[lvm-devel] master - [device/bcache] Initial code drop.
David Teigland
teigland at sourceware.org
Mon Apr 23 13:50:36 UTC 2018
Gitweb: https://sourceware.org/git/?p=lvm2.git;a=commitdiff;h=acb42ec4652450f7cd73440929fd57f1487282bf
Commit: acb42ec4652450f7cd73440929fd57f1487282bf
Parent: 00f1b208a1bf44665ec97a791355b1fcf525a3a7
Author: Joe Thornber <ejt at redhat.com>
AuthorDate: Tue Jan 30 10:46:08 2018 +0000
Committer: David Teigland <teigland at redhat.com>
CommitterDate: Fri Apr 20 11:10:45 2018 -0500
[device/bcache] Initial code drop.
Compiles. Not written tests yet.
---
lib/device/bcache.c | 833 +++++++++++++++++++++++++++++++++++++++++++++++++++
lib/device/bcache.h | 83 +++++
2 files changed, 916 insertions(+), 0 deletions(-)
diff --git a/lib/device/bcache.c b/lib/device/bcache.c
new file mode 100644
index 0000000..1be626c
--- /dev/null
+++ b/lib/device/bcache.c
@@ -0,0 +1,833 @@
+#define _GNU_SOURCE
+
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <libaio.h>
+#include <unistd.h>
+#include <linux/fs.h>
+#include <sys/ioctl.h>
+#include <sys/user.h>
+
+#include "bcache.h"
+#include "dm-logging.h"
+#include "log.h"
+
+#define SECTOR_SHIFT 9L
+
+//----------------------------------------------------------------
+
+// Assumes the list is not empty.
+static inline struct dm_list *_list_pop(struct dm_list *head)
+{
+ struct dm_list *l;
+
+ l = head->n;
+ dm_list_del(l);
+ return l;
+}
+
+//----------------------------------------------------------------
+
+struct control_block {
+ struct dm_list list;
+ void *context;
+ struct iocb cb;
+};
+
+struct cb_set {
+ struct dm_list free;
+ struct dm_list allocated;
+ struct control_block *vec;
+} control_block_set;
+
+static struct cb_set *_cb_set_create(unsigned nr)
+{
+ int i;
+ struct cb_set *cbs = malloc(sizeof(*cbs));
+
+ if (!cbs)
+ return NULL;
+
+ cbs->vec = malloc(nr * sizeof(*cbs->vec));
+ if (!cbs->vec) {
+ free(cbs);
+ return NULL;
+ }
+
+ dm_list_init(&cbs->free);
+ dm_list_init(&cbs->allocated);
+
+ for (i = 0; i < nr; i++)
+ dm_list_add(&cbs->free, &cbs->vec[i].list);
+
+ return cbs;
+}
+
+static bool _cb_set_destroy(struct cb_set *cbs)
+{
+ if (!dm_list_empty(&cbs->allocated)) {
+ // FIXME: I think we should propogate this up.
+ log_error("async io still in flight");
+ return false;
+ }
+
+ free(cbs->vec);
+ free(cbs);
+ return 0;
+}
+
+static struct control_block *_cb_alloc(struct cb_set *cbs, void *context)
+{
+ struct control_block *cb;
+
+ if (dm_list_empty(&cbs->free))
+ return NULL;
+
+ cb = dm_list_item(_list_pop(&cbs->free), struct control_block);
+ cb->context = context;
+ dm_list_add(&cbs->allocated, &cb->list);
+
+ return cb;
+}
+
+static void _cb_free(struct cb_set *cbs, struct control_block *cb)
+{
+ dm_list_del(&cb->list);
+ dm_list_add_h(&cbs->free, &cb->list);
+}
+
+static struct control_block *_iocb_to_cb(struct iocb *icb)
+{
+ return dm_list_struct_base(icb, struct control_block, cb);
+}
+
+//----------------------------------------------------------------
+
+// FIXME: write a sync engine too
+enum dir {
+ DIR_READ,
+ DIR_WRITE
+};
+
+struct io_engine {
+ io_context_t aio_context;
+ struct cb_set *cbs;
+};
+
+static struct io_engine *_engine_create(unsigned max_io)
+{
+ int r;
+ struct io_engine *e = malloc(sizeof(*e));
+
+ if (!e)
+ return NULL;
+
+ e->aio_context = 0;
+ r = io_setup(max_io, &e->aio_context);
+ if (r < 0) {
+ log_warn("io_setup failed");
+ return NULL;
+ }
+
+ e->cbs = _cb_set_create(max_io);
+ if (!e->cbs) {
+ log_warn("couldn't create control block set");
+ free(e);
+ return NULL;
+ }
+
+ return e;
+}
+
+static void _engine_destroy(struct io_engine *e)
+{
+ _cb_set_destroy(e->cbs);
+ io_destroy(e->aio_context);
+ free(e);
+}
+
+static bool _engine_issue(struct io_engine *e, int fd, enum dir d,
+ sector_t sb, sector_t se, void *data, void *context)
+{
+ int r;
+ struct iocb *cb_array[1];
+ struct control_block *cb;
+
+ if (((uint64_t) data) & (PAGE_SIZE - 1)) {
+ log_err("misaligned data buffer");
+ return false;
+ }
+
+ cb = _cb_alloc(e->cbs, context);
+ if (!cb) {
+ log_err("couldn't allocate control block");
+ return false;
+ }
+
+ memset(&cb->cb, 0, sizeof(cb->cb));
+
+ cb->cb.aio_fildes = (int) fd;
+ cb->cb.u.c.buf = data;
+ cb->cb.u.c.offset = sb << SECTOR_SHIFT;
+ cb->cb.u.c.nbytes = (se - sb) << SECTOR_SHIFT;
+ cb->cb.aio_lio_opcode = (d == DIR_READ) ? IO_CMD_PREAD : IO_CMD_PWRITE;
+
+ cb_array[0] = &cb->cb;
+ r = io_submit(e->aio_context, 1, cb_array);
+ if (r < 0) {
+ log_sys_error("io_submit", "");
+ _cb_free(e->cbs, cb);
+ return false;
+ }
+
+ return true;
+}
+
+#define MAX_IO 64
+typedef void complete_fn(void *context, int io_error);
+
+static bool _engine_wait(struct io_engine *e, complete_fn fn)
+{
+ int i, r;
+ struct io_event event[MAX_IO];
+ struct control_block *cb;
+
+ memset(&event, 0, sizeof(event));
+ r = io_getevents(e->aio_context, 1, MAX_IO, event, NULL);
+ if (r < 0) {
+ log_sys_error("io_getevents", "");
+ return false;
+ }
+
+ for (i = 0; i < r; i++) {
+ struct io_event *ev = event + i;
+
+ cb = _iocb_to_cb((struct iocb *) ev->obj);
+
+ if (ev->res == cb->cb.u.c.nbytes)
+ fn((void *) cb->context, 0);
+
+ else if ((int) ev->res < 0)
+ fn(cb->context, (int) ev->res);
+
+ else {
+ log_err("short io");
+ fn(cb->context, -ENODATA);
+ }
+
+ _cb_free(e->cbs, cb);
+ }
+
+ return true;
+}
+
+//----------------------------------------------------------------
+
+#define MIN_BLOCKS 16
+#define WRITEBACK_LOW_THRESHOLD_PERCENT 33
+#define WRITEBACK_HIGH_THRESHOLD_PERCENT 66
+
+//----------------------------------------------------------------
+
+static void *_alloc_aligned(size_t len, size_t alignment)
+{
+ void *result = NULL;
+ int r = posix_memalign(&result, alignment, len);
+ if (r)
+ return NULL;
+
+ return result;
+}
+
+//----------------------------------------------------------------
+
+static bool _test_flags(struct block *b, unsigned bits)
+{
+ return (b->flags & bits) != 0;
+}
+
+static void _set_flags(struct block *b, unsigned bits)
+{
+ b->flags |= bits;
+}
+
+static void _clear_flags(struct block *b, unsigned bits)
+{
+ b->flags &= ~bits;
+}
+
+//----------------------------------------------------------------
+
+enum block_flags {
+ BF_IO_PENDING = (1 << 0),
+ BF_DIRTY = (1 << 1),
+};
+
+struct bcache {
+ int fd;
+ sector_t block_sectors;
+ uint64_t nr_data_blocks;
+ uint64_t nr_cache_blocks;
+
+ struct io_engine *engine;
+
+ void *raw_data;
+ struct block *raw_blocks;
+
+ /*
+ * Lists that categorise the blocks.
+ */
+ unsigned nr_locked;
+ unsigned nr_dirty;
+ unsigned nr_io_pending;
+
+ struct dm_list free;
+ struct dm_list errored;
+ struct dm_list dirty;
+ struct dm_list clean;
+ struct dm_list io_pending;
+
+ /*
+ * Hash table.
+ */
+ unsigned nr_buckets;
+ unsigned hash_mask;
+ struct dm_list *buckets;
+
+ /*
+ * Statistics
+ */
+ unsigned read_hits;
+ unsigned read_misses;
+ unsigned write_zeroes;
+ unsigned write_hits;
+ unsigned write_misses;
+ unsigned prefetches;
+};
+
+//----------------------------------------------------------------
+
+/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
+#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
+
+static unsigned _hash(struct bcache *cache, int fd, uint64_t index)
+{
+ uint64_t h = (index << 10) & fd;
+ h *= GOLDEN_RATIO_PRIME_64;
+ return h & cache->hash_mask;
+}
+
+static struct block *_hash_lookup(struct bcache *cache, int fd, uint64_t index)
+{
+ struct block *b;
+ unsigned h = _hash(cache, fd, index);
+
+ dm_list_iterate_items_gen (b, cache->buckets + h, hash)
+ if (b->index == index)
+ return b;
+
+ return NULL;
+}
+
+static void _hash_insert(struct block *b)
+{
+ unsigned h = _hash(b->cache, b->fd, b->index);
+ dm_list_add_h(b->cache->buckets + h, &b->hash);
+}
+
+static void _hash_remove(struct block *b)
+{
+ dm_list_del(&b->hash);
+}
+
+/*
+ * Must return a power of 2.
+ */
+static unsigned _calc_nr_buckets(unsigned nr_blocks)
+{
+ unsigned r = 8;
+ unsigned n = nr_blocks / 4;
+
+ if (n < 8)
+ n = 8;
+
+ while (r < n)
+ r <<= 1;
+
+ return r;
+}
+
+static int _hash_table_init(struct bcache *cache, unsigned nr_entries)
+{
+ unsigned i;
+
+ cache->nr_buckets = _calc_nr_buckets(nr_entries);
+ cache->hash_mask = cache->nr_buckets - 1;
+ cache->buckets = malloc(cache->nr_buckets * sizeof(*cache->buckets));
+ if (!cache->buckets)
+ return -ENOMEM;
+
+ for (i = 0; i < cache->nr_buckets; i++)
+ dm_list_init(cache->buckets + i);
+
+ return 0;
+}
+
+static void _hash_table_exit(struct bcache *cache)
+{
+ free(cache->buckets);
+}
+
+//----------------------------------------------------------------
+
+static int _init_free_list(struct bcache *cache, unsigned count)
+{
+ unsigned i;
+ size_t block_size = cache->block_sectors << SECTOR_SHIFT;
+ unsigned char *data =
+ (unsigned char *) _alloc_aligned(count * block_size, PAGE_SIZE);
+
+ /* Allocate the data for each block. We page align the data. */
+ if (!data)
+ return -ENOMEM;
+
+ cache->raw_data = data;
+ cache->raw_blocks = malloc(count * sizeof(*cache->raw_blocks));
+
+ if (!cache->raw_blocks)
+ free(cache->raw_data);
+
+ for (i = 0; i < count; i++) {
+ struct block *b = cache->raw_blocks + i;
+ b->cache = cache;
+ b->data = data + (block_size * i);
+ dm_list_add(&cache->free, &b->list);
+ }
+
+ return 0;
+}
+
+static void _exit_free_list(struct bcache *cache)
+{
+ free(cache->raw_data);
+ free(cache->raw_blocks);
+}
+
+static struct block *_alloc_block(struct bcache *cache)
+{
+ struct block *b = dm_list_struct_base(_list_pop(&cache->free), struct block, list);
+ return b;
+}
+
+/*----------------------------------------------------------------
+ * Clean/dirty list management.
+ * Always use these methods to ensure nr_dirty_ is correct.
+ *--------------------------------------------------------------*/
+
+static void _unlink_block(struct block *b)
+{
+ if (_test_flags(b, BF_DIRTY))
+ b->cache->nr_dirty--;
+
+ dm_list_del(&b->list);
+}
+
+static void _link_block(struct block *b)
+{
+ struct bcache *cache = b->cache;
+
+ if (_test_flags(b, BF_DIRTY)) {
+ dm_list_add(&cache->dirty, &b->list);
+ cache->nr_dirty++;
+ } else
+ dm_list_add(&cache->clean, &b->list);
+}
+
+static void _relink(struct block *b)
+{
+ _unlink_block(b);
+ _link_block(b);
+}
+
+/*----------------------------------------------------------------
+ * Low level IO handling
+ *
+ * We cannot have two concurrent writes on the same block.
+ * eg, background writeback, put with dirty, flush?
+ *
+ * To avoid this we introduce some restrictions:
+ *
+ * i) A held block can never be written back.
+ * ii) You cannot get a block until writeback has completed.
+ *
+ *--------------------------------------------------------------*/
+
+/*
+ * |b->list| should be valid (either pointing to itself, on one of the other
+ * lists.
+ */
+static bool _issue_low_level(struct block *b, enum dir d)
+{
+ struct bcache *cache = b->cache;
+ sector_t sb = b->index * cache->block_sectors;
+ sector_t se = sb + cache->block_sectors;
+
+ if (_test_flags(b, BF_IO_PENDING))
+ return false;
+
+ _set_flags(b, BF_IO_PENDING);
+ return _engine_issue(cache->engine, cache->fd, d, sb, se, b->data, b);
+}
+
+static inline bool _issue_read(struct block *b)
+{
+ return _issue_low_level(b, DIR_READ);
+}
+
+static inline bool _issue_write(struct block *b)
+{
+ return _issue_low_level(b, DIR_WRITE);
+}
+
+static void _complete_io(void *context, int err)
+{
+ struct block *b = context;
+ struct bcache *cache = b->cache;
+
+ b->error = err;
+ _clear_flags(b, BF_IO_PENDING);
+ cache->nr_io_pending--;
+
+ /*
+ * b is on the io_pending list, so we don't want to use unlink_block.
+ * Which would incorrectly adjust nr_dirty.
+ */
+ dm_list_del(&b->list);
+
+ if (b->error)
+ dm_list_add(&cache->errored, &b->list);
+
+ else {
+ _clear_flags(b, BF_DIRTY);
+ _link_block(b);
+ }
+}
+
+static int _wait_io(struct bcache *cache)
+{
+ return _engine_wait(cache->engine, _complete_io);
+}
+
+/*----------------------------------------------------------------
+ * High level IO handling
+ *--------------------------------------------------------------*/
+
+static void _wait_all(struct bcache *cache)
+{
+ while (!dm_list_empty(&cache->io_pending))
+ _wait_io(cache);
+}
+
+static void _wait_specific(struct block *b)
+{
+ while (_test_flags(b, BF_IO_PENDING))
+ _wait_io(b->cache);
+}
+
+static unsigned _writeback(struct bcache *cache, unsigned count)
+{
+ unsigned actual = 0;
+ struct block *b, *tmp;
+
+ dm_list_iterate_items_gen_safe (b, tmp, &cache->dirty, list) {
+ if (actual == count)
+ break;
+
+ // We can't writeback anything that's still in use.
+ if (!b->ref_count) {
+ _issue_write(b);
+ actual++;
+ }
+ }
+
+ return actual;
+}
+
+/*----------------------------------------------------------------
+ * High level allocation
+ *--------------------------------------------------------------*/
+
+static struct block *_find_unused_clean_block(struct bcache *cache)
+{
+ struct block *b;
+
+ dm_list_iterate_items (b, &cache->clean) {
+ if (!b->ref_count) {
+ _unlink_block(b);
+ _hash_remove(b);
+ return b;
+ }
+ }
+
+ return NULL;
+}
+
+static struct block *_new_block(struct bcache *cache, block_address index)
+{
+ struct block *b;
+
+ b = _alloc_block(cache);
+ while (!b && cache->nr_locked < cache->nr_cache_blocks) {
+ b = _find_unused_clean_block(cache);
+ if (!b) {
+ if (dm_list_empty(&cache->io_pending))
+ _writeback(cache, 16);
+ _wait_io(cache);
+ }
+ }
+
+ if (b) {
+ dm_list_init(&b->list);
+ dm_list_init(&b->hash);
+ b->flags = 0;
+ b->index = index;
+ b->ref_count = 0;
+ b->error = 0;
+
+ _hash_insert(b);
+ }
+
+ return b;
+}
+
+/*----------------------------------------------------------------
+ * Block reference counting
+ *--------------------------------------------------------------*/
+static void _zero_block(struct block *b)
+{
+ b->cache->write_zeroes++;
+ memset(b->data, 0, b->cache->block_sectors << SECTOR_SHIFT);
+ _set_flags(b, BF_DIRTY);
+}
+
+static void _hit(struct block *b, unsigned flags)
+{
+ struct bcache *cache = b->cache;
+
+ if (flags & (GF_ZERO | GF_DIRTY))
+ cache->write_hits++;
+ else
+ cache->read_hits++;
+
+ _relink(b);
+}
+
+static void _miss(struct bcache *cache, unsigned flags)
+{
+ if (flags & (GF_ZERO | GF_DIRTY))
+ cache->write_misses++;
+ else
+ cache->read_misses++;
+}
+
+static struct block *_lookup_or_read_block(struct bcache *cache,
+ int fd, block_address index,
+ unsigned flags)
+{
+ struct block *b = _hash_lookup(cache, fd, index);
+
+ if (b) {
+ // FIXME: this is insufficient. We need to also catch a read
+ // lock of a write locked block. Ref count needs to distinguish.
+ if (b->ref_count && (flags & (GF_DIRTY | GF_ZERO))) {
+ log_err("concurrent write lock attempted");
+ return NULL;
+ }
+
+ if (_test_flags(b, BF_IO_PENDING)) {
+ _miss(cache, flags);
+ _wait_specific(b);
+
+ } else
+ _hit(b, flags);
+
+ _unlink_block(b);
+
+ if (flags & GF_ZERO)
+ _zero_block(b);
+
+ } else {
+ _miss(cache, flags);
+
+ b = _new_block(cache, index);
+ if (b) {
+ if (flags & GF_ZERO)
+ _zero_block(b);
+
+ else {
+ _issue_read(b);
+ _wait_specific(b);
+
+ // we know the block is clean and unerrored.
+ _unlink_block(b);
+ }
+ }
+ }
+
+ if (b && !b->error) {
+ if (flags & (GF_DIRTY | GF_ZERO))
+ _set_flags(b, BF_DIRTY);
+
+ _link_block(b);
+ return b;
+ }
+
+ return NULL;
+}
+
+static void _preemptive_writeback(struct bcache *cache)
+{
+ // FIXME: this ignores those blocks that are in the error state. Track
+ // nr_clean instead?
+ unsigned nr_available = cache->nr_cache_blocks - (cache->nr_dirty - cache->nr_io_pending);
+ if (nr_available < (WRITEBACK_LOW_THRESHOLD_PERCENT * cache->nr_cache_blocks / 100))
+ _writeback(cache, (WRITEBACK_HIGH_THRESHOLD_PERCENT * cache->nr_cache_blocks / 100) - nr_available);
+
+}
+
+/*----------------------------------------------------------------
+ * Public interface
+ *--------------------------------------------------------------*/
+struct bcache *bcache_create(sector_t block_sectors, unsigned nr_cache_blocks)
+{
+ int r;
+ struct bcache *cache;
+
+ cache = malloc(sizeof(*cache));
+ if (!cache)
+ return NULL;
+
+ cache->block_sectors = block_sectors;
+ cache->nr_cache_blocks = nr_cache_blocks;
+
+ cache->engine = _engine_create(nr_cache_blocks < 1024u ? nr_cache_blocks : 1024u);
+ if (!cache->engine) {
+ free(cache);
+ return NULL;
+ }
+
+ cache->nr_locked = 0;
+ cache->nr_dirty = 0;
+ cache->nr_io_pending = 0;
+
+ dm_list_init(&cache->free);
+ dm_list_init(&cache->errored);
+ dm_list_init(&cache->dirty);
+ dm_list_init(&cache->clean);
+ dm_list_init(&cache->io_pending);
+
+ if (_hash_table_init(cache, nr_cache_blocks)) {
+ _engine_destroy(cache->engine);
+ free(cache);
+ }
+
+ cache->read_hits = 0;
+ cache->read_misses = 0;
+ cache->write_zeroes = 0;
+ cache->write_hits = 0;
+ cache->write_misses = 0;
+ cache->prefetches = 0;
+
+ r = _init_free_list(cache, nr_cache_blocks);
+ if (r) {
+ _engine_destroy(cache->engine);
+ _hash_table_exit(cache);
+ free(cache);
+ }
+
+ return cache;
+}
+
+void bcache_destroy(struct bcache *cache)
+{
+ if (cache->nr_locked)
+ log_warn("some blocks are still locked\n");
+
+ bcache_flush(cache);
+ _wait_all(cache);
+ _exit_free_list(cache);
+ _hash_table_exit(cache);
+ _engine_destroy(cache->engine);
+ free(cache);
+}
+
+void bcache_prefetch(struct bcache *cache, int fd, block_address index)
+{
+ struct block *b = _hash_lookup(cache, fd, index);
+
+ if (!b) {
+ cache->prefetches++;
+
+ b = _new_block(cache, index);
+ if (b)
+ _issue_read(b);
+ }
+}
+
+bool bcache_get(struct bcache *cache, int fd, block_address index,
+ unsigned flags, struct block **result)
+{
+ struct block *b = _lookup_or_read_block(cache, fd, index, flags);
+ if (b) {
+ if (!b->ref_count)
+ cache->nr_locked++;
+ b->ref_count++;
+
+ *result = b;
+ return true;
+ }
+
+ *result = NULL;
+ log_err("couldn't get block");
+ return false;
+}
+
+void bcache_put(struct block *b)
+{
+ if (!b->ref_count) {
+ log_err("ref count on bcache block already zero");
+ return;
+ }
+
+ b->ref_count--;
+ if (!b->ref_count)
+ b->cache->nr_locked--;
+
+ if (_test_flags(b, BF_DIRTY))
+ _preemptive_writeback(b->cache);
+}
+
+int bcache_flush(struct bcache *cache)
+{
+ while (!dm_list_empty(&cache->dirty)) {
+ struct block *b = dm_list_item(_list_pop(&cache->dirty), struct block);
+ if (b->ref_count || _test_flags(b, BF_IO_PENDING))
+ // The superblock may well be still locked.
+ continue;
+
+ _issue_write(b);
+ }
+
+ _wait_all(cache);
+
+ return dm_list_empty(&cache->errored) ? 0 : -EIO;
+}
+
+//----------------------------------------------------------------
+
diff --git a/lib/device/bcache.h b/lib/device/bcache.h
new file mode 100644
index 0000000..1f4262e
--- /dev/null
+++ b/lib/device/bcache.h
@@ -0,0 +1,83 @@
+#ifndef BCACHE_H
+#define BCACHE_H
+
+#include <stdint.h>
+
+#include "libdevmapper.h"
+
+/*----------------------------------------------------------------*/
+
+typedef uint64_t block_address;
+typedef uint64_t sector_t;
+
+struct bcache;
+struct block {
+ /* clients may only access these three fields */
+ int fd;
+ uint64_t index;
+ void *data;
+
+ struct bcache *cache;
+ struct dm_list list;
+ struct dm_list hash;
+
+ unsigned flags;
+ unsigned ref_count;
+ int error;
+};
+
+struct bcache *bcache_create(sector_t block_size, unsigned nr_cache_blocks);
+void bcache_destroy(struct bcache *cache);
+
+enum bcache_get_flags {
+ /*
+ * The block will be zeroed before get_block returns it. This
+ * potentially avoids a read if the block is not already in the cache.
+ * GF_DIRTY is implicit.
+ */
+ GF_ZERO = (1 << 0),
+
+ /*
+ * Indicates the caller is intending to change the data in the block, a
+ * writeback will occur after the block is released.
+ */
+ GF_DIRTY = (1 << 1)
+};
+
+typedef uint64_t block_address;
+
+unsigned bcache_get_max_prefetches(struct bcache *cache);
+
+/*
+ * Use the prefetch method to take advantage of asynchronous IO. For example,
+ * if you wanted to read a block from many devices concurrently you'd do
+ * something like this:
+ *
+ * dm_list_iterate_items (dev, &devices)
+ * bcache_prefetch(cache, dev->fd, block);
+ *
+ * dm_list_iterate_items (dev, &devices) {
+ * if (!bcache_get(cache, dev->fd, block, &b))
+ * fail();
+ *
+ * process_block(b);
+ * }
+ *
+ * It's slightly sub optimal, since you may not run the gets in the order that
+ * they complete. But we're talking a very small difference, and it's worth it
+ * to keep callbacks out of this interface.
+ */
+void bcache_prefetch(struct bcache *cache, int fd, block_address index);
+
+/*
+ * Returns true on success.
+ */
+bool bcache_get(struct bcache *cache, int fd, block_address index,
+ unsigned flags, struct block **result);
+void bcache_put(struct block *b);
+
+int bcache_flush(struct bcache *cache);
+
+/*----------------------------------------------------------------*/
+
+#endif
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