From 82bfb2e7b645c8f228dc3b6d3b27b0b10125ca4f Mon Sep 17 00:00:00 2001 From: Filipe Manana Date: Tue, 14 Feb 2017 17:56:32 +0000 Subject: [PATCH] Btrfs: incremental send, fix unnecessary hole writes for sparse files When using the NO_HOLES feature, during an incremental send we often issue write operations for holes when we should not, because that range is already a hole in the destination snapshot. While that does not change the contents of the file at the receiver, it avoids preservation of file holes, leading to wasted disk space and extra IO during send/receive. A couple examples where the holes are not preserved follows. $ mkfs.btrfs -O no-holes -f /dev/sdb $ mount /dev/sdb /mnt $ xfs_io -f -c "pwrite -S 0xaa 0 4K" /mnt/foo $ xfs_io -f -c "pwrite -S 0xaa 0 4K" -c "pwrite -S 0xbb 1028K 4K" /mnt/bar $ btrfs subvolume snapshot -r /mnt /mnt/snap1 # Now add one new extent to our first test file, increasing its size and # leaving a 1Mb hole between the first extent and this new extent. $ xfs_io -c "pwrite -S 0xbb 1028K 4K" /mnt/foo # Now overwrite the last extent of our second test file. $ xfs_io -c "pwrite -S 0xcc 1028K 4K" /mnt/bar $ btrfs subvolume snapshot -r /mnt /mnt/snap2 $ xfs_io -r -c "fiemap -v" /mnt/snap2/foo /mnt/snap2/foo: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..7]: 25088..25095 8 0x2000 1: [8..2055]: hole 2048 2: [2056..2063]: 24576..24583 8 0x2001 $ xfs_io -r -c "fiemap -v" /mnt/snap2/bar /mnt/snap2/bar: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..7]: 25096..25103 8 0x2000 1: [8..2055]: hole 2048 2: [2056..2063]: 24584..24591 8 0x2001 $ btrfs send /mnt/snap1 -f /tmp/1.snap $ btrfs send -p /mnt/snap1 /mnt/snap2 -f /tmp/2.snap $ umount /mnt # It's not relevant to enable no-holes in the new filesystem. $ mkfs.btrfs -O no-holes -f /dev/sdc $ mount /dev/sdc /mnt $ btrfs receive /mnt -f /tmp/1.snap $ btrfs receive /mnt -f /tmp/2.snap $ xfs_io -r -c "fiemap -v" /mnt/snap2/foo /mnt/snap2/foo: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..7]: 24576..24583 8 0x2000 1: [8..2063]: 25624..27679 2056 0x1 $ xfs_io -r -c "fiemap -v" /mnt/snap2/bar /mnt/snap2/bar: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..7]: 24584..24591 8 0x2000 1: [8..2063]: 27680..29735 2056 0x1 The holes do not exist in the second filesystem and they were replaced with extents filled with the byte 0x00, making each file take 1032Kb of space instead of 8Kb. So fix this by not issuing the write operations consisting of buffers filled with the byte 0x00 when the destination snapshot already has a hole for the respective range. A test case for fstests will follow soon. Signed-off-by: Filipe Manana --- fs/btrfs/send.c | 88 +++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 86 insertions(+), 2 deletions(-) diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c index 712922ea64d2..456c8901489b 100644 --- a/fs/btrfs/send.c +++ b/fs/btrfs/send.c @@ -5306,6 +5306,81 @@ out: return ret; } +static int range_is_hole_in_parent(struct send_ctx *sctx, + const u64 start, + const u64 end) +{ + struct btrfs_path *path; + struct btrfs_key key; + struct btrfs_root *root = sctx->parent_root; + u64 search_start = start; + int ret; + + path = alloc_path_for_send(); + if (!path) + return -ENOMEM; + + key.objectid = sctx->cur_ino; + key.type = BTRFS_EXTENT_DATA_KEY; + key.offset = search_start; + ret = btrfs_search_slot(NULL, root, &key, path, 0, 0); + if (ret < 0) + goto out; + if (ret > 0 && path->slots[0] > 0) + path->slots[0]--; + + while (search_start < end) { + struct extent_buffer *leaf = path->nodes[0]; + int slot = path->slots[0]; + struct btrfs_file_extent_item *fi; + u64 extent_end; + + if (slot >= btrfs_header_nritems(leaf)) { + ret = btrfs_next_leaf(root, path); + if (ret < 0) + goto out; + else if (ret > 0) + break; + continue; + } + + btrfs_item_key_to_cpu(leaf, &key, slot); + if (key.objectid < sctx->cur_ino || + key.type < BTRFS_EXTENT_DATA_KEY) + goto next; + if (key.objectid > sctx->cur_ino || + key.type > BTRFS_EXTENT_DATA_KEY || + key.offset >= end) + break; + + fi = btrfs_item_ptr(leaf, slot, struct btrfs_file_extent_item); + if (btrfs_file_extent_type(leaf, fi) == + BTRFS_FILE_EXTENT_INLINE) { + u64 size = btrfs_file_extent_inline_len(leaf, slot, fi); + + extent_end = ALIGN(key.offset + size, + root->fs_info->sectorsize); + } else { + extent_end = key.offset + + btrfs_file_extent_num_bytes(leaf, fi); + } + if (extent_end <= start) + goto next; + if (btrfs_file_extent_disk_bytenr(leaf, fi) == 0) { + search_start = extent_end; + goto next; + } + ret = 0; + goto out; +next: + path->slots[0]++; + } + ret = 1; +out: + btrfs_free_path(path); + return ret; +} + static int maybe_send_hole(struct send_ctx *sctx, struct btrfs_path *path, struct btrfs_key *key) { @@ -5350,8 +5425,17 @@ static int maybe_send_hole(struct send_ctx *sctx, struct btrfs_path *path, return ret; } - if (sctx->cur_inode_last_extent < key->offset) - ret = send_hole(sctx, key->offset); + if (sctx->cur_inode_last_extent < key->offset) { + ret = range_is_hole_in_parent(sctx, + sctx->cur_inode_last_extent, + key->offset); + if (ret < 0) + return ret; + else if (ret == 0) + ret = send_hole(sctx, key->offset); + else + ret = 0; + } sctx->cur_inode_last_extent = extent_end; return ret; } -- 2.20.1