From: Dennis Zhou <dennis@kernel.org>
To: David Sterba <dsterba@suse.com>, Chris Mason <clm@fb.com>,
Josef Bacik <josef@toxicpanda.com>,
Omar Sandoval <osandov@osandov.com>
Cc: kernel-team@fb.com, linux-btrfs@vger.kernel.org,
Dennis Zhou <dennis@kernel.org>
Subject: [PATCH 20/22] btrfs: add async discard header
Date: Fri, 13 Dec 2019 16:22:29 -0800 [thread overview]
Message-ID: <f20276673ca4890aa3246660c89fd3edbc6b4f53.1576195673.git.dennis@kernel.org> (raw)
In-Reply-To: <cover.1576195673.git.dennis@kernel.org>
In-Reply-To: <cover.1576195673.git.dennis@kernel.org>
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 | 35 +++++++++++++++++++++++++++++++++++
1 file changed, 35 insertions(+)
diff --git a/fs/btrfs/discard.c b/fs/btrfs/discard.c
index fe73814526ef..fe5719797eee 100644
--- a/fs/btrfs/discard.c
+++ b/fs/btrfs/discard.c
@@ -1,4 +1,39 @@
// 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, byte rate limit, and the timeout of max
+ * delay of BTRFS_DISCARD_MAX_DELAY.
+ *
+ * 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>
--
2.17.1
next prev parent reply other threads:[~2019-12-14 0:23 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-14 0:22 [PATCH v6 00/22] btrfs: async discard support Dennis Zhou
2019-12-14 0:22 ` [PATCH 01/22] bitmap: genericize percpu bitmap region iterators Dennis Zhou
2019-12-14 0:22 ` [PATCH 02/22] btrfs: rename DISCARD opt to DISCARD_SYNC Dennis Zhou
2019-12-14 0:22 ` [PATCH 03/22] btrfs: keep track of which extents have been discarded Dennis Zhou
2019-12-14 0:22 ` [PATCH 04/22] btrfs: keep track of cleanliness of the bitmap Dennis Zhou
2019-12-14 0:22 ` [PATCH 05/22] btrfs: add the beginning of async discard, discard workqueue Dennis Zhou
2019-12-14 0:22 ` [PATCH 06/22] btrfs: handle empty block_group removal Dennis Zhou
2019-12-14 0:22 ` [PATCH 07/22] btrfs: discard one region at a time in async discard Dennis Zhou
2019-12-14 0:22 ` [PATCH 08/22] btrfs: add removal calls for sysfs debug/ Dennis Zhou
2019-12-18 11:45 ` Anand Jain
2019-12-14 0:22 ` [PATCH 09/22] btrfs: make UUID/debug have its own kobject Dennis Zhou
2019-12-18 11:45 ` Anand Jain
2019-12-14 0:22 ` [PATCH 10/22] btrfs: add discard sysfs directory Dennis Zhou
2019-12-18 11:45 ` Anand Jain
2019-12-14 0:22 ` [PATCH 11/22] btrfs: track discardable extents for async discard Dennis Zhou
2019-12-14 0:22 ` [PATCH 12/22] btrfs: keep track of discardable_bytes Dennis Zhou
2019-12-14 0:22 ` [PATCH 13/22] btrfs: calculate discard delay based on number of extents Dennis Zhou
2019-12-30 16:50 ` David Sterba
2020-01-02 16:45 ` Dennis Zhou
2019-12-14 0:22 ` [PATCH 14/22] btrfs: add bps discard rate limit Dennis Zhou
2019-12-30 17:58 ` David Sterba
2020-01-02 16:46 ` Dennis Zhou
2019-12-14 0:22 ` [PATCH 15/22] btrfs: limit max discard size for async discard Dennis Zhou
2019-12-30 18:00 ` David Sterba
2019-12-30 18:08 ` David Sterba
2020-01-02 16:48 ` Dennis Zhou
2019-12-14 0:22 ` [PATCH 16/22] btrfs: make max async discard size tunable Dennis Zhou
2019-12-30 18:05 ` David Sterba
2020-01-02 16:50 ` Dennis Zhou
2019-12-14 0:22 ` [PATCH 17/22] btrfs: have multiple discard lists Dennis Zhou
2019-12-14 0:22 ` [PATCH 18/22] btrfs: only keep track of data extents for async discard Dennis Zhou
2019-12-30 17:39 ` David Sterba
2020-01-02 16:55 ` Dennis Zhou
2019-12-14 0:22 ` [PATCH 19/22] btrfs: keep track of discard reuse stats Dennis Zhou
2019-12-30 17:33 ` David Sterba
2020-01-02 16:57 ` Dennis Zhou
2019-12-14 0:22 ` Dennis Zhou [this message]
2019-12-14 0:22 ` [PATCH 21/22] btrfs: increase the metadata allowance for the free_space_cache Dennis Zhou
2019-12-14 0:22 ` [PATCH 22/22] btrfs: make smaller extents more likely to go into bitmaps Dennis Zhou
2019-12-17 14:55 ` [PATCH v6 00/22] btrfs: async discard support David Sterba
2019-12-18 0:06 ` Dennis Zhou
2019-12-19 2:03 ` Dennis Zhou
2019-12-19 20:06 ` David Sterba
2019-12-19 21:22 ` Dennis Zhou
2019-12-19 20:34 ` David Sterba
2019-12-19 21:17 ` Dennis Zhou
2019-12-30 18:13 ` David Sterba
2019-12-30 18:49 ` Dennis Zhou
2020-01-02 13:22 ` David Sterba
-- strict thread matches above, loose matches on Subject: below --
2019-12-09 19:45 [PATCH v5 " Dennis Zhou
2019-12-09 19:46 ` [PATCH 20/22] btrfs: add async discard header Dennis Zhou
2019-11-25 19:46 [PATCH v4 00/22] btrfs: async discard support Dennis Zhou
2019-11-25 19:47 ` [PATCH 20/22] btrfs: add async discard header Dennis Zhou
2019-11-20 21:50 [PATCH v3 00/22] btrfs: async discard support Dennis Zhou
2019-11-20 21:51 ` [PATCH 20/22] btrfs: add async discard header Dennis Zhou
2019-10-23 22:52 [PATCH v2 00/22] btrfs: async discard support Dennis Zhou
2019-10-23 22:53 ` [PATCH 20/22] btrfs: add async discard header Dennis Zhou
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