diff options
Diffstat (limited to 'fs/btrfs/space-info.c')
| -rw-r--r-- | fs/btrfs/space-info.c | 147 |
1 files changed, 147 insertions, 0 deletions
diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c index 01297c5b2666..3b8d8fa8d767 100644 --- a/fs/btrfs/space-info.c +++ b/fs/btrfs/space-info.c @@ -10,6 +10,153 @@ #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. There is a space_info + * for METADATA, SYSTEM, and DATA areas. + * + * 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 + * + * We call into either btrfs_reserve_data_bytes() or + * btrfs_reserve_metadata_bytes(), depending on which we're looking for, with + * num_bytes we want to reserve. + * + * ->reserve + * space_info->bytes_may_reserve += num_bytes + * + * ->extent allocation + * Call btrfs_add_reserved_bytes() which does + * space_info->bytes_may_reserve -= num_bytes + * space_info->bytes_reserved += extent_bytes + * + * ->insert reference + * Call btrfs_update_block_group() which does + * space_info->bytes_reserved -= extent_bytes + * space_info->bytes_used += extent_bytes + * + * MAKING RESERVATIONS, FLUSHING NORMALLY (non-priority) + * + * Assume we are unable to simply make the reservation because we do not have + * enough space + * + * -> __reserve_bytes + * create a reserve_ticket with ->bytes set to our reservation, add it to + * the tail of space_info->tickets, kick async flush thread + * + * ->handle_reserve_ticket + * wait on ticket->wait for ->bytes to be reduced to 0, or ->error to be set + * on the ticket. + * + * -> btrfs_async_reclaim_metadata_space/btrfs_async_reclaim_data_space + * Flushes various things attempting to free up space. + * + * -> btrfs_try_granting_tickets() + * This is called by anything that either subtracts space from + * space_info->bytes_may_use, ->bytes_pinned, etc, or adds to the + * space_info->total_bytes. This loops through the ->priority_tickets and + * then the ->tickets list checking to see if the reservation can be + * completed. If it can the space is added to space_info->bytes_may_use and + * the ticket is woken up. + * + * -> ticket wakeup + * Check if ->bytes == 0, if it does we got our reservation and we can carry + * on, if not return the appropriate error (ENOSPC, but can be EINTR if we + * were interrupted.) + * + * MAKING RESERVATIONS, FLUSHING HIGH PRIORITY + * + * Same as the above, except we add ourselves to the + * space_info->priority_tickets, and we do not use ticket->wait, we simply + * call flush_space() ourselves for the states that are safe for us to call + * without deadlocking and hope for the best. + * + * 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 whether 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) { |