On 09/22/2016 05:25 AM, Wang Xiaoguang wrote:
Since commit b02441999efcc6152b87cd58e7970bb7843f76cf, we don't wait all ordered extents, but I run into some enospc errors when doing large file create and delete test, it's because shrink_delalloc() does not write enough delalloc bytes and wait them finished: From: Miao Xie <mi...@cn.fujitsu.com> Date: Mon, 4 Nov 2013 23:13:25 +0800 Subject: [PATCH] Btrfs: don't wait for the completion of all the ordered extentsIt is very likely that there are lots of ordered extents in the filesytem, if we wait for the completion of all of them when we want to reclaim some space for the metadata space reservation, we would be blocked for a long time. The performance would drop down suddenly for a long time. But since Josef introduced "Btrfs: introduce ticketed enospc infrastructure", shrink_delalloc() starts to be run asynchronously, then If we want to reclaim metadata space, we can try harder, after all, false enospc error is not acceptable. Signed-off-by: Wang Xiaoguang <wangxg.f...@cn.fujitsu.com> --- fs/btrfs/extent-tree.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c index 46c2a37..f7c420b 100644 --- a/fs/btrfs/extent-tree.c +++ b/fs/btrfs/extent-tree.c @@ -4721,7 +4721,7 @@ static void shrink_delalloc(struct btrfs_root *root, u64 to_reclaim, u64 orig, if (trans) return; if (wait_ordered) - btrfs_wait_ordered_roots(root->fs_info, items, + btrfs_wait_ordered_roots(root->fs_info, -1, 0, (u64)-1); return; } @@ -4775,6 +4775,14 @@ skip_async: } delalloc_bytes = percpu_counter_sum_positive( &root->fs_info->delalloc_bytes); + if (loops == 2) { + /* + * Try to write all current delalloc bytes and wait all + * ordered extents to have a last try. + */ + to_reclaim = delalloc_bytes; + items = -1; + } } }
The problem is if the outstanding ordered extents aren't enough to actually return the space we need we end up flushing and waiting longer when we should have just committed the transaction. Think for example if we are slowly writing to a few files and rapidly removing thousands of files. In this case all of our space is tied up in pinned, so we'd be better off not waiting on ordered extents and instead committing the transaction.
I think instead what we should do is have a priority set, so instead of doing commit_cycles in btrfs_async_reclaim_metadata_space, we instead have priority, and set it to say 3. Then we pass this priority down to all of the flushers, and use it as a multiplier in delalloc for the number of items we'll wait on. Once we hit priority 0 we wait for all the things. This way we do the easy pass first and hope it works, if not we try harder the next time through, etc until we throw all caution to the wind and wait for anything we can find. Thanks,
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