From crash to testcase

From Crash to Testcase: a Debugging Primer
(Or: how to get your bugs fixed really quickly and be loved by developers)
Roel Van de Paar
QA Lead, Percona
22 April 1:30pm - 4:30pm @ Ballroom D

From Crash to Testcase: a Debugging Primer Roel Van de Paar, Percona
2
Database Issues Cheat SheetDatabase Issues Cheat Sheet

3
Oops! My ServerOops! My Server CRASHED!CRASHED!
Or did it?Or did it?

4
Application Error or not?Application Error or not?
●
Application side: Signs that database server is alive?Application side: Signs that database server is alive?
●
Server side: Check mysql CLI (Command Line Interface)Server side: Check mysql CLI (Command Line Interface)
●
Check Error Log (data_dir/host_name.err)Check Error Log (data_dir/host_name.err)
– Start at end and work your way up
– Happy day if the last line reads “Writing a core file”
●
Check for cores (data_dir/core.pid, system locations)Check for cores (data_dir/core.pid, system locations)

5
Application Errors: You're It!Application Errors: You're It!
●
Bad News?Bad News?
– You need to fix it
– “original developer” has
“left the building”
●
Good News?Good News?
– Your A+ developer
incorporated app logs
“Pay me now or pay me later”

6
Debugging Your AppDebugging Your App

7
Related Misconfigurations:Related Misconfigurations:
Buffer & File Sizing, Communication ErrorsBuffer & File Sizing, Communication Errors
●
Comms IssueComms Issue
– Hardware
– Comms buffer settings on server set to small etc.
● max_allowed_packet http://dev.mysql.com/doc/refman/5.6/en/packet-too-large.html
●
Other non-comms IssuesOther non-comms Issues
– Buffer settings on server
● Example: [ERROR] mysqld: Sort aborted
– Can be due to a small sort_buffer_size
● Example:InnoDB: ERROR: the age of the last checkpoint is 724774680, InnoDB:
which exceeds the log group capacity 724770200.
– Due to innodb_log_file_size being too small
● Example: per-session var set too high thereby causing slowness, OOM etc.

8
Severity/Error LevelSeverity/Error Level

9
Server Crash?Server Crash?

10
Always check the Error LogAlways check the Error Log 1st1st

11
Error Log Analysis?Error Log Analysis?
● 2013-03-10 06:58:50 19481 [Note] /ssd/Percona-Server-5.6.8-alpha60.2-313-
debug.Linux.x86_64/bin/mysqld-debug: ready for connections.
Version: '5.6.10-alpha60.2-debug-log' socket: '/ssd/198649/current1_6/tmp/master.sock'
port: 13100 Percona Server with XtraDB (GPL), Release alpha60.2, Revision 313-debug
mysqld-debug: /ssd/ps56-univ-log-archive-qa/Percona-Server-5.6.8-
alpha60.2/sql/protocol.cc:518: void Protocol::end_statement(): Assertion `0' failed.
04:00:32 UTC - mysqld got signal 6 ;
● 2013-03-17 16:17:44 7f45c9e96700 InnoDB: Operating system error number 2 in a file
operation. InnoDB: The error means the system cannot find the path specified.
2013-03-17 16:17:44 7f45c9e96700 InnoDB: File name
/tmp/1363526254145352487/ib_log_archive_0000000000045568
2013-03-17 16:17:44 7f45c9e96700 InnoDB: File operation call: 'open' returned OS error 71.
2013-03-17 16:17:44 7f45c9e96700 InnoDB: Cannot continue operation.
2013-03-17 16:17:44 7f45c9e96700 InnoDB: Assertion failure in thread 13993771698764813 in
file os0file.cc line 62
InnoDB: Failing assertion: 0

12
Error Log Analysis?Error Log Analysis?
● Version: '5.6.10-alpha60.2-debug-log' socket: '/ssd/198649/current1_4/tmp/master.sock' port:
13060 Percona Server with XtraDB (GPL), Release alpha60.2, Revision 313-debug
2013-03-13 02:20:59 8466 [ERROR] InnoDB: Unable to lock
/ssd/tmp/ib_log_archive_0000000687614464, error: 11
2013-03-13 02:20:59 8466 [Note] InnoDB: Check that you do not already have another mysqld
process using the same InnoDB data or log files.
InnoDB: Cannot create or open archive log file /ssd/tmp/ib_log_archive_0000000687614464.
InnoDB: Cannot continue operation.
InnoDB: Check that the log archive directory exists,
InnoDB: you have access rights to it, and
InnoDB: there is space available.
● 2013-03-11 03:34:23 7f9866dca700 InnoDB: Assertion failure in thread 140292537493248 in file
row0purge.cc line 459
InnoDB: Failing assertion: 0x20UL & rec_get_info_bits( btr_cur_get_rec(btr_cur),
dict_table_is_comp(index->table))

13
Error Log Analysis: Initial TipsError Log Analysis: Initial Tips
●
RRR++: Read! Read! Read! Read Again! (And once more)RRR++: Read! Read! Read! Read Again! (And once more)
– Why?
●
What's the problem?What's the problem?
– Assert vs. Error vs. Crash vs. OS vs. OOM vs. Sigx vs.
Halt vs. Kill vs. Corruption vs. Deadlocks vs. Buffer &
File sizing vs. Communication Errors vs. SQL Errors vs.
Warnings vs. 3rd
Party Messages vs. …

14
Analyzing the Error Log &Analyzing the Error Log & allall it containsit contains
:$ / RRR++:$ / RRR++
Research++Research++

15
WhichWhich QueryQuery caused trouble?caused trouble?
●
Crashing query: In Error log:Crashing query: In Error log:
– Query (3ff000002300): select f1 from t2 limit 5
●
Faulting query:Faulting query:
– 130325 6:07:46 [ERROR] mysqld: Sort aborted:
Query execution was interrupted

16
Resolving StacksResolving Stacks
[roel@localhost log]$ grep "mysqld(_" master.err | sed 's/^.*mysqld//'
(_ZN8Protocol13end_statementEv+0x1db)[0x525ce7]
(_Z16dispatch_command19enum_server_commandP3THDPcj+0x1496)[0x5a2e6d]
(_Z10do_commandP3THD+0x284)[0x5a3702]
(_Z24do_handle_one_connectionP3THD+0x121)[0x648f1d]
[roel@localhost log]$ grep "mysqld(_" master.err | sed 's/^.*mysqld//' | c++filt
(Protocol::end_statement()+0x1db)[0x525ce7]
(dispatch_command(enum_server_command, THD*, char*, unsigned int)+0x1496)[0x5a2e6d]
(do_command(THD*)+0x284)[0x5a3702]
(do_handle_one_connection(THD*)+0x121)[0x648f1d]

17
SideTour:SideTour: WhichWhich QueryQuery caused trouble: LEScaused trouble: LES
●
LES: Last Executed StatementLES: Last Executed Statement
– LES Error log ExtractionLES Error log Extraction
/server/bin/mysqld(handle_one_connection+0x52)[0x649010]
/lib64/libpthread.so.0[0x333d007851]
/lib64/libc.so.6(clone+0x6d)[0x333cce890d]
Trying to get some variables.
Some pointers may be invalid and cause the dump to abort.
Query (7fe000009188): select `c3`,`c4` from `qa07` limit 10
– LES gdb ExtractionLES gdb Extraction
Select 'do_command' frame in crashing thread using thread & frame,
then use: p thd->query_string.string.str
http://www.mysqlperformanceblog.com/2012/09/09/obtain-last-executed-statement-from-optimized-core-dump/
Demo: /ssd/Percona-Server-5.5.29-rel30.0--debug.Linux.x86_64/data4

18
AssertAssertions: Generalions: General
●
Assert: “I, developer x, assert that at this point y, x=0Assert: “I, developer x, assert that at this point y, x=0
(as an example) should not be the case.”(as an example) should not be the case.”
– RRR++: SE, File, Line, Vars, TimeRRR++: SE, File, Line, Vars, Time
121204 7:45:06 InnoDB: Assertion failure in thread 1390 in file row0upd.c line 2023
InnoDB: Failing assertion: btr_pcur_restore_position(thr_get_trx(thr)->fake_changes ?
BTR_SEARCH_TREE : BTR_MODIFY_TREE, pcur, mtr)
– RRR++: Are you a dev?RRR++: Are you a dev?
130127 0:20:37 InnoDB: Assertion failure in thread 1396 in file row0sel.c line 115
InnoDB: Failing assertion: prefix_len >= sec_len

19
AssertAssertions: Typeions: Type
●
Server AssertionServer Assertion
mysqld: /mysql-5.5/sql/sql_string.cc:37: bool
String::real_alloc(uint32): Assertion `arg_length > length' failed.
●
InnoDB/XtraDB/Other SE Assertion (Seen most often)InnoDB/XtraDB/Other SE Assertion (Seen most often)
InnoDB: Error: Waited for 600 secs for hash index ref_count (1)
to drop to 0. index: "c32" table: "test/#sql2-4b20-a"
121203 3:48:15 InnoDB: Assertion failure in thread 352803136 in
file dict0dict.c line 1883

20
Error LogError Log AnalysisAnalysis ExampleExample
● 2013-03-10 06:58:50 19481 [Note] /ssd/Percona-Server-5.6.8-alpha60.2-313-
debug.Linux.x86_64/bin/mysqld-debug: ready for connections.
Version: '5.6.10-alpha60.2-debug-log' socket:
'/ssd/198649/current1_6/tmp/master.sock' port: 13100 Percona Server with XtraDB
(GPL), Release alpha60.2, Revision 313-debug
mysqld-debug: /ssd/ps56-univ-log-archive-qa/Percona-Server-5.6.8-
alpha60.2/sql/protocol.cc:518: void Protocol::end_statement(): Assertion `0' failed.
04:00:32 UTC - mysqld got signal 6 ;
● 2013-03-17 16:17:44 7f45c9e96700 InnoDB: Operating system error number 2 in a file
operation. InnoDB: The error means the system cannot find the path specified.
2013-03-17 16:17:44 7f45c9e96700 InnoDB: File name
/tmp/1363526254145352487/ib_log_archive_0000000000045568
2013-03-17 16:17:44 7f45c9e96700 InnoDB: File operation call: 'open' returned OS
error 71.
2013-03-17 16:17:44 7f45c9e96700 InnoDB: Cannot continue operation.
2013-03-17 16:17:44 7f45c9e96700 InnoDB: Assertion failure in thread
13993771698764813 in file os0file.cc line 62
InnoDB: Failing assertion: 0

21
Error LogError Log AnalysisAnalysis ExampleExample
● Version: '5.6.10-alpha60.2-debug-log' socket: '/ssd/198649/current1_4/tmp/master.sock'
port: 13060 Percona Server with XtraDB (GPL), Release alpha60.2, Revision 313-debug
2013-03-13 02:20:59 8466 [ERROR] InnoDB: Unable to lock
/ssd/tmp/ib_log_archive_0000000687614464, error: 11
2013-03-13 02:20:59 8466 [Note] InnoDB: Check that you do not already have another
mysqld process using the same InnoDB data or log files.
InnoDB: Cannot create or open archive log file
/ssd/tmp/ib_log_archive_0000000687614464.
InnoDB: Cannot continue operation.
InnoDB: Check that the log archive directory exists,
InnoDB: you have access rights to it, and
InnoDB: there is space available.
● 2013-03-11 03:34:23 7f9866dca700 InnoDB: Assertion failure in thread 140292537493248
in file row0purge.cc line 459
InnoDB: Failing assertion: 0x20UL & rec_get_info_bits( btr_cur_get_rec(btr_cur),
dict_table_is_comp(index->table))

22
ErrorErrorss
● 130325 5:54:07 [ERROR] Can't open the mysql.plugin table. Please run mysql_upgrade to create it.
130325 5:54:07 [ERROR] Fatal error: Can't open and lock privilege tables: Table 'mysql.host' doesn't exist
● 130223 12:44:05 InnoDB: Error: Write to file ./apr/fr1 failed at offset 13.
InnoDB: 49152 bytes should have been written, only 0 were written.
InnoDB: Operating system error number 9.
InnoDB: Check that your OS and file system support files of this size.
InnoDB: Check also that the disk is not full or a disk quota exceeded.
InnoDB: Error number 9 means 'Bad file descriptor'.
● 130325 5:36:11 [ERROR] /ssd/Server/bin/mysqld: Incorrect information in file: './test/v.frm'
● 130325 6:07:46 [ERROR] /ssd/Server/bin/mysqld: Sort aborted: Query execution was interrupted
● 130325 6:10:07 [ERROR] /ssd/Server/bin/mysqld: Sort aborted: Server shutdown in progress

23
CrashCrasheses
● thread 10 (LWP 23954):
+bt
#0 0x0000000004e3969c in pthread_kill () from /lib64/libpthread.so.0
#1 0x00000000007e2779 in my_write_core (sig=11) at /ssd/QA-16274-5.5/Percona-Server-
5.5.28-rel29.3/mysys/stacktrace.c:433
#2 0x00000000006ab0ea in handle_fatal_signal (sig=11) at /ssd/QA-16274-5.5/Percona-
Server-5.5.28-rel29.3/sql/signal_handler.cc:249
#3 <signal handler called>
#4 rbt_free_node (node=0x0, nil=0x1040f170) at /ssd/QA-16274-5.5/Percona-Server-5.5.28-
rel29.3/storage/innobase/ut/ut0rbt.c:731
#5 0x00000000009935e9 in rbt_free_node (node=0x1040f1e0, nil=0x1040f170) at /ssd/QA-
16274-5.5/Percona-Server-5.5.28-rel29.3/storage/innobase/ut/ut0rbt.c:731
● https://bugs.launchpad.net/percona-server/+bug/1111226 (Crash, Valgrind, Error Log)

24
OSOS//HardwareHardware Related MessageRelated Message((ss))

25
OSOS Related IssuesRelated Issues
●
https://bugs.launchpad.net/percona-server/+bug/806975https://bugs.launchpad.net/percona-server/+bug/806975
●
OS errors: PerrorOS errors: Perror
– <base_dir>/bin/perror

26
OOMOOM
● CLI:
ERROR 5 (HY000): Out of memory (Needed 128992 bytes)
● Error Log:
110531 17:12:08 [ERROR] /home/philips/bzr/mysql-55-
eb/sql/mysqld: Out of memory (Needed 129872 bytes)
● Use Valgrind [Memcheck, Massif]!
● https://bugs.launchpad.net/percona-server/+bug/1042946
– Could cause OOM
– Valgrind [Massif] helps

27
SigSig=x &=x & KillKill -x-x
●
Signal NumbersSignal Numbers
– Sig=x Signal Results Note
– 4 SIGILL Core Illegal Instruction
– 6 SIGABRT Core Abort signal by abort()
– 8 SIGFPE Core Floating Point Exception
– 11 SIGSEGV Core Invalid Memory Reference
●
Tip: you can use for example 'kill -11' to get a core dump atTip: you can use for example 'kill -11' to get a core dump at
any given point, for example I've used this when seeing aany given point, for example I've used this when seeing a
memory allocation issue.memory allocation issue.

28
HaltHaltss
●
Different from Sig=x.Different from Sig=x.
– Server just “halts”
●
Different from unplanned shutdownDifferent from unplanned shutdown
– Server just “halts”
●
Query loggingQuery logging
●
Error log informationError log information
●
(gdb breakpoints on exit functions)(gdb breakpoints on exit functions)

29
DatabaseDatabase CorruptionCorruption
●
Standard recovery (Not corruption):Standard recovery (Not corruption):
130327 11:02:02 InnoDB: highest supported file format is Barracuda.
InnoDB: The log sequence number in ibdata files does not match
InnoDB: the log sequence number in the ib_logfiles!
130327 11:02:02 InnoDB: Database was not shut down normally!
InnoDB: Starting crash recovery.
InnoDB: Reading tablespace information from the .ibd files...
InnoDB: Restoring possible half-written data pages from the doublewrite buffer...
●
Data Corruption:Data Corruption:
120117 1:22:00 InnoDB: Starting an apply batch of log records to the database...
InnoDB: Progress in percents: 0 1 2 3 4 5[...] 99
InnoDB: Apply batch completed
120117 1:22:02 InnoDB: Rolling back trx with id A01D1001, 13 rows to undo
InnoDB: Dropping table with id 54885 in recovery if it exists
InnoDB: Error: trying to load index PRIMARY for table nr92/#sql2-2e46-316ce0
InnoDB: but the index tree has been freed!
InnoDB: Rolling back of trx id A01D1001 completed

30
DeadlocksDeadlocks
●
Deadlocks are funDeadlocks are fun
●
User initiated vs actual server deadlockUser initiated vs actual server deadlock
– User Initiated:
mysql> select * from t1 where a = 2;
#with corresponding other session
ERROR 1205 (HY000): Lock wait timeout exceeded;
try restarting transaction
– Server deadlock
Programming deadlock

31
(3(3rdrd
) Party Messages (RQG, Valgrind) vs.) Party Messages (RQG, Valgrind) vs.
●
Other items which may write to the error logOther items which may write to the error log
- RQG
- Valgrind
- InnoDB status monitor
●
Valgrind Example:Valgrind Example:
==12667== Thread 15:
==12667== Invalid read of size 8
==12667== at 0x93D473: lock_rec_block_validate (lock0lock.c:4969)
==12667== by 0x93D8D0: lock_print_info_all_transactions (lock0lock.c:5113)
==12667== by 0x862BAC: srv_printf_innodb_monitor (srv0srv.c:2263)
==12667== by 0x862DA5: srv_monitor_thread (srv0srv.c:2580)
==12667== by 0x4E34850: start_thread (in /lib64/libpthread-2.12.so)
==12667== by 0x19FCA6FF: ???
==12667== Address 0x16220c48 is 664 bytes inside a block of size 872 free'd
==12667== at 0x4C2695D: free (vg_replace_malloc.c:366)
==12667== by 0x952579: mem_area_free (mem0pool.c:519)
[...]

32
SideTour:SideTour: Googling++Googling++
(Demo)(Demo)

33
Considering InnoDB Status & RecoveryConsidering InnoDB Status & Recovery
●
This is for 'Production' (non-test) systems onlyThis is for 'Production' (non-test) systems only
●
innodb_force_recoveryinnodb_force_recovery
http://dev.mysql.com/doc/refman/5.6/en/forcing-innodb-recovery.htmlhttp://dev.mysql.com/doc/refman/5.6/en/forcing-innodb-recovery.html

34
Crash Severity SummaryCrash Severity Summary

35
Core Dumps: Locations & SetupCore Dumps: Locations & Setup

36
Core Dumps: Analysis: using gdbCore Dumps: Analysis: using gdb
●
LES exampleLES example
●
Google search exampleGoogle search example
●
thread apply all btthread apply all bt
●
Cheat SheetCheat Sheet

37
Core Dumps: Analysis: using WinDbgCore Dumps: Analysis: using WinDbg
●
DemoDemo

38
Valgrind: IntroductionValgrind: Introduction

39
RQG: IntroductionRQG: Introduction
(Demo)(Demo)

40
SideTour: Bash Scripting Fun!SideTour: Bash Scripting Fun!
(Demo)(Demo)

41
SQL ErrorsSQL Errors
●
Place?Place?
– MySQL CLI
– Error Log
● Usually higher severity then in the CLI
– Application

42
Query SimplificationQuery Simplification
●
Objective?Objective?
– Good testcase: Reduce a crashing/failing query to the
minimum length and complexity required to still obtain
the “desired” crash/error/issue (QA/Debugging)
– Optimizing the query: Reduce a query to obtain the
same result without altering it's functionality or future
results with changed data (Support/Optimization)

43
Query Simplification: ClausesQuery Simplification: Clauses
mysql> SELECT * FROM t1 WHERE a>1 LIMIT 20;mysql> SELECT * FROM t1 WHERE a>1 LIMIT 20;
mysql> SELECT * FROM t1;mysql> SELECT * FROM t1;

44
Query Simplification: Query SplitQuery Simplification: Query Split
mysql> SELECT * FROM (SELECT a FROM t1) AS res1mysql> SELECT * FROM (SELECT a FROM t1) AS res1
WHERE a > 2;WHERE a > 2;
mysql> SELECT a FROM t1;mysql> SELECT a FROM t1;
mysql> SELECT a FROM t1 WHERE a > 2;mysql> SELECT a FROM t1 WHERE a > 2;

45
Query Simplification: Generalize DataQuery Simplification: Generalize Data
mysql> SELECT "There was once a little bug" INTO @a;mysql> SELECT "There was once a little bug" INTO @a;
mysql> SELECT "a" INTO @a;mysql> SELECT "a" INTO @a;
mysql> SELECT 1 INTO @a;mysql> SELECT 1 INTO @a;

46
Query Simplification: Move Data into QueryQuery Simplification: Move Data into Query
●
Often involves changes of results, but that matters “little”Often involves changes of results, but that matters “little”
(reproducibility factor may change) if the issue still(reproducibility factor may change) if the issue still
reproduces.reproduces.
mysql> SELECT a FROM t1; a=columnmysql> SELECT a FROM t1; a=column
mysql> SELECT "a" FROM t1; a=?mysql> SELECT "a" FROM t1; a=?
mysql> SELECT 1 FROM t1; 1=digit 1 (x rows)mysql> SELECT 1 FROM t1; 1=digit 1 (x rows)
mysql> SELECT 1;mysql> SELECT 1;

47
Query Simplification: Limit nr of FieldsQuery Simplification: Limit nr of Fields
mysql> SELECT a,b,x,z FROM t1;mysql> SELECT a,b,x,z FROM t1;
mysql> SELECT * FROM t1;mysql> SELECT * FROM t1;
mysql> SELECT 1 FROM t1;mysql> SELECT 1 FROM t1;

48
Query Simplification: Simplify TableQuery Simplification: Simplify Table
mysql> ALTER TABLE t1 DROP COLUMN b;mysql> ALTER TABLE t1 DROP COLUMN b;

49
Test Case ProductionTest Case Production
●
Strategies:Strategies:
– Bring server up -> re-run crashing query
● mysqldump -> add query -> “Simplify the query”
– Use randgen or Gypsy with grammars based on
crashing query (or usually used queries if the
crashing query is not known)
– Run Valgrind for some time on the server to check
for programming errors

50
SideTour:SideTour: Logging Bugs++Logging Bugs++

51
SideTour:SideTour: A note on threading & reproducibilityA note on threading & reproducibility
●
1 Thread?1 Thread?
– Usually easy to reproduce
● But not always: timing, OS slicing, SE dives etc.
● Impossible to 100% match timing
●
Many threads?Many threads?
– Usually hard to reproduce (exception: RQG/Gypsy)
● Dev core analysis is usually quickest way forward

52
Percona!Percona!

53
Connect++Connect++

54
ConnectConnect++++
roel.vandepaar@percona.com

From crash to testcase

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