btrfs: add async discard implementation overview

Give a brief overview for how async discard is implemented.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/discard.c

@@ -12,6 +12,45 @@
 #include "discard.h"
 #include "free-space-cache.h"
 
+/*
+ * This contains the logic to handle async discard.
+ *
+ * Async discard manages trimming of free space outside of transaction commit.
+ * Discarding is done by managing the block_groups on a LRU list based on free
+ * space recency.  Two passes are used to first prioritize discarding extents
+ * and then allow for trimming in the bitmap the best opportunity to coalesce.
+ * The block_groups are maintained on multiple lists to allow for multiple
+ * passes with different discard filter requirements.  A delayed work item is
+ * used to manage discarding with timeout determined by a max of the delay
+ * incurred by the iops rate limit, the byte rate limit, and the max delay of
+ * BTRFS_DISCARD_MAX_DELAY.
+ *
+ * Note, this only keeps track of block_groups that are explicitly for data.
+ * Mixed block_groups are not supported.
+ *
+ * The first list is special to manage discarding of fully free block groups.
+ * This is necessary because we issue a final trim for a full free block group
+ * after forgetting it.  When a block group becomes unused, instead of directly
+ * being added to the unused_bgs list, we add it to this first list.  Then
+ * from there, if it becomes fully discarded, we place it onto the unused_bgs
+ * list.
+ *
+ * The in-memory free space cache serves as the backing state for discard.
+ * Consequently this means there is no persistence.  We opt to load all the
+ * block groups in as not discarded, so the mount case degenerates to the
+ * crashing case.
+ *
+ * As the free space cache uses bitmaps, there exists a tradeoff between
+ * ease/efficiency for find_free_extent() and the accuracy of discard state.
+ * Here we opt to let untrimmed regions merge with everything while only letting
+ * trimmed regions merge with other trimmed regions.  This can cause
+ * overtrimming, but the coalescing benefit seems to be worth it.  Additionally,
+ * bitmap state is tracked as a whole.  If we're able to fully trim a bitmap,
+ * the trimmed flag is set on the bitmap.  Otherwise, if an allocation comes in,
+ * this resets the state and we will retry trimming the whole bitmap.  This is a
+ * tradeoff between discard state accuracy and the cost of accounting.
+ */
+
 /* This is an initial delay to give some chance for block reuse */
 #define BTRFS_DISCARD_DELAY		(120ULL * NSEC_PER_SEC)
 #define BTRFS_DISCARD_UNUSED_DELAY	(10ULL * NSEC_PER_SEC)