diff mbox series

[08/12] btrfs: add async discard header

Message ID 6b051445e34dadaa0ed1cd7a5efdd19eddbae887.1577999991.git.dennis@kernel.org
State New, archived
Headers show
Series btrfs: async discard follow up | expand

Commit Message

Dennis Zhou Jan. 2, 2020, 9:26 p.m. UTC
Give a brief overview for how async discard is implemented.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
 fs/btrfs/discard.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)
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diff --git a/fs/btrfs/discard.c b/fs/btrfs/discard.c
index bc6d4344397d..d5a89e3755ed 100644
--- a/fs/btrfs/discard.c
+++ b/fs/btrfs/discard.c
@@ -1,4 +1,42 @@ 
 // SPDX-License-Identifier: GPL-2.0
+ * 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
+ *
+ * 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.
+ */
 #include <linux/jiffies.h>
 #include <linux/kernel.h>