[537] Fast File System. Tyler Harter
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1 [537] Fast File System Tyler Harter
2 File-System Case Studies Local - FFS: Fast File System - LFS: Log-Structured File System Network - NFS: Network File System - AFS: Andrew File System
3 File-System Case Studies Local - FFS: Fast File System [today] - LFS: Log-Structured File System Network - NFS: Network File System - AFS: Andrew File System
4 Review Basic FS
5 Basic FS Structures (on disk) Operations
6 Structure Overview Core Performance Super Block
7 Structure Overview Core Performance Super Block Data Block
8 Structure Overview Core Performance Super Block Data Block Inode Table
9 Structure Overview Core Performance Super Block Data Block directories indirects Inode Table
10 Structure Overview Core Performance Super Block Data Block directories indirects Inode Table Data Bitmap Inode Bitmap
11 Layout super block inodes Data Blocks 0 N
12 Layout super block bit maps inodes Data Blocks 0 N
13 Layout super block bit maps inodes Data Blocks 0 N inodes data blocks
14 Layout super block bit maps inodes Data Blocks 0 N inodes data blocks regular data directories indirect blocks
15 Basic FS Structures (on disk) Operations
16 create /foo/bar data inode root foo bar root foo bitmap bitmap inode inode inode data data
17 create /foo/bar [traverse] data inode root foo bar root foo bitmap bitmap inode inode inode data data read read
18 create /foo/bar [traverse] data inode root foo bar root foo bitmap bitmap inode inode inode data data read read read read
19 create /foo/bar [traverse] data inode root foo bar root foo bitmap bitmap inode inode inode data data read read read read foo inode: we have perm foo data: bar doesn t exist
20 create /foo/bar data inode root foo bar root foo bitmap bitmap inode inode inode data data read read read read
21 create /foo/bar [allocate inode] data inode root foo bar root foo bitmap bitmap inode inode inode data data read write read read read read
22 create /foo/bar [populate inode] data inode root foo bar root foo bitmap bitmap inode inode inode data data read write read read read write read read
23 create /foo/bar [add bar to /foo] data inode root foo bar root foo bitmap bitmap inode inode inode data data read write read read write read write read read write
24 append to /foo/bar data inode root foo bar root foo bitmap bitmap inode inode inode data data bar data
25 append to /foo/bar [block full? yes] data inode root foo bar root foo bitmap bitmap inode inode inode data data bar data read
26 append to /foo/bar [allocate block] data inode root foo bar root foo bitmap bitmap inode inode inode data data bar data read write read
27 append to /foo/bar [point to block] data inode root foo bar root foo bitmap bitmap inode inode inode data data bar data read write read write
28 append to /foo/bar [write to block] data inode root foo bar root foo bitmap bitmap inode inode inode data data bar data read write read write write
29 append to /foo/bar [write to block] dir entries data inode root foo bar root foo bitmap bitmap inode inode inode data data file bar data read write read write write
30 Review Locality
31 Locality Types address address time time
32 Locality Types address Temporal Locality address Spatial Locality time time
33 Locality Usefulness What types of locality are useful for a cache? What types of locality are useful for a disk?
34 Order Matters Now address address time time
35 Order Matters Now address Slow address Fast time time
36 Policy: Choose Inode, Data Blocks S i d I I I I I 0 7 D D D D D D D D 8 15 D D D D D D D D D D D D D D D D 24 31
37 Bad File Layout inode S i d I I I I I D D D D D D D D D D D D D D D D D D D D D D D D 24 31
38 Better File Layout inode S i d I I I I I D D D D D D D D 8 15 D D D D D D D D D D D D D D D D 24 31
39 Best File Layout S i d I I I I I 0 7 inode D D D D D D D D 8 15 D D D D D D D D D D D D D D D D 24 31
40 Fast File System
41 System Building noob approach 1. get idea 2. build it!
42 System Building noob approach 1. get idea 2. build it! pro approach 1. identify state of the art 2. measure it, identify problems 3. get idea 4. build it!
43 System Building noob approach 1. get idea 2. build it! pro approach 1. identify state of the art 2. measure it, identify problems 3. get idea 4. build it! measure then build
44 Old FS State of the art: original UNIX file system.
45 Layout super block inodes Data Blocks 0 N Free lists are embedded in inodes, data blocks. Data blocks are 512 bytes.
46 Old FS State of the art: original UNIX file system.
47 Old FS State of the art: original UNIX file system. Measure throughput for file reads/writes. Compare to theoretical max, which is
48 Old FS State of the art: original UNIX file system. Measure throughput for file reads/writes. Compare to theoretical max, which is disk bandwidth
49 Old FS State of the art: original UNIX file system. Measure throughput for file reads/writes. Compare to theoretical max, which is disk bandwidth Old UNIX file system: only 2% of potential. Why?
50 Measurement 1 What is performance before/after aging?
51 Measurement 1 What is performance before/after aging? New FS: 17.5% of disk bandwidth Few weeks old: 3% of disk bandwidth
52 Measurement 1 What is performance before/after aging? New FS: 17.5% of disk bandwidth Few weeks old: 3% of disk bandwidth FS is probably becoming fragmented over time. Free list makes contiguous chunks hard to find.
53 Measurement 1 What is performance before/after aging? New FS: 17.5% of disk bandwidth Few weeks old: 3% of disk bandwidth hacky solution: occasional defrag FS is probably becoming fragmented over time. Free list makes contiguous chunks hard to find.
54 Measurement 2 How does block size affect performance? Try doubling it!
55 Measurement 2 How does block size affect performance? Try doubling it! Performance more than doubled.
56 Measurement 2 How does block size affect performance? Try doubling it! Performance more than doubled.
57 Measurement 2 How does block size affect performance? Try doubling it! Performance more than doubled. Logically adjacent blocks are probably not physically adjacent.
58 Measurement 2 How does block size affect performance? Try doubling it! Performance more than doubled. Logically adjacent blocks are probably not physically adjacent.
59 Measurement 2 How does block size affect performance? Try doubling it! Performance more than doubled. Logically adjacent blocks are probably not physically adjacent. Smaller blocks cause more indirect I/O.
60 Old FS Summary Observations: - long distance between inodes/data - inodes in single dir not close to one another - small blocks (512 bytes) - blocks laid out poorly - free list becomes scrambled, causes random alloc Result: 2% of potential performance! (and worse over time)
61 Problem: old FS treats disk like RAM!
62 Solution: a disk-aware FS
63 Design Questions How to use big blocks without wasting space? How to place data on disk?
64 Technique 1: Bitmaps super block inodes Data Blocks 0 N
65 Technique 1: Bitmaps super block bitmaps inodes Data Blocks 0 N Use bitmaps instead of free list. Provides more flexibility, with more global view.
66 Bitmaps Techniques
67 Technique 2: Groups fast super block bitmaps inodes Data Blocks 0 N before: whole disk
68 Technique 2: Groups slow super block bitmaps inodes Data Blocks 0 N before: whole disk
69 Technique 2: Groups slower super block bitmaps inodes Data Blocks 0 N before: whole disk
70 Technique 2: Groups slowest super block bitmaps inodes Data Blocks 0 N before: whole disk
71 Technique 2: Groups super block bitmaps inodes Data Blocks 0 N before: whole disk
72 Technique 2: Groups super block bitmaps inodes Data Blocks 0 G now: one (smallish) group
73 Technique 2: Groups S B I D S B I D S B I D 0 group 1 G 2G group 2 group 3 3G zoom out
74 Technique 2: Groups fast fast fast S B I D S B I D S B I D 0 group 1 G 2G group 2 group 3 3G strategy: allocate inodes and data blocks in same group.
75 Groups In FFS, groups were ranges of cylinders - called cylinder group In ext2-4, groups are ranges of blocks - called block group
76 Groups In FFS, groups were ranges of cylinders - called cylinder group In ext2-4, groups are ranges of blocks - called block group
77 Techniques Bitmaps Locality groups
78 Technique 3: Super Rotation S B I D S B I D S B I D 0 group 1 G 2G group 2 group 3 3G
79 Technique 3: Super Rotation S B I D S B I D S B I D 0 group 1 G 2G group 2 group 3 3G Is it useful to have multiple super blocks?
80 Technique 3: Super Rotation S B I D S B I D S B I D 0 group 1 G 2G group 2 group 3 3G Is it useful to have multiple super blocks? Yes, if some (but not all) fail.
81 Problem
82 Problem All super-block copies are on the top platter. What if it dies?
83 Problem All super-block copies are on the top platter. What if it dies? solution: for each group, store super-block at different offset.
84 Techniques Bitmaps Locality groups Rotated super
85 Block Size Doubling the block size for the old FS over doubled performance. Strategy: choose block size so we never have to read more than two indirect blocks to find a data block (2 levels of indirection max). Want 4GB files.
86 Techniques Bitmaps Locality groups Rotated super Large blocks
87 Large Blocks Why not make blocks huge?
88 Most file are very small.
89 Large Blocks Why not make blocks huge? Lots of waste in remainder of blocks. 50 Plot summarizes table 1 of FFS paper Time vs. Space Tradeoffs Percent Block Size
90 Solution: Fragments Hybrid! Introduce fragment for files that use parts of blocks. Only tail of file uses fragments.
91 Fragment Example Block size = 4096 Fragment size = 1024 bits: blk1 blk2 blk3 blk4
92 How to Decide Whether addr refers to block or fragment is inferred by the file size. What about when files grow? Must copy fragments to new block if there s not room to grow.
93 file, size 5KB file, size 2KB AAAA B A B
94 file, size 6KB file, size 2KB AAAA B A B A append A to first file
95 file, size 6KB file, size 2KB AAAA B A B A
96 file, size 7KB file, size 2KB AAAA B A B A A append A to first file
97 file, size 7KB file, size 2KB AAAA B A B A A
98 file, size 8KB file, size 2KB AAAA B B AAAA append A to first file, copy to fragments to new block.
99 Optimal Write Size Writing less than a block is inefficient. Solution: new API exposes optimal write size. For pipes and sockets, the new call returns the buffer size. The stdio library uses this call.
100 Techniques Bitmaps Locality groups Rotated super Large blocks Fragments
101 Smart Policy S B I D S B I D S B I D 0 group 1 G 2G group 2 group 3 3G Where should new inodes and data blocks go?
102 Strategy Put related pieces of data near each other.
103 Strategy Put related pieces of data near each other. Rules: 1. Put directory entries near directory inodes 2. Put inodes near directory entries 3. Put data blocks near inodes
104 Strategy Put related pieces of data near each other. Rules: 1. Put directory entries near directory inodes 2. Put inodes near directory entries 3. Put data blocks near inodes Sound good?
105 Challenge The file system is one big tree. All directories and files have a common root. In some sense, all data in the same FS is related.
106 Challenge The file system is one big tree. All directories and files have a common root. In some sense, all data in the same FS is related. Trying to put everything near everything else will leave us with the same mess we started with.
107 Revised Strategy Put more-related pieces of data near each other. Put less-related pieces of data far from each other.
108 Revised Strategy Put more-related pieces of data near each other. Put less-related pieces of data far from each other. FFS developers used their best judgement.
109 FFS: Two-Level Allocator Level 1: decide which group Level 2: decide where in group
110 B1 file inode B2 inode dir B3 pointer related dir inode B1 B2
111 B1 file inode B2 inode dir B3 many pointer related dir inode B1 B2
112 Where to cut the tree and start growing into another group? B1 file inode B2 inode dir B3 many pointer related dir inode B1 B2
113 FFS puts dir inodes in a new group. B1 file inode B2 inode dir B3 many pointer related break dir inode B1 B2
114 ls is fast on directories with many files. B1 file inode B2 inode dir B3 many pointer related break dir inode B1 B2
115 Preferences File inodes: allocate in same group with dir Dir inodes: allocate in new group with fewer inodes than the average group First data block: allocate near inode Other data blocks: allocate near previous block
116 Problem: Large Files A single large file can use nearly all of a group. This displaces data for many small files. It s better to do one seek for the large file than one seek for each of many small files.
117 B1 file inode B2 inode dir B3 many pointer related break dir inode B1 B2
118 Define large as requiring an indirect. B1 file inode B2 inode dir Ind B3 many break dir inode B1 B4 pointer related B2
119 Starting at indirect (e.g., after 48 KB), put blocks in a new block group. B1 file inode B2 break inode dir Ind break B3 many break dir inode B1 B4 pointer related B2
120 Preferences File inodes: allocate in same group with dir Dir inodes: allocate in new group with fewer inodes than the average group First data block: allocate near inode Other data blocks: allocate near previous block Large file data blocks: after 48KB, go to new group. Move to another group (w/ fewer than avg blocks) every subsequent 1MB.
121 Preferences File inodes: allocate in same group with dir Dir inodes: allocate in new group with fewer inodes than the average group First data block: allocate near inode Other data blocks: allocate near previous block Large file data blocks: after 48KB, go to new group. Move to another group (w/ fewer than avg blocks) every subsequent 1MB.
122 Group Descriptor (aka Summary Block) super block bitmaps inodes Data Blocks 0 G
123 Group Descriptor (aka Summary Block) super block summary bitmaps inodes Data Blocks 0 G how many free inodes, data blocks?
124 Bitmaps Locality groups Rotated super Large blocks Fragments Smart allocation Techniques
125 Conclusion First disk-aware file system. FFS inspired modern files systems, including ext2 and ext3. FFS also introduced several new features: - long file names - atomic rename - symbolic links
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