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diff --git a/Documentation/admin-guide/device-mapper/dm-pcache.rst b/Documentation/admin-guide/device-mapper/dm-pcache.rst new file mode 100644 index 000000000000..09d327ef4b14 --- /dev/null +++ b/Documentation/admin-guide/device-mapper/dm-pcache.rst @@ -0,0 +1,202 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================= +dm-pcache — Persistent Cache +================================= + +*Author: Dongsheng Yang <dongsheng.yang@linux.dev>* + +This document describes *dm-pcache*, a Device-Mapper target that lets a +byte-addressable *DAX* (persistent-memory, “pmem”) region act as a +high-performance, crash-persistent cache in front of a slower block +device. The code lives in `drivers/md/dm-pcache/`. + +Quick feature summary +===================== + +* *Write-back* caching (only mode currently supported). +* *16 MiB segments* allocated on the pmem device. +* *Data CRC32* verification (optional, per cache). +* Crash-safe: every metadata structure is duplicated (`PCACHE_META_INDEX_MAX + == 2`) and protected with CRC+sequence numbers. +* *Multi-tree indexing* (indexing trees sharded by logical address) for high PMem parallelism +* Pure *DAX path* I/O – no extra BIO round-trips +* *Log-structured write-back* that preserves backend crash-consistency + + +Constructor +=========== + +:: + + pcache <cache_dev> <backing_dev> [<number_of_optional_arguments> <cache_mode writeback> <data_crc true|false>] + +========================= ==================================================== +``cache_dev`` Any DAX-capable block device (``/dev/pmem0``…). + All metadata *and* cached blocks are stored here. + +``backing_dev`` The slow block device to be cached. + +``cache_mode`` Optional, Only ``writeback`` is accepted at the + moment. + +``data_crc`` Optional, default to ``false`` + + * ``true`` – store CRC32 for every cached entry + and verify on reads + * ``false`` – skip CRC (faster) +========================= ==================================================== + +Example +------- + +.. code-block:: shell + + dmsetup create pcache_sdb --table \ + "0 $(blockdev --getsz /dev/sdb) pcache /dev/pmem0 /dev/sdb 4 cache_mode writeback data_crc true" + +The first time a pmem device is used, dm-pcache formats it automatically +(super-block, cache_info, etc.). + + +Status line +=========== + +``dmsetup status <device>`` (``STATUSTYPE_INFO``) prints: + +:: + + <sb_flags> <seg_total> <cache_segs> <segs_used> \ + <gc_percent> <cache_flags> \ + <key_head_seg>:<key_head_off> \ + <dirty_tail_seg>:<dirty_tail_off> \ + <key_tail_seg>:<key_tail_off> + +Field meanings +-------------- + +=============================== ============================================= +``sb_flags`` Super-block flags (e.g. endian marker). + +``seg_total`` Number of physical *pmem* segments. + +``cache_segs`` Number of segments used for cache. + +``segs_used`` Segments currently allocated (bitmap weight). + +``gc_percent`` Current GC high-water mark (0-90). + +``cache_flags`` Bit 0 – DATA_CRC enabled + Bit 1 – INIT_DONE (cache initialised) + Bits 2-5 – cache mode (0 == WB). + +``key_head`` Where new key-sets are being written. + +``dirty_tail`` First dirty key-set that still needs + write-back to the backing device. + +``key_tail`` First key-set that may be reclaimed by GC. +=============================== ============================================= + + +Messages +======== + +*Change GC trigger* + +:: + + dmsetup message <dev> 0 gc_percent <0-90> + + +Theory of operation +=================== + +Sub-devices +----------- + +==================== ========================================================= +backing_dev Any block device (SSD/HDD/loop/LVM, etc.). +cache_dev DAX device; must expose direct-access memory. +==================== ========================================================= + +Segments and key-sets +--------------------- + +* The pmem space is divided into *16 MiB segments*. +* Each write allocates space from a per-CPU *data_head* inside a segment. +* A *cache-key* records a logical range on the origin and where it lives + inside pmem (segment + offset + generation). +* 128 keys form a *key-set* (kset); ksets are written sequentially in pmem + and are themselves crash-safe (CRC). +* The pair *(key_tail, dirty_tail)* delimit clean/dirty and live/dead ksets. + +Write-back +---------- + +Dirty keys are queued into a tree; a background worker copies data +back to the backing_dev and advances *dirty_tail*. A FLUSH/FUA bio from the +upper layers forces an immediate metadata commit. + +Garbage collection +------------------ + +GC starts when ``segs_used >= seg_total * gc_percent / 100``. It walks +from *key_tail*, frees segments whose every key has been invalidated, and +advances *key_tail*. + +CRC verification +---------------- + +If ``data_crc is enabled`` dm-pcache computes a CRC32 over every cached data +range when it is inserted and stores it in the on-media key. Reads +validate the CRC before copying to the caller. + + +Failure handling +================ + +* *pmem media errors* – all metadata copies are read with + ``copy_mc_to_kernel``; an uncorrectable error logs and aborts initialisation. +* *Cache full* – if no free segment can be found, writes return ``-EBUSY``; + dm-pcache retries internally (request deferral). +* *System crash* – on attach, the driver replays ksets from *key_tail* to + rebuild the in-core trees; every segment’s generation guards against + use-after-free keys. + + +Limitations & TODO +================== + +* Only *write-back* mode; other modes planned. +* Only FIFO cache invalidate; other (LRU, ARC...) planned. +* Table reload is not supported currently. +* Discard planned. + + +Example workflow +================ + +.. code-block:: shell + + # 1. Create devices + dmsetup create pcache_sdb --table \ + "0 $(blockdev --getsz /dev/sdb) pcache /dev/pmem0 /dev/sdb 4 cache_mode writeback data_crc true" + + # 2. Put a filesystem on top + mkfs.ext4 /dev/mapper/pcache_sdb + mount /dev/mapper/pcache_sdb /mnt + + # 3. Tune GC threshold to 80 % + dmsetup message pcache_sdb 0 gc_percent 80 + + # 4. Observe status + watch -n1 'dmsetup status pcache_sdb' + + # 5. Shutdown + umount /mnt + dmsetup remove pcache_sdb + + +``dm-pcache`` is under active development; feedback, bug reports and patches +are very welcome! |