.h>
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#include <linux/bitfield.h>
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#include "pcache_internal.h"
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struct pcache_segment_info {
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struct pcache_meta_header header;
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__u32 flags;
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__u32 next_seg;
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};
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#define PCACHE_SEG_INFO_FLAGS_HAS_NEXT BIT(0)
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#define PCACHE_SEG_INFO_FLAGS_TYPE_MASK GENMASK(4, 1)
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#define PCACHE_SEGMENT_TYPE_CACHE_DATA 1
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static inline bool segment_info_has_next(struct pcache_segment_info *seg_info)
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{
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return (seg_info->flags & PCACHE_SEG_INFO_FLAGS_HAS_NEXT);
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}
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static inline void segment_info_set_type(struct pcache_segment_info *seg_info, u8 type)
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{
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seg_info->flags &= ~PCACHE_SEG_INFO_FLAGS_TYPE_MASK;
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seg_info->flags |= FIELD_PREP(PCACHE_SEG_INFO_FLAGS_TYPE_MASK, type);
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}
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static inline u8 segment_info_get_type(struct pcache_segment_info *seg_info)
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{
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return FIELD_GET(PCACHE_SEG_INFO_FLAGS_TYPE_MASK, seg_info->flags);
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}
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struct pcache_segment_pos {
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struct pcache_segment *segment; /* Segment associated with the position */
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u32 off; /* Offset within the segment */
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};
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struct pcache_segment_init_options {
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u8 type;
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u32 seg_id;
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u32 data_off;
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struct pcache_segment_info *seg_info;
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};
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struct pcache_segment {
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struct pcache_cache_dev *cache_dev;
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void *data;
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u32 data_size;
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u32 seg_id;
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struct pcache_segment_info *seg_info;
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};
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int segment_copy_to_bio(struct pcache_segment *segment,
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u32 data_off, u32 data_len, struct bio *bio, u32 bio_off);
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int segment_copy_from_bio(struct pcache_segment *segment,
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u32 data_off, u32 data_len, struct bio *bio, u32 bio_off);
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static inline void segment_pos_advance(struct pcache_segment_pos *seg_pos, u32 len)
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{
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BUG_ON(seg_pos->off + len > seg_pos->segment->data_size);
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seg_pos->off += len;
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}
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void pcache_segment_init(struct pcache_cache_dev *cache_dev, struct pcache_segment *segment,
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struct pcache_segment_init_options *options);
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#endif /* _PCACHE_SEGMENT_H */
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