[PATCH 3/3] btrfs: describe the space reservation system in general

Josef Bacik
2020-02-03T20:44:47Z

Add another comment to cover how the space reservation system works
generally. This covers the actual reservation flow, as well as how
flushing is handled.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
fs/btrfs/space-info.c | 128 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 128 insertions(+)

diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
index d3befc536a7f..6de1fbe2835a 100644
--- a/fs/btrfs/space-info.c
+++ b/fs/btrfs/space-info.c
@@ -10,6 +10,134 @@
#include "transaction.h"
#include "block-group.h"

+/*
+ * HOW DOES SPACE RESERVATION WORK
+ *
+ * If you want to know about delalloc specifically, there is a separate comment
+ * for that with the delalloc code. This comment is about how the whole system
+ * works generally.
+ *
+ * BASIC CONCEPTS
+ *
+ * 1) space_info. This is the ultimate arbiter of how much space we can use.
+ * There's a description of the bytes_ fields with the struct declaration,
+ * refer to that for specifics on each field. Suffice it to say that for
+ * reservations we care about total_bytes - SUM(space_info->bytes_) when
+ * determining if there is space to make an allocation.
+ *
+ * 2) block_rsv's. These are basically buckets for every different type of
+ * metadata reservation we have. You can see the comment in the block_rsv
+ * code on the rules for each type, but generally block_rsv->reserved is how
+ * much space is accounted for in space_info->bytes_may_use.
+ *
+ * 3) btrfs_calc*_size. These are the worst case calculations we used based
+ * on the number of items we will want to modify. We have one for changing
+ * items, and one for inserting new items. Generally we use these helpers to
+ * determine the size of the block reserves, and then use the actual bytes
+ * values to adjust the space_info counters.
+ *
+ * MAKING RESERVATIONS, THE NORMAL CASE
+ *
+ * Things wanting to make reservations will calculate the size that they want
+ * and make a reservation request. If there is sufficient space, and there
+ * are no current reservations pending, we will adjust
+ * space_info->bytes_may_use by this amount.
+ *
+ * Once we allocate an extent, we will add that size to ->bytes_reserved and
+ * subtract the size from ->bytes_may_use. Once that extent is written out we
+ * subtract that value from ->bytes_reserved and add it to ->bytes_used.
+ *
+ * If there is an error at any point the reserver is responsible for dropping
+ * its reservation from ->bytes_may_use.
+ *
+ * MAKING RESERVATIONS, FLUSHING
+ *
+ * If we are unable to satisfy our reservation, or if there are pending
+ * reservations already, we will create a reserve ticket and add ourselves to
+ * the appropriate list. This is controlled by btrfs_reserve_flush_enum. For
+ * simplicity sake this boils down to two cases, priority and normal.
+ *
+ * 1) Priority. These reservations are important and have limited ability to
+ * flush space. For example, the relocation code currently tries to make a
+ * reservation under a transaction commit, thus it cannot wait on anything
+ * that may want to commit the transaction. These tasks will add themselves
+ * to the priority list and thus get any new space first, and then they can
+ * flush space directly in their own context that is safe for them to do
+ * without causing a deadlock.
+ *
+ * 2) Normal. These reservations can wait forever on anything, because the do
+ * not hold resources that they would deadlock on. These tickets simply go to
+ * sleep and start an async thread that will flush space on their behalf.
+ * Every time one of the ->bytes_* counters is adjusted for the space info, we
+ * will check to see if there is enough space to satisfy the requests (in
+ * order) on either of our lists. If there is enough space we will set the
+ * ticket->bytes = 0, and wake the task up. If we flush a few times and fail
+ * to make any progress we will wake up all of the tickets and fail them all.
+ *
+ * THE FLUSHING STATES
+ *
+ * Generally speaking we will have two cases for each state, a "nice" state
+ * and a "ALL THE THINGS" state. In btrfs we delay a lot of work in order to
+ * reduce the locking over head on the various trees, and even to keep from
+ * doing any work at all in the case of delayed refs. Each of these delayed
+ * things however hold reservations, and so letting them run allows us to
+ * reclaim space so we can make new reservations.
+ *
+ * FLUSH_DELAYED_ITEMS
+ * Every inode has a delayed item to update the inode. Take a simple write
+ * for example, we would update the inode item at write time to update the
+ * mtime, and then again at finish_ordered_io() time in order to update the
+ * isize or bytes. We keep these delayed items to coalesce these operations
+ * into a single operation done on demand. These are an easy way to reclaim
+ * metadata space.
+ *
+ * FLUSH_DELALLOC
+ * Look at the delalloc comment to get an idea of how much space is reserved
+ * for delayed allocation. We can reclaim some of this space simply by
+ * running delalloc, but usually we need to wait for ordered extents to
+ * reclaim the bulk of this space.
+ *
+ * FLUSH_DELAYED_REFS
+ * We have a block reserve for the outstanding delayed refs space, and every
+ * delayed ref operation holds a reservation. Running these is a quick way
+ * to reclaim space, but we want to hold this until the end because COW can
+ * churn a lot and we can avoid making some extent tree modifications if we
+ * are able to delay for as long as possible.
+ *
+ * ALLOC_CHUNK
+ * We will skip this the first time through space reservation, because of
+ * overcommit and we don't want to have a lot of useless metadata space when
+ * our worst case reservations will likely never come true.
+ *
+ * RUN_DELAYED_IPUTS
+ * If we're freeing inodes we're likely freeing checksums, file extent
+ * items, and extent tree items. Loads of space could be freed up by these
+ * operations, however they won't be usable until the transaction commits.
+ *
+ * COMMIT_TRANS
+ * may_commit_transaction() is the ultimate arbiter on wether we commit the
+ * transaction or not. In order to avoid constantly churning we do all the
+ * above flushing first and then commit the transaction as the last resort.
+ * However we need to take into account things like pinned space that would
+ * be freed, plus any delayed work we may not have gotten rid of in the case
+ * of metadata.
+ *
+ * OVERCOMMIT
+ * Because we hold so many reservations for metadata we will allow you to
+ * reserve more space than is currently free in the currently allocate
+ * metadata space. This only happens with metadata, data does not allow
+ * overcommitting.
+ *
+ * You can see the current logic for when we allow overcommit in
+ * btrfs_can_overcommit(), but it only applies to unallocated space. If there
+ * is no unallocated space to be had, all reservations are kept within the
+ * free space in the allocated metadata chunks.
+ *
+ * Because of overcommitting, you generally want to use the
+ * btrfs_can_overcommit() logic for metadata allocations, as it does the right
+ * thing with or without extra unallocated space.
+ */
+
u64 __pure btrfs_space_info_used(struct btrfs_space_info *s_info,
bool may_use_included)
{
--
2.24.1