#ifndef _BCACHE_JOURNAL_H
  #define _BCACHE_JOURNAL_H
  
  /*
   * THE JOURNAL:
   *
   * The journal is treated as a circular buffer of buckets - a journal entry
   * never spans two buckets. This means (not implemented yet) we can resize the
   * journal at runtime, and will be needed for bcache on raw flash support.
   *
   * Journal entries contain a list of keys, ordered by the time they were
   * inserted; thus journal replay just has to reinsert the keys.
   *
   * We also keep some things in the journal header that are logically part of the
   * superblock - all the things that are frequently updated. This is for future
   * bcache on raw flash support; the superblock (which will become another
   * journal) can't be moved or wear leveled, so it contains just enough
   * information to find the main journal, and the superblock only has to be
   * rewritten when we want to move/wear level the main journal.
   *
   * Currently, we don't journal BTREE_REPLACE operations - this will hopefully be
   * fixed eventually. This isn't a bug - BTREE_REPLACE is used for insertions
   * from cache misses, which don't have to be journaled, and for writeback and
   * moving gc we work around it by flushing the btree to disk before updating the
   * gc information. But it is a potential issue with incremental garbage
   * collection, and it's fragile.
   *
   * OPEN JOURNAL ENTRIES:
   *
   * Each journal entry contains, in the header, the sequence number of the last
   * journal entry still open - i.e. that has keys that haven't been flushed to
   * disk in the btree.
   *
   * We track this by maintaining a refcount for every open journal entry, in a
   * fifo; each entry in the fifo corresponds to a particular journal
   * entry/sequence number. When the refcount at the tail of the fifo goes to
   * zero, we pop it off - thus, the size of the fifo tells us the number of open
   * journal entries
   *
   * We take a refcount on a journal entry when we add some keys to a journal
   * entry that we're going to insert (held by struct btree_op), and then when we
   * insert those keys into the btree the btree write we're setting up takes a
   * copy of that refcount (held by struct btree_write). That refcount is dropped
   * when the btree write completes.
   *
   * A struct btree_write can only hold a refcount on a single journal entry, but
   * might contain keys for many journal entries - we handle this by making sure
   * it always has a refcount on the _oldest_ journal entry of all the journal
   * entries it has keys for.
   *
   * JOURNAL RECLAIM:
   *
   * As mentioned previously, our fifo of refcounts tells us the number of open
   * journal entries; from that and the current journal sequence number we compute
   * last_seq - the oldest journal entry we still need. We write last_seq in each
   * journal entry, and we also have to keep track of where it exists on disk so
   * we don't overwrite it when we loop around the journal.
   *
   * To do that we track, for each journal bucket, the sequence number of the
   * newest journal entry it contains - if we don't need that journal entry we
   * don't need anything in that bucket anymore. From that we track the last
   * journal bucket we still need; all this is tracked in struct journal_device
   * and updated by journal_reclaim().
   *
   * JOURNAL FILLING UP:
   *
   * There are two ways the journal could fill up; either we could run out of
   * space to write to, or we could have too many open journal entries and run out
   * of room in the fifo of refcounts. Since those refcounts are decremented
   * without any locking we can't safely resize that fifo, so we handle it the
   * same way.
   *
   * If the journal fills up, we start flushing dirty btree nodes until we can
   * allocate space for a journal write again - preferentially flushing btree
   * nodes that are pinning the oldest journal entries first.
   */
  
  /*
   * Only used for holding the journal entries we read in btree_journal_read()
   * during cache_registration
   */
  struct journal_replay {
  	struct list_head	list;
  	atomic_t		*pin;
  	struct jset		j;
  };
  
  /*
   * We put two of these in struct journal; we used them for writes to the
   * journal that are being staged or in flight.
   */
  struct journal_write {
  	struct jset		*data;
  #define JSET_BITS		3
  
  	struct cache_set	*c;
  	struct closure_waitlist	wait;
  	bool			need_write;
  };
  
  /* Embedded in struct cache_set */
  struct journal {
  	spinlock_t		lock;
  	/* used when waiting because the journal was full */
  	struct closure_waitlist	wait;
  	struct closure		io;
  	int			io_in_flight;
  	struct delayed_work	work;
  
  	/* Number of blocks free in the bucket(s) we're currently writing to */
  	unsigned		blocks_free;
  	uint64_t		seq;
  	DECLARE_FIFO(atomic_t, pin);
  
  	BKEY_PADDED(key);
  
  	struct journal_write	w[2], *cur;
  };
  
  /*
   * Embedded in struct cache. First three fields refer to the array of journal
   * buckets, in cache_sb.
   */
  struct journal_device {
  	/*
  	 * For each journal bucket, contains the max sequence number of the
  	 * journal writes it contains - so we know when a bucket can be reused.
  	 */
  	uint64_t		seq[SB_JOURNAL_BUCKETS];
  
  	/* Journal bucket we're currently writing to */
  	unsigned		cur_idx;
  
  	/* Last journal bucket that still contains an open journal entry */
  	unsigned		last_idx;
  
  	/* Next journal bucket to be discarded */
  	unsigned		discard_idx;
  
  #define DISCARD_READY		0
  #define DISCARD_IN_FLIGHT	1
  #define DISCARD_DONE		2
  	/* 1 - discard in flight, -1 - discard completed */
  	atomic_t		discard_in_flight;
  
  	struct work_struct	discard_work;
  	struct bio		discard_bio;
  	struct bio_vec		discard_bv;
  
  	/* Bio for journal reads/writes to this device */
  	struct bio		bio;
  	struct bio_vec		bv[8];
  };
  
  #define journal_pin_cmp(c, l, r)				\
  	(fifo_idx(&(c)->journal.pin, (l)) > fifo_idx(&(c)->journal.pin, (r)))
  
  #define JOURNAL_PIN	20000
  
  #define journal_full(j)						\
  	(!(j)->blocks_free || fifo_free(&(j)->pin) <= 1)
  
  struct closure;
  struct cache_set;
  struct btree_op;
  struct keylist;
  
  atomic_t *bch_journal(struct cache_set *, struct keylist *, struct closure *);
  void bch_journal_next(struct journal *);
  void bch_journal_mark(struct cache_set *, struct list_head *);
  void bch_journal_meta(struct cache_set *, struct closure *);
  int bch_journal_read(struct cache_set *, struct list_head *);
  int bch_journal_replay(struct cache_set *, struct list_head *);
  
  void bch_journal_free(struct cache_set *);
  int bch_journal_alloc(struct cache_set *);
  
  #endif /* _BCACHE_JOURNAL_H */