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<small>''(Updated: Feb 2010)''</small>
__TOC__
The procedures below may be useful to administrators or developers debugging a Lustre™ file system.
The procedures below may be useful to administrators or developers debugging a Lustre™ file system.


== Understanding the Lustre debug messaging format ==
== Understanding the Lustre debug messaging format ==


Lustre debug messages are categorized by originating subsystem, message type, and location in the source code.  For a list of subsystems and message types, see [http://wiki.lustre.org/manual/LustreManual18_HTML/LustreDebugging.html#50532482_pgfId-1291339|Section 23.1 ''Lustre Debug Messages''] in the [[Lustre Documentation|''Lustre Operations Manual'']].
Lustre debug messages are categorized by originating subsystem, message type, and location in the source code.  For a list of subsystems and message types, see [http://wiki.lustre.org/manual/LustreManual20_HTML/LustreDebugging.html#50438274_pgfId-1295747 Section 28.2.1.1: ''Lustre Debug Messages''] in the [http://wiki.lustre.org/manual/LustreManual20_HTML/index.html ''Lustre Operations Manual''].


'''''Note:''''' For a  current list of subsystems and debug message types, see lnet/include/libcfs/libcfs.h '''[[Keep this note?]]'''
'''''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 [http://wiki.lustre.org/manual/LustreManual18_HTML/LustreDebugging.html#50532482_pgfId-1291365|Section 23.1.1 ''Format of Lustre Debug Messages''] in the [[Lustre Documentation|''Lustre Operations Manual'']].
The elements of a Lustre debug message are described in [http://wiki.lustre.org/manual/LustreManual20_HTML/LustreDebugging.html#50438274_pgfId-1295842 Section 28.2.1.2: ''Format of Lustre Debug Messages''] in the [http://wiki.lustre.org/manual/LustreManual20_HTML/index.html ''Lustre Operations Manual''].


== Using the ''lctl'' tool to view debug messages==


==== Using the Lustre debug daemon ====
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 more information about the ''lctl'' tool, see [http://wiki.lustre.org/manual/LustreManual20_HTML/LustreDebugging.html#50438274_pgfId-1295889 Section 28.2.2: ''Using the lctl Tool to View Debug Messages''] and [http://wiki.lustre.org/manual/LustreManual20_HTML/SystemConfigurationUtilities_HTML.html#50438219_pgfId-1318224 Section 36.3: ''lctl''] in the [http://wiki.lustre.org/manual/LustreManual20_HTML/index.html ''Lustre Operations Manual''].
The '''debug_daemon''' allows users to control the[[ Lustre kernel debug daemon]] to dump the ''debug_kernel'' buffer to a user-specified file. This functionality is built using a kernel thread on top of ''debug_kernel''. ''debug_kernel'', another sub-command of lctl, continues to work in parallel with the ''debug_daemon'' command.


''Debug_daemon'' is highly dependent on the filesystem write speed. If the Lustre file system is under heavy system load, the filesystem write operation may not be fast enough to flush out all of the debug_buffer and continue to CDEBUG to the debug_buffer. Debug_daemon is to put '''DEBUG MARKER: Trace buffer full''' into the debug_buffer, indicating that debug_buffer is overlapping itself before debug_daemon flushes data to a file.
A sample ''lctl'' run is shown in [http://wiki.lustre.org/manual/LustreManual20_HTML/LustreDebugging.html#50438274_pgfId-1295915 Section 28.2.2.1: ''Sample lctl Run''] in the [http://wiki.lustre.org/manual/LustreManual20_HTML/index.html ''Lustre Operations Manual''].


The user can use lctl control to start or stop the Lustre daemon from dumping the debug_buffer to a file. Users can also temporarily pause daemon from dumping the file. It can be controlled using the '''debug_daemon''' sub-command to ''lctl''.
==== Using the ''lctl'' debug daemon option to dump the buffer to a file====
The debug_daemon option is used by ''lctl'' to control the dumping of the ''debug_kernel'' buffer to a user-specified file. For more information about the debug daemon including a list of commands, see [http://wiki.lustre.org/manual/LustreManual20_HTML/LustreDebugging.html#50438274_pgfId-1295930 Section 28.2.3: ''Dumping the Buffer to a File (debug_daemon)''] in the [http://wiki.lustre.org/manual/LustreManual20_HTML/index.html ''Lustre Operations Manual''].


==== Debug daemon commands ====
== Controlling the information written to the kernel debug log ==
* debug_daemon start [{file} {megabytes}] - Initiates the debug_daemon to start dumping debug_buffer into a file. The file can be a system default file,as shown in ''/proc/sys/lnet/debug_path''.  
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. For more information and examples, see [http://wiki.lustre.org/manual/LustreManual20_HTML/LustreDebugging.html#50438274_pgfId-1295954 Section 28.2.4: ''Controlling Information Written to the Kernel Debug Log''] and [http://wiki.lustre.org/manual/LustreManual20_HTML/LustreProc.html#50438271_pgfId-1290883 Section 31.3: ''Debug''] in the [http://wiki.lustre.org/manual/LustreManual20_HTML/index.html ''Lustre Operations Manual''].


The default patch [[path?]] after Lustre boots up is ''/tmp/lustre-log-$HOSTNAME''.
== Using ''strace'' to trace system calls ==
The ''strace'' utility provided with the Linux distribution enables a system call to be traced by intercepting all the system calls made by a process and recording the system call name, arguments, and return values. For more details and examples, see [http://wiki.lustre.org/manual/LustreManual20_HTML/LustreDebugging.html#50438274_pgfId-1295970 Section 28.2.5: ''Troubleshooting with strace''] in the [http://wiki.lustre.org/manual/LustreManual20_HTML/index.html ''Lustre Operations Manual''].


Users can specify a new file name for debug_daemon to output debug_buffer. The new file name will show up in ''/proc/sys/lnet/debug_path''.
== Looking at disk content ==


''megabytes'' is the limitation of the file size in megabytes.  
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 via the ''getstripe'' sub-command. For more information, see see [http://wiki.lustre.org/manual/LustreManual20_HTML/LustreDebugging.html#50438274_pgfId-1295983 Section 28.2.6: ''Looking at Disk Content''] in the [http://wiki.lustre.org/manual/LustreManual20_HTML/index.html ''Lustre Operations Manual''] ].


The daemon will wrap around and dump data to the beginning of the file when the output file size exceeds the user-specified file size limit. To decode the dumped file to ASCII run:
== Finding the Lustre UUID of an OST ==
{|
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.
|-
|
#lctl debug_file {file}  > {newfile}


The output is already internally sorted by lctl using quicksort.
|}


* debug_daemon stop - Completely shuts down the debug_daemon operation and flushes the file output. Otherwise, debug_daemon is shut down as part of Lustre file system shutdown process. Users can restart debug_daemon by using ''start'' command after each issued ''stop'' command.


Below is an example using the '''debug_daemon''' to dump debug logs to a 10MB file:
== Printing debug messages to the console ==
 
To dump debug messages to the console (''/var/log/messages''), set the corresponding debug mask in the ''printk'' flag:
#~/utils/lctl
 
To start the daemon dumping 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 yet another daemon with an unlimited file size, enter:
lctl > debug_daemon start /tmp/unlimited
 
The following line is added to end of each output file:
“*** End of debug_daemon trace log ***”
 
==== Controlling the Kernel Debug Log ====
The amount of information printed to the kernel debug logs can be controlled by masks in  ''/proc/sys/lnet/subsystem_debug'' and ''/proc/sys/lnet/debug''. The subsystem_debug mask is for controlling the subsystems (e.g. obdfilter, net, portals, OSC, etc) and the debug mask is for controlling the debug types that would be written out to the log (e.g. info, error, trace, alloc, etc).
 
* ''sysctl -w lnet.debug=0'' - Turns off Lustre debugging completely
* ''sysctl -w lnet.debug=-1'' - Enables full debugging
* ''sysctl -w lnet.debug=net'' - Turns on logging of messages related to network communications.
* ''sysctl -w lnet.debug=+net'' - Turns on logging of messages related to network communications, in addition to existing debug flags.
* ''sysctl -w lnet.debug=-net'' - Turns off logging of messages related to network communications, leaving debug flags alone.
The different options are listed in ''lnet/include/libcfs/libcfs.h''.
 
== The lctl Tool ==
Debug messages useful for troubleshooting are sprinkled throughout the Lustre source code. As described earlier, we have sub-divided the debug messages into a number of sub-systems and types. This allows us to filter and show only the messages we are interested in.
 
The lctl tool is very useful to enable this filtering and manipulate the logs to extract the useful information from them. The lctl command can be used directly from the command-line to execute a single command, or it can be used interactively to execute multiple commands in a single invocation.
 
In this section, we describe the lctl commands which can be used effectively to obtain the required information from the logs. More detailed help on lctl can be found in the [http://manual.lustre.org/manual/LustreManual16_HTML/SystemConfigurationUtilitiesMan8.html#50491437_pgfId-1294257 lctl] [[reference section]] [[in the Lustre manual]].
 
Note that the debug messages displayed with lctl are also subject to the kernel debug masks above! In other words, the filters are additive.
 
* '''Listing all the sub-systems/types.''' Use the following command to obtain a list of all the types and sub-systems:
lctl debug_list <''subs'' | ''types''>
 
* '''Filtering.''' As mentioned before, the whole key to debugging such a system is limiting the debug log to what is really important. The filter command is the primary means of doing this:
 
lctl filter <''subsystem name'' | ''debug type''> '' 
 
:When lctl filters, it removes those lines from the displayed output only--this does not affect the contents of the debug log in the kernel's memory. As a result, you can print the log many times with different filtering levels without worrying about losing data.
 
* '''Enabling the required debug messages.''' In order to show debug messages belonging to certain type or sub-system, use the show command as shown below:
lctl show <''subsystem name'' | ''debug type''>
 
* '''Displaying the Log.''' Once you've setup your filters, debug_kernel pulls the data from the kernel logs, filters it appropriately, and displays or saves it as per the options specified in the command below: <br> ''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 any later time too.
 
* '''Filter a log on disk.''' If you already have a debug log saved to disk (probably from a crash), debug_file will process that:
lctl debug_file <''input filename''> [''output filename'']
 
* '''Marking the Log.''' During the course of a debug session you may want to add markers or breaks to the log for many reasons: ''lctl mark [marker text]''  The marker text defaults to the current date and time. Three lines similar to these will be inserted into the debug log:
:<pre>
;  ***************************************************************
;      DEBUG MARKER: Tue Mar 5 16:06:44 EST 2002
;  ***************************************************************
</pre>
 
* '''Clearing the Log.''' When you're ready to completely flush the kernel debug buffer, run the command:
:<pre>
; lctl clear
</pre>
 
A sample run of lctl is shown below:
<pre>
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.
</pre>
 
== Use of strace for Troubleshooting ==
''strace'' is a system call trace utility made available by the operating system. It can be used to trace a system call, intercept all the systems calls made by a process, and record the system call name, arguments, and return values. This tool is useful when trying to troubleshoot a failed system call.
 
The ''strace'' can be invoked for a system call as shown below:
 
$strace <''system-call''> <''args''>
 
A system call might sometimes fork child processes, we need to use the ''-f'' option in such case to trace the child processes also:
 
$strace -f <''system-call''> <''args''>
 
We can redirect the ''strace'' output to a file that maybe examined at a later time:
 
$strace -o <''filename''> <''system-call''> <''args''>
 
We can use the ''-ff'' option along with ''-o'' to cause the trace output to be saved in ''filename.pid'', where ''pid'' is the process id of the process being traced.
 
Use ''-ttt'' 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 User mode Linux, it's a good idea to save the traces on the host machine by using the command shown below; here we assume that the hostfs is mounted on  ''/r'':
 
$strace -o /r/tmp/vi.strace
 
== Looking at Disk Content ==
In Lustre, the information about objects that make a file is stored in the extended attributes of the file inode. This will contain a list of all the object ids and their locations (that is, which OST device stores them). Lustre has a tool called the '''lfs''' that can be used to obtain this information for a given file name via the '''getstripe''' subcommand. A corresponding '''lfs setstripe''' command can be used to specify the striping attributes of 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. So, to obtain this information for the file ''/mnt/lustre/frog'' in Lustre filesystem, we would do:
 
{|
|-
| 
  $lfs getstripe /mnt/lustre/frog
  $
    OBDs:
      0 : OSC_localhost_UUID
      1 : OSC_localhost_2_UUID
      2 : OSC_localhost_3_UUID
    obdidx        objid
      0            17
      1            4
|}
 
The ''' debugfs''' tool is provided by the e2fsprogs package; it can be used for interactive debugging of an ext3/ldiskfs filesystem. It can either be used just to check the status, or it can be used to modify some information in the filesystem, maybe to correct the filesystem state. In Lustre, all the objects that belong to a file are stored in an underlying ldiskfs filesystem on the OST's.  The filesystem uses the object id's as the file names. So, once the object id's are known, '''debugfs''' can be used to obtain the attributes of all the objects from the different OST's. A sample run for the ''/mnt/lustre/frog'' file used in the example above 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
|}
More help on '''debugfs''' can be obtained from the '''man pages'''.
 
== Finding the lustre UUID of an OST ==
To find the Lustre UUID of an obdfilter disk (in the event, perhaps, you mix up the cables on your OST devices or the SCSI bus numbering suddenly changes and your SCSI devices get new names), use debugfs to get the last_rcvd file ''debugfs -R "dump last_rcvd /tmp/last_rcvf"''
 
== Tcpdump ==
[[Obsolete?]]
Lustre provides a modified version of tcpdump that can be used to decode the complete Lustre message packet. '''[NOTE:''' It is currently
out-of-date and unusable] The tool has more developed support
to read packets from clients to OSTs; the functionality to decode the packets between clients and MDSs is not well developed yet.
 
== Printing to /var/log/messages ==
To dump debug messages to the console set the corresponding debug mask in the printk flag:
<pre>
<pre>
sysctl -w lnet.printk=-1
sysctl -w lnet.printk=-1
</pre>
</pre>


This slows down the system dramatically.  It is also possible to selectively enable or disable this for particular flags:
This slows down the system dramatically, so it is also possible to selectively enable or disable this capability for particular flags:
<pre>
<pre>
sysctl -w lnet.printk=+vfstrace
sysctl -w lnet.printk=+vfstrace
Line 203: Line 47:
</pre>
</pre>


In newer versions of Lustre (1.4.12, 1.6.5) it is even possible to disable '''warning''', '''error''', and '''console''' messages, though it is strongly recommended to have something like ''lctl debug_daemon'' running to capture this data to a local filesystem for debugging purposes.
It is possible to disable '''warning''', '''error''', and '''console''' messages, though it is strongly recommended to have something like ''lctl debug_daemon'' running to capture this data to a local file system for debugging purposes.


== Tracing Lock Traffic ==
== Tracing lock traffic ==
We have built a special debug type category for tracing lock traffic:
Lustre has a specific debug type category for tracing lock traffic. Use:


{| border=1 cellspacing=1
<pre>
|-
lctl> filter all_types
| 
lctl> show dlmtrace
lctl> filter all_types <br/>
lctl> show dlmtrace <br/>
lctl> debug_kernel [filename]
lctl> debug_kernel [filename]
|}
</pre>
 
[[NOTES]] Put at end - used only in extreme cases when we are helping them trace a bad problem but useful for admin to know.

Latest revision as of 08:08, 20 January 2011

(Updated: Feb 2010)

The procedures below may be useful to administrators or developers debugging a Lustre™ file system.

Understanding the Lustre debug messaging format

Lustre debug messages are categorized by originating subsystem, message type, and location in the source code. For a list of subsystems and message types, see Section 28.2.1.1: Lustre Debug Messages in the Lustre Operations Manual.

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 Section 28.2.1.2: Format of Lustre Debug Messages in the Lustre Operations Manual.

Using the lctl tool to view debug messages

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 more information about the lctl tool, see Section 28.2.2: Using the lctl Tool to View Debug Messages and Section 36.3: lctl in the Lustre Operations Manual.

A sample lctl run is shown in Section 28.2.2.1: Sample lctl Run in the Lustre Operations Manual.

Using the lctl debug daemon option to dump the buffer to a file

The debug_daemon option is used by lctl to control the dumping of the debug_kernel buffer to a user-specified file. For more information about the debug daemon including a list of commands, see Section 28.2.3: Dumping the Buffer to a File (debug_daemon) in the Lustre Operations Manual.

Controlling the information written to the kernel debug log

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. For more information and examples, see Section 28.2.4: Controlling Information Written to the Kernel Debug Log and Section 31.3: Debug in the Lustre Operations Manual.

Using strace to trace system calls

The strace utility provided with the Linux distribution enables a system call to be traced by intercepting all the system calls made by a process and recording the system call name, arguments, and return values. For more details and examples, see Section 28.2.5: Troubleshooting with strace in the Lustre Operations Manual.

Looking at disk content

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 via the getstripe sub-command. For more information, see see Section 28.2.6: Looking at Disk Content in the Lustre Operations Manual ].

Finding the Lustre UUID of an OST

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.


Printing debug messages to the console

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, so it is also possible to selectively enable or disable this capability for particular flags: 

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 running to capture this data to a local file system for debugging purposes.

Tracing lock traffic

Lustre has a specific debug type category for tracing lock traffic. Use: 

lctl> filter all_types
lctl> show dlmtrace
lctl> debug_kernel [filename]
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