From e2fd83064a9bae368ce1c88a0cb9aee64ad4e124 Mon Sep 17 00:00:00 2001 From: Filipe Manana Date: Tue, 17 Jan 2023 11:21:39 +0000 Subject: btrfs: skip backref walking during fiemap if we know the leaf is shared During fiemap, when checking if a data extent is shared we are doing the backref walking even if we already know the leaf is shared, which is a waste of time since if the leaf shared then the data extent is also shared. So skip the backref walking when we know we are in a shared leaf. The following test was measures the gains for a case where all leaves are shared due to a snapshot: $ cat test.sh #!/bin/bash DEV=/dev/sdj MNT=/mnt/sdj umount $DEV &> /dev/null mkfs.btrfs -f $DEV # Use compression to quickly create files with a lot of extents # (each with a size of 128K). mount -o compress=lzo $DEV $MNT # 40G gives 327680 extents, each with a size of 128K. xfs_io -f -c "pwrite -S 0xab -b 1M 0 40G" $MNT/foobar # Add some more files to increase the size of the fs and extent # trees (in the real world there's a lot of files and extents # from other files). xfs_io -f -c "pwrite -S 0xcd -b 1M 0 20G" $MNT/file1 xfs_io -f -c "pwrite -S 0xef -b 1M 0 20G" $MNT/file2 xfs_io -f -c "pwrite -S 0x73 -b 1M 0 20G" $MNT/file3 # Create a snapshot so all the extents become indirectly shared # through subtrees, with a generation less than or equals to the # generation used to create the snapshot. btrfs subvolume snapshot -r $MNT $MNT/snap1 # Unmount and mount again to clear cached metadata. umount $MNT mount -o compress=lzo $DEV $MNT start=$(date +%s%N) # The filefrag tool uses the fiemap ioctl. filefrag $MNT/foobar end=$(date +%s%N) dur=$(( (end - start) / 1000000 )) echo "fiemap took $dur milliseconds (metadata not cached)" echo start=$(date +%s%N) filefrag $MNT/foobar end=$(date +%s%N) dur=$(( (end - start) / 1000000 )) echo "fiemap took $dur milliseconds (metadata cached)" umount $MNT The results were the following on a non-debug kernel (Debian's default kernel config). Before this patch: (...) /mnt/sdi/foobar: 327680 extents found fiemap took 1821 milliseconds (metadata not cached) /mnt/sdi/foobar: 327680 extents found fiemap took 399 milliseconds (metadata cached) After this patch: (...) /mnt/sdi/foobar: 327680 extents found fiemap took 591 milliseconds (metadata not cached) /mnt/sdi/foobar: 327680 extents found fiemap took 123 milliseconds (metadata cached) That's a speedup of 3.1x and 3.2x. Signed-off-by: Filipe Manana Signed-off-by: David Sterba --- fs/btrfs/backref.c | 21 ++++++++++++++++++++- 1 file changed, 20 insertions(+), 1 deletion(-) (limited to 'fs/btrfs/backref.c') diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c index f846fec08c86..90e40d5ceccd 100644 --- a/fs/btrfs/backref.c +++ b/fs/btrfs/backref.c @@ -1872,6 +1872,8 @@ int btrfs_is_data_extent_shared(struct btrfs_inode *inode, u64 bytenr, .have_delayed_delete_refs = false, }; int level; + bool leaf_cached; + bool leaf_is_shared; for (int i = 0; i < BTRFS_BACKREF_CTX_PREV_EXTENTS_SIZE; i++) { if (ctx->prev_extents_cache[i].bytenr == bytenr) @@ -1893,6 +1895,23 @@ int btrfs_is_data_extent_shared(struct btrfs_inode *inode, u64 bytenr, walk_ctx.time_seq = elem.seq; } + ctx->use_path_cache = true; + + /* + * We may have previously determined that the current leaf is shared. + * If it is, then we have a data extent that is shared due to a shared + * subtree (caused by snapshotting) and we don't need to check for data + * backrefs. If the leaf is not shared, then we must do backref walking + * to determine if the data extent is shared through reflinks. + */ + leaf_cached = lookup_backref_shared_cache(ctx, root, + ctx->curr_leaf_bytenr, 0, + &leaf_is_shared); + if (leaf_cached && leaf_is_shared) { + ret = 1; + goto out_trans; + } + walk_ctx.ignore_extent_item_pos = true; walk_ctx.trans = trans; walk_ctx.fs_info = fs_info; @@ -1901,7 +1920,6 @@ int btrfs_is_data_extent_shared(struct btrfs_inode *inode, u64 bytenr, /* -1 means we are in the bytenr of the data extent. */ level = -1; ULIST_ITER_INIT(&uiter); - ctx->use_path_cache = true; while (1) { bool is_shared; bool cached; @@ -1972,6 +1990,7 @@ int btrfs_is_data_extent_shared(struct btrfs_inode *inode, u64 bytenr, ctx->prev_extents_cache_slot = slot; } +out_trans: if (trans) { btrfs_put_tree_mod_seq(fs_info, &elem); btrfs_end_transaction(trans); -- cgit v1.2.3-73-gaa49b