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[76.182.20.124]) by smtp.gmail.com with ESMTPSA id fu22-20020a05622a5d9600b004054b435f8csm672791qtb.65.2023.09.08.08.32.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 08 Sep 2023 08:32:05 -0700 (PDT) Date: Fri, 8 Sep 2023 11:32:05 -0400 From: Josef Bacik To: fdmanana@kernel.org Cc: linux-btrfs@vger.kernel.org Subject: Re: [PATCH 21/21] btrfs: always reserve space for delayed refs when starting transaction Message-ID: <20230908153205.GV1977092@perftesting> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Fri, Sep 08, 2023 at 01:09:23PM +0100, fdmanana@kernel.org wrote: > From: Filipe Manana > > When starting a transaction (or joining an existing one with > btrfs_start_transaction()), we reserve space for the number of items we > want to insert in a btree, but we don't do it for the delayed refs we > will generate while using the transaction to modify (COW) extent buffers > in a btree or allocate new extent buffers. Basically how it works: > > 1) When we start a transaction we reserve space for the number of items > the caller wants to be inserted/modified/deleted in a btree. This space > goes to the transaction block reserve; > > 2) If the delayed refs block reserve is not full, its size is greater > than the amount of its reserved space, and the flush method is > BTRFS_RESERVE_FLUSH_ALL, then we attempt to reserve more space for > it corresponding to the number of items the caller wants to > insert/modify/delete in a btree; > > 3) The size of the delayed refs block reserve is increased when a task > creates delayed refs after COWing an extent buffer, allocating a new > one or deleting (freeing) an extent buffer. This happens after the > the task started or joined a transaction, whenever it calls > btrfs_update_delayed_refs_rsv(); > > 4) The delayed refs block reserve is then refilled by anyone calling > btrfs_delayed_refs_rsv_refill(), either during unlink/truncate > operations or when someone else calls btrfs_start_transaction() with > a 0 number of items and flush method BTRFS_RESERVE_FLUSH_ALL; > > 5) As a task COWs or allocates extent buffers, it consumes space from the > transaction block reserve. When the task releases its transaction > handle (btrfs_end_transaction()) or it attempts to commit the > transaction, it releases any remaining space in the transaction block > reserve that it did not use, as not all space may have been used (due > to pessimistic space calculation) by calling btrfs_block_rsv_release() > which will try to add that unused space to the delayed refs block > reserve (if its current size is greater than its reserved space). > That transferred space may not be enough to completely fulfill the > delayed refs block reserve. > > Plus we have some tasks that will attempt do modify as many leaves > as they can before getting -ENOSPC (and then reserving more space and > retrying), such as hole punching and extent cloning which call > btrfs_replace_file_extents(). Such tasks can generate therefore a > high number of delayed refs, for both metadata and data (we can't > know in advance how many file extent items we will find in a range > and therefore how many delayed refs for dropping references on data > extents we will generate); > > 6) If a transaction starts its commit before the delayeds refs block > reserve is refilled, for example by the transaction kthread or by > someone who called btrfs_join_transaction() before starting the > commit, then when running delayed references if we don't have enough > reserved space in the delayed refs block reserve, we will consume > space from the global block reserve. > > Now this doesn't make a lot of sense because: > > 1) We should reserve space for delayed references when starting the > transaction, since we have no guarantees the delayed refs block > reserve will be refilled; > > 2) If no refill happens then we will consume from the global block reserve > when running delayed refs during the transaction commit; > > 3) If we have a bunch of tasks calling btrfs_start_transaction() with a > number of items greater than zero and at the time the delayed refs > reserve is full, then we don't reserve any space at > btrfs_start_transaction() for the delayed refs that will be generated > by a task, and we can therefore end up using a lot of space from the > global reserve when running the delayed refs during a transaction > commit; > > 4) There are also other operations that result in bumping the size of the > delayed refs reserve, such as creating and deleting block groups, as > well as the need to update a block group item because we allocated or > freed an extent from the respective block group; > > 5) If we have a significant gap betweent the delayed refs reserve's size > and its reserved space, two very bad things may happen: > > 1) The reserved space of the global reserve may not be enough and we > fail the transaction commit with -ENOSPC when running delayed refs; > > 2) If the available space in the global reserve is enough it may result > in nearly exhausting it. If the fs has no more unallocated device > space for allocating a new block group and all the available space > in existing metadata block groups is not far from the global > reserve's size before we started the transaction commit, we may end > up in a situation where after the transaction commit we have too > little available metadata space, and any future transaction commit > will fail with -ENOSPC, because although we were able to reserve > space to start the transaction, we were not able to commit it, as > running delayed refs generates some more delayed refs (to update the > extent tree for example) - this includes not even being able to > commit a transaction that was started with the goal of unlinking a > file, removing an empty data block group or doing reclaim/balance, > so there's no way to release metadata space. > > In the worst case the next time we mount the filesystem we may > also fail with -ENOSPC due to failure to commit a transaction to > cleanup orphan inodes. This later case was reported and hit by > someone running a SLE (SUSE Linux Enterprise) distribution for > example - where the fs had no more unallocated space that could be > used to allocate a new metadata block group, and the available > metadata space was about 1.5M, not enough to commit a transaction > to cleanup an orphan inode (or do relocation of data block groups > that were far from being full). > > So improve on this situation by always reserving space for delayed refs > when calling start_transaction(), and if the flush method is > BTRFS_RESERVE_FLUSH_ALL, also try to refill the delayeds refs block > reserve if it's not full. The space reserved for the delayed refs is added > to a local block reserve that is part of the transaction handle, and when > a task updates the delayed refs block reserve size, after creating a > delayed ref, the space is transferred from that local reserve to the > global delayed refs reserve (fs_info->delayed_refs_rsv). In case the > local reserve does not have enough space, which may happen for tasks > that generate a variable and potentially large number of delayed refs > (such as the hole punching and extent cloning cases mentioned before), > we transfer any available space and then rely on the current behaviour > of hoping some other task refills the delayed refs reserve or fallback > to the global block reserve. > > Signed-off-by: Filipe Manana Reviewed-by: Josef Bacik Thanks, Josef