All File Systems Are Not Created Equal: On the Complexity of Crafting Crash-Consistent Applications
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1 All File Systems Are Not Created Equal: On the Compleity of Crafting Crash-Consistent Applications Thanumalayan Sankaranarayana Pillai Vijay Chidambaram Ramnatthan Alagappan, Samer Al-Kiswany Andrea Arpaci-Dusseau, Remzi Arpaci-Dusseau
2 Crash Consistency Maintaining data invariants across system crash - E: database transactions should be atomic Important in many systems - Databases - File Systems - Key-Value Stores
3 File-System Crash Consistency Many techniques to ensure file system remains consistent after a crash - Journaling - Copy-on-write - Soft Updates Techniques ensure internal file-system structures are consistent 3
4 Application-Level Crash Consistency Many applications run over file systems - E: SQLite, LevelDB, Git, etc. 4
5 Application-Level Crash Consistency Many applications run over file systems - E: SQLite, LevelDB, Git, etc. Provide user guarantees across system crashes - E: SQLite ts provide ACID guarantees Interact with file systems via POSIX calls 4
6 Crash Recovery is Hard Databases took a long time to get it right - Commercial database - System R (98) - Crash Recovery algorithm - ARIES (99) - ARIES proved correct (997) Applications must achieve high performance and correctness - Mutating persistent state synchronously too slow - Leads to comple update protocols 5
7 Belief: all POSIX file systems implement calls the same way Application using POSIX interface should work on any POSIX-compliant file system POSIX specifies what happens in memory - Does not specify what happens on crash 6
8 Eample Goal: atomically update multiple blocks of file /dir/file my foo /dir/file my bar Initial state Final state Solution: use write-ahead-logging Protocol:. Write to log (/dir/log). Update /dir/file 3. Delete /dir/log On crash: read log, do steps and 3 7
9 Eample. Write log open( /dir/log, O_CREAT O_WRONLY O_TRUNC) pwrite( /dir/log, log_entry, 0, 6). Update /dir/file pwrite( /dir/file, data, 3, 3) 3. Delete /dir/log unlink( /dir/log ) 8
10 Eample open( /dir/log, O_CREAT O_WRONLY O_TRUNC) pwrite( /dir/log, log_entry, 0, 6) 3 pwrite( /dir/file, bar, 3, 3) 4 unlink( /dir/log ) Works in et3 data journaling mode Possible disk states after a crash: /dir/file /dir/log my foo After After /dir/file /dir/log my foo 3, 3, bar Middle of 3 /dir/file /dir/log my boo 3, 3, bar After 4 /dir/file my bar 9
11 Eample open( /dir/log, O_CREAT O_WRONLY O_TRUNC) pwrite( /dir/log, log_entry, 0, 6) 3 pwrite( /dir/file, bar, 3, 3) 4 unlink( /dir/log ) Fails in et3 ordered mode! /dir/file /dir/log my boo 0
12 Eample open( /dir/log, O_CREAT O_WRONLY O_TRUNC) pwrite( /dir/log, log_entry, 0, 6) 3 pwrite( /dir/file, bar, 3, 3) 4 unlink( /dir/log ) Fails in et3 ordered mode! /dir/file /dir/log my boo 0
13 Eample open( /dir/log, O_CREAT O_WRONLY O_TRUNC) pwrite( /dir/log, log_entry, 0, 6).5 fsync( /dir/log ) 3 pwrite( /dir/file, bar, 3, 3) 3.5 fsync( /dir/file ) 4 unlink( /dir/log ) Fails in et3 ordered mode! /dir/file /dir/log my boo 0
14 Eample open( /dir/log, O_CREAT O_WRONLY O_TRUNC) pwrite( /dir/log, log_entry, 0, 6).5 fsync( /dir/log ) 3 pwrite( /dir/file, bar, 3, 3) 3.5 fsync( /dir/file ) 4 unlink( /dir/log ) Fails in et3 ordered mode! Fails in et3 writeback mode! /dir/file /dir/log /dir/file /dir/log my boo my foo &%^ 0
15 Eample open( /dir/log, O_CREAT O_WRONLY O_TRUNC) pwrite( /dir/log, log_entry + csum, 0, 6).5 fsync( /dir/log ) 3 pwrite( /dir/file, bar, 3, 3) 3.5 fsync( /dir/file ) 4 unlink( /dir/log ) Fails in et3 ordered mode! Fails in et3 writeback mode! /dir/file /dir/log /dir/file /dir/log my boo my foo &%^ 0
16 Eample open( /dir/log, O_CREAT O_WRONLY O_TRUNC) pwrite( /dir/log, log_entry + csum, 0, 6).5 fsync( /dir/log ) 3 pwrite( /dir/file, bar, 3, 3) 3.5 fsync( /dir/file ) 4 unlink( /dir/log ) Fails in et3 ordered mode! Fails in et3 writeback mode! /dir/file /dir/log /dir/file /dir/log my boo my foo &%^ May fail in new POSIX file system /dir/file my boo 0
17 Eample open( /dir/log, O_CREAT O_WRONLY O_TRUNC) pwrite( /dir/log, log_entry + csum, 0, 6).5 fsync( /dir/log ).7 fsync( /dir ) 3 pwrite( /dir/file, bar, 3, 3) 3.5 fsync( /dir/file ) 4 unlink( /dir/log ) Fails in et3 ordered mode! Fails in et3 writeback mode! /dir/file /dir/log /dir/file /dir/log my boo my foo &%^ May fail in new POSIX file system /dir/file my boo 0
18 How do file-systems vary in implementing POSIX calls? Built Block-Order-Breaker(BOB) and analyzed 6 file systems How do applications maintain crash consistency? Built Application-Level Intelligent Crash Eplorer (ALICE) and analyzed applications
19 Outline Introduction Background Analyzing file systems with BOB Analyzing applications with ALICE Application Study Conclusion and Future Work
20 POSIX Standard POSIX standard is etremely weak Eample: POSIX fsync() need not flush data From the man page for Mac OS X fsync(): Specifically, if the drive loses power or the OS crashes, the application may find that only some or none of their data was written. Flushing data to disk requires F_FULLFSYNC fcntl 3
21 Unwritten Standard Developers coded to an unwritten standard - Based on et3 (default Linu fs for many years) Widely held belief: - Correct behavior == et3 behavior - POSIX guarantees not widely known 4
22 All was well until et4 introduced delayed allocation - data writes could be persisted after rename() Changed guarantees given to applications Broke hundreds of applications - write (tmp); rename(tmp, old) led to zero length files Caused wide-spread data loss 5
23 Application developers: your file system is broken! FS developers: your application is broken! It doesn t follow POSIX! 6
24 File-system developers added code to bring back old behavior in certain cases All this could have been avoided if application requirements are known to developers Our tool, ALICE, allows developers to do this 7
25 Outline Introduction Background Analyzing file systems with BOB Analyzing applications with ALICE Application Study Conclusion and Future Work 8
26 Analyzing File Systems File systems implement POSIX calls differently Study behavior via Persistence Properties: - define how system calls are persisted - affect which on-disk states are possible after a system crash - two classes: atomicity and ordering 9
27 Persistence Properties: Eample Consider the following code snippet: Atomicity write(f, AA ) write(f, BB ) f f Only size updated f AA f A f YY f BB f B f ZZ Ordering f AA f AA f f f BB f BB 0
28 Block-Order-Breaker Built Block-Order-Breaker (BOB) to study persistence properties Methodology: - Re-order block IO to construct legal disk images - Inspect file-system state on constructed images - Test whether persistence properties hold
29 Block-Order-Breaker (BOB) write (K) Test workload designed to stress persistence property File System W W F W3 Capture block-level trace of writes, flushes, barriers Initial Disk State
30 Block-Order-Breaker (BOB) Reconstruct crash state W W F W3 States limited by flushes and barriers Initial Disk State 3
31 Block-Order-Breaker (BOB) Reconstruct crash state W States limited by flushes and barriers Initial Disk State 3
32 Block-Order-Breaker (BOB) Reconstruct crash state W States limited by flushes and barriers Crashed Disk State 3
33 Block-Order-Breaker (BOB) File System Mount file system on crashed disk state Reconstruct crash state W States limited by flushes and barriers Crashed Disk State 3
34 Block-Order-Breaker (BOB) Is entire write() data present? File System Check if persistence property violated Mount file system on crashed disk state Reconstruct crash state W States limited by flushes and barriers Crashed Disk State 3
35 Caveats BOB can used to find: - Which properties don t hold - The eact case where the property fails BOB does not eplore all workloads BOB cannot be used to prove a file system has a specific persistence property 4
36 Studying File Systems Uses BOB to study how properties varied over file systems Studied si file systems - et, et3, et4, btrfs, fs, reiserfs - A total of 6 configurations 5
37 Study Results: Atomicity et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync 6
38 Study Results: Atomicity et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync Single Sector write(5) atomic? 6
39 Study Results: Atomicity et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync Single Sector 6
40 Study Results: Atomicity et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync Single Sector Multi Sector write(gb) atomic? 6
41 Study Results: Atomicity et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync Single Sector Multi Sector 6
42 Study Results: Atomicity et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync Single Sector Multi Sector Append Content open(file, O_APPEND) write(k) atomic? 6
43 Study Results: Atomicity et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync Single Sector Multi Sector Append Content 6
44 Study Results: Atomicity et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync Single Sector Multi Sector Append Content rename(old, new) atomic? Directory Operation 6
45 Study Results: Atomicity Single Sector Multi Sector Append Content Directory Operation et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync 6
46 Study Results: Ordering et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync 7
47 Study Results: Ordering Overwrite -> any op et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync write(4k) -> rename() 7
48 Study Results: Ordering Overwrite -> any op et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync 7
49 Study Results: Ordering Overwrite -> any op et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync Append -> any op 7
50 Study Results: Ordering Overwrite -> any op et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync Append -> any op Dir op -> any op 7
51 Study Results: Ordering Overwrite -> any op et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync Append -> any op Dir op -> any op Append(f) -> rename(f) 7
52 Study Results: Ordering Overwrite -> any op et async et sync et3 writeback et3 ordered et3 journal et4 writeback et4 ordered et4 no-delalloc et4 journal btrfs fs default fs wsync Append -> any op Dir op -> any op Append(f) -> rename(f) 7
53 File-System Study Results Persistence properties vary widely among file systems - Even within different configurations of same file system Applications should not rely on them Testing application correctness on single file system is not enough 8
54 Outline Introduction Background Analyzing file systems with BOB Analyzing applications with ALICE Application Study Conclusion and Future Work 9
55 Application-Level Intelligent Crash Eplorer (ALICE) ALICE: tool to find Crash Vulnerabilities Application Crash Vulnerabilities - code that depends on specific persistence properties for correct behavior - e: if file system doesn't persist two system calls calls in order, it leads to data corruption 30
56 ALICE Methodology Construct crash state by violating a single persistence property Run application on crash state (allow recovery) Eamine application state If application inconsistent, it depended on persistence property violated in crash state 3
57 ALICE Overview Application Workload git add file FS Abstract Persistence Model Application Checker git status ERROR creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) System-Call Trace Crash State Constructor Crash State Crash State Crash State 3
58 ALICE Overview Application Workload git add file FS Abstract Persistence Model Application Checker git status ERROR creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) System-Call Trace Crash State Constructor Crash State Crash State Crash State 33
59 Tracing the Workload Run the application workload Collect the system-call traces System calls converted into logical operations: - Abstract away current file offset, fd, etc - Group writev(), pwrite() etc into a single type of operation 34
60 ALICE Overview Application Workload git add file FS Abstract Persistence Model Application Checker git status ERROR creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) System-Call Trace Crash State Constructor Crash State Crash State Crash State 35
61 Constructing Crash States ALICE constructs crash states by applying a subset of operations to the initial disk image Initial Disk State creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) Crash State 36
62 Constructing Crash States Persistence Properties Violated:. Atomicity across system calls Method: apply prefi of operations creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm). Atomicity within system calls 37
63 Constructing Crash States Persistence Properties Violated:. Atomicity across system calls Method: apply prefi of operations. Atomicity within system calls Method: apply prefi + partial operation creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) 37
64 Constructing Crash States Persistence Properties Violated:. Atomicity across system calls Method: apply prefi of operations. Atomicity within system calls Method: apply prefi + partial operation creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) creat(inde.lock) creat(tmp) append(tmp, 5) append(tmp, 5) fsync(tmp) link(tmp, perm) 37
65 Constructing Crash States Persistence Properties Violated:. Atomicity across system calls Method: apply prefi of operations. Atomicity within system calls Method: apply prefi + partial operation creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) creat(inde.lock) creat(tmp) append(tmp, 5) append(tmp, 5) fsync(tmp) link(tmp, perm) 37
66 Constructing Crash States Persistence Properties Violated: 3. Ordering among system calls creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) Method: ignore an operation, apply prefi 38
67 Constructing Crash States Persistence Properties Violated: 3. Ordering among system calls creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) Method: ignore an operation, apply prefi 38
68 ALICE Overview Application Workload git add file FS Abstract Persistence Model Application Checker git status ERROR creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) System-Call Trace Crash State Constructor Crash State Crash State Crash State 39
69 FS Abstract Persistence Model Each file system implements persistence properties differently - E: et4 orders writes of a file before its rename APM defines which crash states are permitted APM defines atomicity and ordering constraints APM allow ALICE to model file-system behavior without file-system implementation 40
70 ALICE Overview Application Workload git add file FS Abstract Persistence Model Application Checker git status ERROR creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) System-Call Trace Crash State Constructor Crash State Crash State Crash State 4
71 Finding Crash Vulnerabilities Identify persistence property violated creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) Identify system calls involved Identify source code lines involved 4
72 ALICE Overview Application Workload git add file FS Abstract Persistence Model Application Checker git status ERROR creat(inde.lock) creat(tmp) append(tmp, 4K) fsync(tmp) link(tmp, perm) System-Call Trace Crash State Constructor Crash State Crash State Crash State 43
73 ALICE Limitations Not complete - does not eecute all code paths in application - does not eplore all crash states - does not test combinations of persistence property violations (e: atomicity + ordering) Cannot prove an update protocol is correct 44
74 Outline Introduction Background Analyzing file systems with BOB Analyzing applications with ALICE Application Study Conclusion and Future Work 45
75 Application Study Used ALICE to study eleven applications Version Control Systems Key-Value Stores GDBM LMDB Relational Databases Distributed Systems ZooKeeper Virtualization Platforms Player
76 Study Goals Analyzed applications using weak APM - Minimum constraints on possible crash states Sought to answer: - Which persistence properties do applications depend upon? - What are the consequences of vulnerabilities? - How many vulnerabilities occur on today s file systems? Did not seek to compare applications 47
77 Study: Setup What is correct behavior for an application? - We use guarantees in documentation - In case of no documentation, we assume typical user epectations ( committed data is durable ) Configurations change guarantees - We test each configuration separately - Tested 34 configurations across applications Post-crash, we run all appropriate application recovery mechanisms 48
78 Eample: Git mkdir(o/) creat(o//tmp_y) append(o//tmp_y) fsync(o//tmp_y) link(o//tmp_y, o//y) unlink(o//tmp_y) store object do(store object) creat(branch.lock) append(branch.lock) append(branch.lock) append(logs/branch) append(logs/head) rename(branch.lock,/branch) stdout( finished commit ) git commit 49
79 Eample: Git mkdir(o/) creat(o//tmp_y) append(o//tmp_y) fsync(o//tmp_y) link(o//tmp_y, o//y) unlink(o//tmp_y) store object Atomicity do(store object) creat(branch.lock) append(branch.lock) append(branch.lock) append(logs/branch) append(logs/head) rename(branch.lock,/branch) stdout( finished commit ) [ ] git commit 49
80 Eample: Git Ordering mkdir(o/) creat(o//tmp_y) append(o//tmp_y) fsync(o//tmp_y) link(o//tmp_y, o//y) unlink(o//tmp_y) store object do(store object) creat(branch.lock) append(branch.lock) append(branch.lock) append(logs/branch) append(logs/head) rename(branch.lock,/branch) stdout( finished commit ) git commit 49
81 Eample: Git Ordering mkdir(o/) creat(o//tmp_y) append(o//tmp_y) fsync(o//tmp_y) link(o//tmp_y, o//y) unlink(o//tmp_y) store object do(store object) creat(branch.lock) append(branch.lock) append(branch.lock) append(logs/branch) append(logs/head) rename(branch.lock,/branch) stdout( finished commit ) git commit 49
82 Eample: Git mkdir(o/) creat(o//tmp_y) append(o//tmp_y) fsync(o//tmp_y) link(o//tmp_y, o//y) unlink(o//tmp_y) store object do(store object) creat(branch.lock) append(branch.lock) append(branch.lock) append(logs/branch) append(logs/head) rename(branch.lock,/branch) stdout( finished commit ) git commit Durability 49
83 Vulnerability Types Multi-call atomicity Ordering Single-call atomicity Durability Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player #vulnerabilties 50
84 Vulnerability Types Multi-call atomicity Ordering Single-call atomicity Durability Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player #vulnerabilties 50
85 Vulnerability Types Multi-call atomicity Ordering Single-call atomicity Durability Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player #vulnerabilties 50
86 Vulnerability Types Multi-call atomicity Ordering Single-call atomicity Durability Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player #vulnerabilties 50
87 Vulnerability Types Multi-call atomicity Ordering Single-call atomicity Durability Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player #vulnerabilties 50
88 Vulnerability Types Multi-call atomicity Ordering Single-call atomicity Durability Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player 3 60 vulnerabilities across applications #vulnerabilties 50
89 Vulnerability Consequences Silent Errors Data Loss Cannot Open Failed reads/writes Misc Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player #vulnerabilties 5
90 Vulnerability Consequences Silent Errors Data Loss Cannot Open Failed reads/writes Misc Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player #vulnerabilties 5
91 Vulnerability Consequences Silent Errors Data Loss Cannot Open Failed reads/writes Misc Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player #vulnerabilties 5
92 Vulnerability Consequences Silent Errors Data Loss Cannot Open Failed reads/writes Misc Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player #vulnerabilties 5
93 Vulnerability Consequences Silent Errors Data Loss Cannot Open Failed reads/writes Misc Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player #vulnerabilties 5
94 Vulnerability Consequences Silent Errors Data Loss Cannot Open Failed reads/writes Misc Git Mercurial LevelDB-.0 LevelDB-.5 GDBM LMDB PostgreSQL HSQLDB SQLite HDFS ZooKeeper VMWare Player 3 3 Many vulnerabilities result in data loss, silent errors, and failed reads/writes #vulnerabilties 5
95 Vulnerabilities on Current File Systems #vulnerabilities Weak APM et3-writeback et3-ordered et3-journal et4-ordered btrfs 5
96 Vulnerabilities on Current File Systems #vulnerabilities Weak APM et3-writeback et3-ordered et3-journal et4-ordered btrfs 5
97 Vulnerabilities on Current File Systems #vulnerabilities Weak APM et3-writeback et3-ordered et3-journal et4-ordered btrfs 5
98 Vulnerabilities on Current File Systems #vulnerabilities Weak APM et3-writeback et3-ordered et3-journal et4-ordered btrfs 5
99 Vulnerabilities on Current File Systems #vulnerabilities Every current file system eposes at least one vulnerability; btrfs eposes more than half 6 0 Weak APM et3-writeback et3-ordered et3-journal et4-ordered btrfs 7 3 5
100 Observations Applications very careful in overwriting user data - None required atomicity for multi-block overwrites Applications not as careful in appending to logs - Multi-block appends require prefi atomicity - E: write( ABC ) should result in A / AB / ABC Atomicity across system calls doesn't seem useful 53
101 Observations Update protocols spread over layers and files - E: HSQLDB has 3 consecutive fsync() calls Recovery code is poorly written and untested - E: LevelDB recovery does not correct errors Documentation unclear or misleading - SQLite by default does not provide durability - GDBM_SYNC does not ensure durability 54
102 Reporting Vulnerabilities Developers generally suspicious when we reported vulnerabilities - Dev #: Maybe it is the disk - Dev #: File systems don't behave that way Tough to reproduce without tools like ALICE Developers acting on five vulnerabilities - Vulnerabilities in LevelDB, HDFS, ZooKeeper 55
103 Outline Introduction Background Analyzing file systems with BOB Analyzing applications with ALICE Application Study Conclusion and Future Work 56
104 Summary Built BOB to study persistence properties - Studied 6 configurations of 6 file systems - Persistence properties vary widely Built ALICE to study application-level crashconsistency protocols - Studied applications - Found 60 vulnerabilities across all applications 57
105 Application-Level Consistency in the Cloud Cloud computing and software-defined storage make this problem worse - Increased storage-stack diversity - Multiple storage media, file systems, etc. Applications deployed in multiple environments - Cant rely on specific persistence properties 58
106 Portability in the Cloud Need to match application requirements to storage-stack guarantees Challenges: - specifying application requirements - computing how layers build on each other to provide guarantees - checking if requirements are met 59
107 Matching Applications to Stacks Use a formal language (like Isar) to specify application requirements Specify high-level design of stack layers in Isar Use proof-assistants (like Isabelle) to verify that requirements are provided by stack Do this on-the-fly as storage stacks are constructed 60
108 Benefits of Matching Currently, applications are coarsely matched to storage stacks Stacks provide either too much or too little Verifying application correctness allows construction of optimal stacks - Cheapest stack that satisfies application requirements 6
109 Conclusion Applications are moving to the cloud - For performance - For ease-of use or availability - correctness is often forgotten To unlock true potential of cloud, portable applications need to be created Figuring out application requirements is the first step towards this vision 6
110 Thank You Source code Software/alice/ Questions? SOSP 3
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