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Scaling the Btrfs Free Space Cache
Omar SandovalVault 2016
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Outline
Background
Design Background
Identifying the Problem
SolutionThe Free Space Tree
Results
Questions
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Background
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What is Btrfs?
• General-purpose filesystem• Advanced features
• First-class snapshots• Checksumming of all data and metadata• Transparent compression• Integrated multi-device support (RAID)• Incremental backups with send/receive
• Actively developed
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Contributors$ git log --since=”January 1, 2015” --pretty=format:”%an” --no-merges -- fs/btrfs
Filipe Manana Dan Carpenter Borislav Petkov Mike Snitzer
David Sterba Forrest Liu Andreas Gruenbacher Mel Gorman
Zhao Lei Yang Dongsheng Alex Lyakas Matthew Wilcox
Qu Wenruo Tsutomu Itoh Zach Brown Konstantin Khebnikov
Anand Jain Michal Hocko Yauhen Kharuzhy Jan Kara
Josef Bacik Kirill A. Shutemov Yaowei Bai Guenter Roeck
Omar Sandoval Jiri Kosina Vladimir Davydov Greg Kroah-Hartman
Chris Mason Daniel Dressler Tom Van Braeckel Geert Uytterhoeven
Zhaolei Wang Shilong Sudip Mukherjee Gabríel Arthúr Pétursson
Liu Bo Satoru Takeuchi Shilong Wang Dmitry Monakhov
Chandan Rajendra Naohiro Aota Shaohua Li Deepa Dinamani
Dongsheng Yang Luis de Bethencourt Shan Hai Davide Italiano
Alexandru Moise Kinglong Mee Sebastian Andrzej Siewior Dave Jones
Mark Fasheh Kent Overstreet Sasha Levin Darrick J. Wong
Eric Sandeen Justin Maggard Sam Tygier Colin Ian King
Jeff Mahoney Jens Axboe Robin Ruede Chengyu Song
Byongho Lee Holger Hoffstätte Rasmus Villemoes chandan r
Christoph Hellwig Gui Hecheng Quentin Casasnovas Ashish Samant
Al Viro Fabian Frederick Pranith Kumar Arnd Bergmann
chandan David Howells Oleg Nesterov Adam Buchbinder
Geliang Tang Christian Engelmayer NeilBrown
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Btrfs at Facebook
• GlusterFS• Testing on other tiers
• Web servers• Build machines• More...
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Design Background
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Copy-on-Write B-trees
• B+-trees with no leaf chaining• Lock coupling• Relaxed balancing contraints, proactive balancing• Lazy reference-counting• Update in memory, write out in periodic commits• Due to O. Rodeh
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Copy-on-Write B-tree Insertion
1, 4, 10
1, 2 5, 6, 7 10, 11
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Copy-on-Write B-tree Insertion
1, 4, 10
1, 2 5, 6, 7 10, 11
1, 4, 10
10, 11, 19
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Copy-on-Write B-tree Deletion
1, 4, 10
1, 2 5, 6, 7 10, 11
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Copy-on-Write B-tree Deletion
1, 4, 10
1, 2 5, 6, 7 10, 11
1, 4, 10
5, 7
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Btrfs B-trees
• Generic data structure• Three main structures:
• Block header: header of every block in the B-tree, contains checksum, flags, filesystem ID,generation, etc.
• Key : comprises 64-bit object ID, 8-bit type, and 64-bit offset• Item: comprises key, 32-bit offset, and 32-bit size
• Nodes contain only keys and block pointers to children• Leaves contain array of items and data
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B-tree Nodes
struct btrfs_disk_keyobjectid=256 type=INODE_ITEM offset=0
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B-tree Nodes
struct btrfs_key_ptrstruct btrfs_disk_key
256 INODE_ITEM 0blockptr=46976991232 generation=51107
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B-tree Nodes
struct btrfs_node
struct btrfs_headerstruct btrfs_key_ptr
struct btrfs_disk_key256 INODE_ITEM 0
46976991232 51107
struct btrfs_key_ptrstruct btrfs_disk_key
261 DIR_ITEM 35889527346976319488 51107
...
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B-tree Leaves
struct btrfs_itemstruct btrfs_disk_key
256 INODE_ITEM 0offset=16123 size=160
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B-tree Leaves
struct btrfs_leaf
struct btrfs_headerstruct btrfs_item
struct btrfs_disk_key256 INODE_ITEM 0
16123 160
struct btrfs_itemstruct btrfs_disk_key
256 INODE_REF 25616111 12
... struct btrfs_inode_ref struct btrfs_inode_item
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B-tree Structure
struct btrfs_nodestruct btrfs_header struct btrfs_key_ptr struct btrfs_key_ptr ...
... ...
struct btrfs_nodestruct btrfs_header struct btrfs_key_ptr struct btrfs_key_ptr ...
struct btrfs_leafstruct btrfs_header struct btrfs_item struct btrfs_item ... data data
...
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B-trees Used in Btrfs
• Root tree• FS trees• Extent tree• Checksum tree• Chunk tree, device tree• Relocation tree, log tree, quota tree, UUID tree
• Free space tree: subject of this talk!
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B-trees Used in Btrfs
• Root tree• FS trees• Extent tree• Checksum tree• Chunk tree, device tree• Relocation tree, log tree, quota tree, UUID tree• Free space tree: subject of this talk!
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Identifying the Problem
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Symptoms
• Large (tens of terabytes), busy filesystems on GlusterFS• Stalls during commits, sometimes seconds long• Narrowed down the problem to write-out of the free space cache
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Terminology
• Chunk: physical unit of allocation, 1 GBfor data, 256 MB for metadata
• Block group: logical unit of allocation,comprises one or more chunks
• Extent: range of filesystem used for aparticular purpose
Chunks
Disk 1 Disk 2
Block group
Extent
Figure: Basic terminology
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Extent Tree
• Tracks allocated space with references• Three types of items:
• Block group items• Extent items• Metadata items
EXTENT_ITEM EXTENT_ITEM
BLOCK_GROUP_ITEM
Figure: Data block group
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Extent Tree
• Tracks allocated space with references• Three types of items:
• Block group items• Extent items• Metadata items
METADATA_ITEM METADATA_ITEM METADATA_ITEM
BLOCK_GROUP_ITEM
Figure: Metadata block group
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Free Space Cache
• Track free space in memory after loading a block group• Red-black tree• Hybrid of extents + bitmaps
• Loading directly from the extent tree is slow• Instead, dump the in-memory cache to disk
• Special free space inodes, one per block group• Compact, very fast to load• Has to be to written out in entirety
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Delayed Refs
• Don’t want to update extent tree on diskfor every operation
• Instead, track updates in memory(red-black tree)
• Run at commit time as a batch
32
16 128
8 24
add 2 drop 1 add 1
Figure: Delayed refs tree
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Diagnosis
• Large, busy filesystem can dirty many block groups• Have to write out the free space cache for each one• Run during the critical section of the commit
• Blocks other operations
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Solution
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False Start
• Start writing out free space cache outside of critical section• Redo block groups that got dirtied again during that window• Still possible for a lot to get dirtied again
• Back to square one
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The Free Space Tree
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Key Ideas
• Use another B-tree instead• Inverse of the extent tree
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On-Disk Format
• Per-block group free space info• Extent count• Flags
• Free space extents• Start• Length
• Free space bitmaps• Start• Length• Bitmap
FREE_SPACE_EXTENT FREE_SPACE_EXTENT
EXTENT_ITEM EXTENT_ITEM
Figure: Free space tree extents
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On-Disk Format
• Per-block group free space info• Extent count• Flags
• Free space extents• Start• Length
• Free space bitmaps• Start• Length• Bitmap
FREE_SPACE_BITMAP FREE_SPACE_BITMAP
EXTENT_ITEM EXTENT_ITEM
Figure: Free space tree bitmaps
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Maintaining the Free Space Tree
• Piggy-back updates on delayed-refs• Get batching for free
• Convert between extents or bitmaps• Thresholds for whichever is more space-efficient• Buffer to avoid thrashing between formats
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Results
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Measuring Commit Overhead
• Use fallocate and unlink to dirty a lot of block groups• Sparse filesystem image on loop device to allow for exaggerated sizes (50TB of dirtiedspace)
• Measure time spent committing, esp. in critical section
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Commit Overhead
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
Free space cache Free space tree No cache
Tim
e s
pent in
critical section p
er
tran
sactio
n (
ms)
Free space commit overhead
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Commit Overhead
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Free space tree No cache
Tim
e s
pent in
critical section p
er
tran
sactio
n (
ms)
Free space commit overhead
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Measuring Load Overhead
• Create a fragmented filesystem with fs_mark
• Unmount, remount• Run fs_mark again• Measure time spent loading space cache
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Load Overhead
0
50
100
150
200
250
Free space cache Free space tree No cache
Tota
l tim
e s
pent cachin
g b
lock g
roup
s (
ms)
Free space loading overhead
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Try It Out
mount -o space_cache=v2(Since Linux v4.5)
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Questions
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