| Lustre 2.0 Operations Manual |    
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| C H A P T E R 28 |
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Lustre Debugging |
This chapter describes tips and information to debug Lustre, and includes the following sections:
A variety of diagnostic and analysis tools are available to debug issues with the Lustre software. Some of these are provided in Linux distributions, while others have been developed and are made available by the Lustre project.
The following in-kernel debug mechanisms are incorporated into the Lustre software:
The following tools are also provided with the Lustre software:
The tools described in this section are provided in the Linux kernel or are available at an external website. For information about using some of these tools for Lustre debugging, see Lustre Debugging Procedures and Lustre Debugging for Developers.
Some general debugging tools provided as a part of the standard Linux distro are:
The following logging and data collection tools can be used to collect information for debugging Lustre kernel issues:
The tools described in this section may be useful for debugging Lustre in a development environment.
A virtual machine is often used to create an isolated development and test environment. Some commonly-used virtual machines are:
A variety of debuggers and analysis tools are available including:
The procedures below may be useful to administrators or developers debugging a Lustre files system.
Lustre debug messages are categorized by originating sybsystem, message type, and locaton in the source code. For a list of subsystems and message types, see Lustre Debug Messages.
| Note - For a current list of subsystems and debug message types, see lnet/include/libcfs/libcfs.h in the Lustre tree |
The elements of a Lustre debug message are described in Format of Lustre Debug Messages.
Each Lustre debug message has the tag of the subsystem it originated in, the message type, and the location in the source code. The subsystems and debug types used in Lustre are as follows:
mdc, mds, osc, ost, obdclass, obdfilter, llite, ptlrpc, portals, lnd, ldlm, lov
Lustre uses the CDEBUG and CERROR macros to print the debug or error messages. To print the message, the CDEBUG macro uses portals_debug_msg (portals/linux/oslib/debug.c). The message format is described below, along with an example.
Lustre debug messages are maintained in a buffer, with the maximum buffer size specified (in MBs) by the debug_mb parameter (/proc/sys/lnet/debug_mb). The buffer is circular, so debug messages are kept until the allocated buffer limit is reached, and then the first messages are overwritten.
The lctl tool allows debug messages to be filtered based on subsystems and message types to extract information useful for troubleshooting from a kernel debug log. For a command reference, see lctl.
lctl > debug_list <subs | types>
lctl > filter <subsystem name | debug type>
lctl > show <subsystem name | debug type>
debug_kernel pulls the data from the kernel logs, filters it appropriately, and displays or saves it as per the specified options
lctl > debug_kernel [output filename]
If the debugging is being done on User Mode Linux (UML), it might be useful to save the logs on the host machine so that they can be used at a later time.
lctl > debug_file <input filename> [output filename]
During the debug session, you can add markers or breaks to the log for any reason:
lctl > mark [marker text]
The marker text defaults to the current date and time in the debug log (similar to the example shown below):
DEBUG MARKER: Tue Mar 5 16:06:44 EST 2002
lctl > clear
| Note - Debug messages displayed with lctl are also subject to the kernel debug masks; the filters are additive. |
Below is a sample run using the lctl command.
bash-2.04# ./lctl lctl > debug_kernel /tmp/lustre_logs/log_all Debug log: 324 lines, 324 kept, 0 dropped. lctl > filter trace Disabling output of type "trace" lctl > debug_kernel /tmp/lustre_logs/log_notrace Debug log: 324 lines, 282 kept, 42 dropped. lctl > show trace Enabling output of type "trace" lctl > filter portals Disabling output from subsystem "portals" lctl > debug_kernel /tmp/lustre_logs/log_noportals Debug log: 324 lines, 258 kept, 66 dropped.
The debug_daemon option is used by lctl to control the dumping of the debug_kernel buffer to a user-specified file. This functionality uses a kernel thread on top of debug_kernel, which works in parallel with the debug_daemon command.
The debug_daemon is highly dependent on file system write speed. File system write operations may not be fast enough to flush out all of the debug_buffer if the Lustre file system is under heavy system load and continues to CDEBUG to the debug_buffer. The debug_daemon will write the message DEBUG MARKER: Trace buffer full into the debug_buffer to indicate the debug_buffer contents are overlapping before the debug_daemon flushes data to a file.
Users can use lctl control to start or stop the Lustre daemon from dumping the debug_buffer to a file. Users can also temporarily hold daemon from dumping the file. Use of the debug_daemon sub-command to lctl can provide the same function.
This section describes lctl debug_daemon commands.
To initiate the debug_daemon to start dumping debug_buffer into a file., enter
$ lctl debug_daemon start [{file} {megabytes}]
The file can be a system default file, as shown in /proc/sys/lnet/debug_path. After Lustre starts, the default path is /tmp/lustre-log-$HOSTNAME. Users can specify a new filename for debug_daemon to output debug_buffer. The new file name shows up in /proc/sys/lnet/debug_path. Megabytes is the limitation of the file size in MBs.
The daemon wraps around and dumps data to the beginning of the file when the output file size is over the limit of the user-specified file size. To decode the dumped file to ASCII and order the log entries by time, run:
lctl debug_file {file} > {newfile}
The output is internally sorted by the lctl command using quicksort.
To completely shut down the debug_daemon operation and flush the file output, enter:
debug_daemon stop
Otherwise, debug_daemon is shut down as part of the Lustre file system shutdown process. Users can restart debug_daemon by using start command after each stop command issued.
This is an example using debug_daemon with the interactive mode of lctl to dump debug logs to a 10 MB file.
#~/utils/lctl
To start the daemon to dump debug_buffer into a 40 MB /tmp/dump file, enter:
lctl > debug_daemon start /trace/log 40
To completely shut down the daemon, enter:
lctl > debug_daemon stop
To start another daemon with an unlimited file size, enter:
lctl > debug_daemon start /tmp/unlimited
The text message *** End of debug_daemon trace log *** appears at the end of each output file.
Masks are provided in /proc/sys/lnet/subsystem_debug and /proc/sys/lnet/debug to be used with the systctl command to determine what information is to be written to the debug log. The subsystem_debug mask determines the information written to the log based on the subsystem (such as iobdfilter, net, portals, or OSC). The debug mask controls information based on debug type (such as info, error, trace, or alloc).
To turn off Lustre debugging completely:
sysctl -w lnet.debug=0
To turn on full Lustre debugging:
sysctl -w lnet.debug=-1
To turn on logging of messages related to network communications:
sysctl -w lnet.debug=net
To turn on logging of messages related to network communications and existing debug flags:
sysctl -w lnet.debug=+net
To turn off network logging with changing existing flags:
sysctl -w lnet.debug=-net
The various options available to print to kernel debug logs are listed in lnet/include/libcfs/libcfs.h
The strace utility provided with the Linux distribution enables system calls to be traced by intercepting all the system calls made by a process and recording the system call name, aruguments, and return values.
To invoke strace on a program, enter:
$ strace <program> <args>
Sometimes, a system call may fork child processes. In this situation, use the -f option of strace to trace the child processes:
$ strace -f <program> <args>
To redirect the strace output to a file, enter:
$ strace -o <filename> <program> <args>
Use the -ff option, along with -o, to save the trace output in filename.pid, where pid is the process ID of the process being traced. Use the -ttt option to timestamp all lines in the strace output, so they can be correlated to operations in the lustre kernel debug log.
If the debugging is done in UML, save the traces on the host machine. In this example, hostfs is mounted on /r:
$ strace -o /r/tmp/vi.strace
In Lustre, the inodes on the metadata server contain extended attributes (EAs) that store information about file striping. EAs contain a list of all object IDs and their locations (that is, the OST that stores them). The lfs tool can be used to obtain this information for a given file using the getstripe subcommand. Use a corresponding lfs setstripe command to specify striping attributes for a new file or directory.
The lfs getstripe utility is written in C; it takes a Lustre filename as input and lists all the objects that form a part of this file. To obtain this information for the file /mnt/lustre/frog in Lustre file system, run:
$ lfs getstripe /mnt/lustre/frog $ obdix objid 0 17 1 4
The debugfs tool is provided in the e2fsprogs package. It can be used for interactive debugging of an ldiskfs file system. The debugfs tool can either be used to check status or modify information in the file system. In Lustre, all objects that belong to a file are stored in an underlying ldiskfs file system on the OSTs. The file system uses the object IDs as the file names. Once the object IDs are known, use the debugfs tool to obtain the attributes of all objects from different OSTs.
A sample run for the /mnt/lustre/frog file used in the above example is shown here:
$ debugfs -c /tmp/ost1 debugfs: cd O debugfs: cd 0 /* for files in group 0 */ debugfs: cd d<objid % 32> debugfs: stat <objid> /* for getattr on object */ debugfs: quit ## Suppose object id is 36, then follow the steps below: $ debugfs /tmp/ost1 debugfs: cd O debugfs: cd 0 debugfs: cd d4 /* objid % 32 */ debugfs: stat 36 /* for getattr on obj 4*/ debugfs: dump 36 /tmp/obj.36 /* dump contents of obj 4 */ debugfs: quit
To determine the Lustre UUID of an obdfilter disk (for example, if you mix up the cables on your OST devices or the SCSI bus numbering suddenly changes and the SCSI devices get new names), use debugfs to get the last_rcvd file.
To dump debug messages to the console (/var/log/messages), set the corresponding debug mask in the printk flag:
sysctl -w lnet.printk=-1
This slows down the system dramatically. It is also possible to selectively enable or disable this capability for particular flags using:
sysctl -w lnet.printk=+vfstrace sysctl -w lnet.printk=-vfstrace
It is possible to disable warning, error , and console messages, though it is strongly recommended to have something like lctl debug_daemon runing to capture this data to a local file system for debugging purposes.
Lustre has a specific debug type category for tracing lock traffic. Use:
lctl> filter all_types lctl> show dlmtrace lctl> debug_kernel [filename]
The procedures in this section may be useful to developers debugging Lustre code.
The debugging infrastructure provides a number of macros that can be used in Lustre source code to aid in debugging or reporting serious errors.
To use these macros, you will need to set the DEBUG_SUBSYSTEM variable at the top of the file as shown below:
#define DEBUG_SUBSYSTEM S_PORTALS
A list of available macros with descritions is provided in the table below.
Each service maintains a request history, which can be useful for first occurrence troubleshooting.
Ptlrpc is an RPC protocol layered on LNET that deals with stateful servers and has semantics and built-in support for recovery.
A prlrpc request history works as follows:
1. Request_in_callback() adds the new request to the service's request history.
2. When a request buffer becomes idle, it is added to the service's request buffer history list.
3. Buffers are culled from the service's request buffer history if it has grown above
req_buffer_history_max and its reqs are removed from the service's request history.
Request history is accessed and controlled using the following /proc files under the service directory:
Number of request buffers currently in the history
Maximum number of request buffers to keep
Requests in the history include "live" requests that are currently being handled. Each line in req_history looks like:
<seq>:<target NID>:<client ID>:<xid>:<length>:<phase> <svc specific>
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Service-specific request printout. Currently, the only service that does this is the OST (which prints the opcode if the message has been unpacked successfully |
Memory leaks can occur in code when memory has been allocated and then not freed once it is no longer required. The leak_finder.pl program provides a way to find memory leaks.
Before running this program, you must turn on debugging to collect all malloc and free entries. Run:
sysctl -w lnet.debug=+malloc
Then complete the following steps:
1. Dump the log into a user-specified log file using lctl (see Using the lctl Tool to View Debug Messages).
2. Run the leak finder on the newly-created log dump:
perl leak_finder.pl <ascii-logname>
malloced 8bytes at a3116744 (called pathcopy) (lprocfs_status.c:lprocfs_add_vars:80) freed 8bytes at a3116744 (called pathcopy) (lprocfs_status.c:lprocfs_add_vars:80)
The tool displays the following output to show the leaks found:
Leak:32bytes allocated at a23a8fc(service.c:ptlrpc_init_svc:144,debug file line 241)
| Lustre 2.0 Operations Manual | 821-2076-10 |
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