configure Kdump in centos (to recover data)

1	================================================================
2	Documentation for Kdump - The kexec-based Crash Dumping Solution
3	================================================================
4	
5	This document includes overview, setup and installation, and analysis
6	information.
7	
8	Overview
9	========
10	
11	Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
12	dump of the system kernel's memory needs to be taken (for example, when
13	the system panics). The system kernel's memory image is preserved across
14	the reboot and is accessible to the dump-capture kernel.
15	
16	You can use common commands, such as cp and scp, to copy the
17	memory image to a dump file on the local disk, or across the network to
18	a remote system.
19	
20	Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
21	and s390x architectures.
22	
23	When the system kernel boots, it reserves a small section of memory for
24	the dump-capture kernel. This ensures that ongoing Direct Memory Access
25	(DMA) from the system kernel does not corrupt the dump-capture kernel.
26	The kexec -p command loads the dump-capture kernel into this reserved
27	memory.
28	
29	On x86 machines, the first 640 KB of physical memory is needed to boot,
30	regardless of where the kernel loads. Therefore, kexec backs up this
31	region just before rebooting into the dump-capture kernel.
32	
33	Similarly on PPC64 machines first 32KB of physical memory is needed for
34	booting regardless of where the kernel is loaded and to support 64K page
35	size kexec backs up the first 64KB memory.
36	
37	For s390x, when kdump is triggered, the crashkernel region is exchanged
38	with the region [0, crashkernel region size] and then the kdump kernel
39	runs in [0, crashkernel region size]. Therefore no relocatable kernel is
40	needed for s390x.
41	
42	All of the necessary information about the system kernel's core image is
43	encoded in the ELF format, and stored in a reserved area of memory
44	before a crash. The physical address of the start of the ELF header is
45	passed to the dump-capture kernel through the elfcorehdr= boot
46	parameter. Optionally the size of the ELF header can also be passed
47	when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
48	
49	
50	With the dump-capture kernel, you can access the memory image, or "old
51	memory," in two ways:
52	
53	- Through a /dev/oldmem device interface. A capture utility can read the
54	  device file and write out the memory in raw format. This is a raw dump
55	  of memory. Analysis and capture tools must be intelligent enough to
56	  determine where to look for the right information.
57	
58	- Through /proc/vmcore. This exports the dump as an ELF-format file that
59	  you can write out using file copy commands such as cp or scp. Further,
60	  you can use analysis tools such as the GNU Debugger (GDB) and the Crash
61	  tool to debug the dump file. This method ensures that the dump pages are
62	  correctly ordered.
63	
64	
65	Setup and Installation
66	======================
67	
68	Install kexec-tools
69	-------------------
70	
71	1) Login as the root user.
72	
73	2) Download the kexec-tools user-space package from the following URL:
74	
75	http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz
76	
77	This is a symlink to the latest version.
78	
79	The latest kexec-tools git tree is available at:
80	
81	git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
82	and
83	http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
84	
85	There is also a gitweb interface available at
86	http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git
87	
88	More information about kexec-tools can be found at
89	http://www.kernel.org/pub/linux/utils/kernel/kexec/README.html
90	
91	3) Unpack the tarball with the tar command, as follows:
92	
93	   tar xvpzf kexec-tools.tar.gz
94	
95	4) Change to the kexec-tools directory, as follows:
96	
97	   cd kexec-tools-VERSION
98	
99	5) Configure the package, as follows:
100	
101	   ./configure
102	
103	6) Compile the package, as follows:
104	
105	   make
106	
107	7) Install the package, as follows:
108	
109	   make install
110	
111	
112	Build the system and dump-capture kernels
113	-----------------------------------------
114	There are two possible methods of using Kdump.
115	
116	1) Build a separate custom dump-capture kernel for capturing the
117	   kernel core dump.
118	
119	2) Or use the system kernel binary itself as dump-capture kernel and there is
120	   no need to build a separate dump-capture kernel. This is possible
121	   only with the architectures which support a relocatable kernel. As
122	   of today, i386, x86_64, ppc64 and ia64 architectures support relocatable
123	   kernel.
124	
125	Building a relocatable kernel is advantageous from the point of view that
126	one does not have to build a second kernel for capturing the dump. But
127	at the same time one might want to build a custom dump capture kernel
128	suitable to his needs.
129	
130	Following are the configuration setting required for system and
131	dump-capture kernels for enabling kdump support.
132	
133	System kernel config options
134	----------------------------
135	
136	1) Enable "kexec system call" in "Processor type and features."
137	
138	   CONFIG_KEXEC=y
139	
140	2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
141	   filesystems." This is usually enabled by default.
142	
143	   CONFIG_SYSFS=y
144	
145	   Note that "sysfs file system support" might not appear in the "Pseudo
146	   filesystems" menu if "Configure standard kernel features (for small
147	   systems)" is not enabled in "General Setup." In this case, check the
148	   .config file itself to ensure that sysfs is turned on, as follows:
149	
150	   grep 'CONFIG_SYSFS' .config
151	
152	3) Enable "Compile the kernel with debug info" in "Kernel hacking."
153	
154	   CONFIG_DEBUG_INFO=Y
155	
156	   This causes the kernel to be built with debug symbols. The dump
157	   analysis tools require a vmlinux with debug symbols in order to read
158	   and analyze a dump file.
159	
160	Dump-capture kernel config options (Arch Independent)
161	-----------------------------------------------------
162	
163	1) Enable "kernel crash dumps" support under "Processor type and
164	   features":
165	
166	   CONFIG_CRASH_DUMP=y
167	
168	2) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems".
169	
170	   CONFIG_PROC_VMCORE=y
171	   (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
172	
173	Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
174	--------------------------------------------------------------------
175	
176	1) On i386, enable high memory support under "Processor type and
177	   features":
178	
179	   CONFIG_HIGHMEM64G=y
180	   or
181	   CONFIG_HIGHMEM4G
182	
183	2) On i386 and x86_64, disable symmetric multi-processing support
184	   under "Processor type and features":
185	
186	   CONFIG_SMP=n
187	
188	   (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
189	   when loading the dump-capture kernel, see section "Load the Dump-capture
190	   Kernel".)
191	
192	3) If one wants to build and use a relocatable kernel,
193	   Enable "Build a relocatable kernel" support under "Processor type and
194	   features"
195	
196	   CONFIG_RELOCATABLE=y
197	
198	4) Use a suitable value for "Physical address where the kernel is
199	   loaded" (under "Processor type and features"). This only appears when
200	   "kernel crash dumps" is enabled. A suitable value depends upon
201	   whether kernel is relocatable or not.
202	
203	   If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
204	   This will compile the kernel for physical address 1MB, but given the fact
205	   kernel is relocatable, it can be run from any physical address hence
206	   kexec boot loader will load it in memory region reserved for dump-capture
207	   kernel.
208	
209	   Otherwise it should be the start of memory region reserved for
210	   second kernel using boot parameter "crashkernel=Y@X". Here X is
211	   start of memory region reserved for dump-capture kernel.
212	   Generally X is 16MB (0x1000000). So you can set
213	   CONFIG_PHYSICAL_START=0x1000000
214	
215	5) Make and install the kernel and its modules. DO NOT add this kernel
216	   to the boot loader configuration files.
217	
218	Dump-capture kernel config options (Arch Dependent, ppc64)
219	----------------------------------------------------------
220	
221	1) Enable "Build a kdump crash kernel" support under "Kernel" options:
222	
223	   CONFIG_CRASH_DUMP=y
224	
225	2)   Enable "Build a relocatable kernel" support
226	
227	   CONFIG_RELOCATABLE=y
228	
229	   Make and install the kernel and its modules.
230	
231	Dump-capture kernel config options (Arch Dependent, ia64)
232	----------------------------------------------------------
233	
234	- No specific options are required to create a dump-capture kernel
235	  for ia64, other than those specified in the arch independent section
236	  above. This means that it is possible to use the system kernel
237	  as a dump-capture kernel if desired.
238	
239	  The crashkernel region can be automatically placed by the system
240	  kernel at run time. This is done by specifying the base address as 0,
241	  or omitting it all together.
242	
243	  crashkernel=256M@0
244	  or
245	  crashkernel=256M
246	
247	  If the start address is specified, note that the start address of the
248	  kernel will be aligned to 64Mb, so if the start address is not then
249	  any space below the alignment point will be wasted.
250	
251	
252	Extended crashkernel syntax
253	===========================
254	
255	While the "crashkernel=size[@offset]" syntax is sufficient for most
256	configurations, sometimes it's handy to have the reserved memory dependent
257	on the value of System RAM -- that's mostly for distributors that pre-setup
258	the kernel command line to avoid a unbootable system after some memory has
259	been removed from the machine.
260	
261	The syntax is:
262	
263	    crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
264	    range=start-[end]
265	
266	    'start' is inclusive and 'end' is exclusive.
267	
268	For example:
269	
270	    crashkernel=512M-2G:64M,2G-:128M
271	
272	This would mean:
273	
274	    1) if the RAM is smaller than 512M, then don't reserve anything
275	       (this is the "rescue" case)
276	    2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
277	    3) if the RAM size is larger than 2G, then reserve 128M
278	
279	
280	
281	Boot into System Kernel
282	=======================
283	
284	1) Update the boot loader (such as grub, yaboot, or lilo) configuration
285	   files as necessary.
286	
287	2) Boot the system kernel with the boot parameter "crashkernel=Y@X",
288	   where Y specifies how much memory to reserve for the dump-capture kernel
289	   and X specifies the beginning of this reserved memory. For example,
290	   "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
291	   starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
292	
293	   On x86 and x86_64, use "crashkernel=64M@16M".
294	
295	   On ppc64, use "crashkernel=128M@32M".
296	
297	   On ia64, 256M@256M is a generous value that typically works.
298	   The region may be automatically placed on ia64, see the
299	   dump-capture kernel config option notes above.
300	
301	   On s390x, typically use "crashkernel=xxM". The value of xx is dependent
302	   on the memory consumption of the kdump system. In general this is not
303	   dependent on the memory size of the production system.
304	
305	Load the Dump-capture Kernel
306	============================
307	
308	After booting to the system kernel, dump-capture kernel needs to be
309	loaded.
310	
311	Based on the architecture and type of image (relocatable or not), one
312	can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
313	of dump-capture kernel. Following is the summary.
314	
315	For i386 and x86_64:
316		- Use vmlinux if kernel is not relocatable.
317		- Use bzImage/vmlinuz if kernel is relocatable.
318	For ppc64:
319		- Use vmlinux
320	For ia64:
321		- Use vmlinux or vmlinuz.gz
322	For s390x:
323		- Use image or bzImage
324	
325	
326	If you are using a uncompressed vmlinux image then use following command
327	to load dump-capture kernel.
328	
329	   kexec -p <dump-capture-kernel-vmlinux-image> \
330	   --initrd=<initrd-for-dump-capture-kernel> --args-linux \
331	   --append="root=<root-dev> <arch-specific-options>"
332	
333	If you are using a compressed bzImage/vmlinuz, then use following command
334	to load dump-capture kernel.
335	
336	   kexec -p <dump-capture-kernel-bzImage> \
337	   --initrd=<initrd-for-dump-capture-kernel> \
338	   --append="root=<root-dev> <arch-specific-options>"
339	
340	Please note, that --args-linux does not need to be specified for ia64.
341	It is planned to make this a no-op on that architecture, but for now
342	it should be omitted
343	
344	Following are the arch specific command line options to be used while
345	loading dump-capture kernel.
346	
347	For i386, x86_64 and ia64:
348		"1 irqpoll maxcpus=1 reset_devices"
349	
350	For ppc64:
351		"1 maxcpus=1 noirqdistrib reset_devices"
352	
353	For s390x:
354		"1 maxcpus=1 cgroup_disable=memory"
355	
356	Notes on loading the dump-capture kernel:
357	
358	* By default, the ELF headers are stored in ELF64 format to support
359	  systems with more than 4GB memory. On i386, kexec automatically checks if
360	  the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
361	  So, on non-PAE systems, ELF32 is always used.
362	
363	  The --elf32-core-headers option can be used to force the generation of ELF32
364	  headers. This is necessary because GDB currently cannot open vmcore files
365	  with ELF64 headers on 32-bit systems.
366	
367	* The "irqpoll" boot parameter reduces driver initialization failures
368	  due to shared interrupts in the dump-capture kernel.
369	
370	* You must specify <root-dev> in the format corresponding to the root
371	  device name in the output of mount command.
372	
373	* Boot parameter "1" boots the dump-capture kernel into single-user
374	  mode without networking. If you want networking, use "3".
375	
376	* We generally don' have to bring up a SMP kernel just to capture the
377	  dump. Hence generally it is useful either to build a UP dump-capture
378	  kernel or specify maxcpus=1 option while loading dump-capture kernel.
379	
380	* For s390x there are two kdump modes: If a ELF header is specified with
381	  the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
382	  is done on all other architectures. If no elfcorehdr= kernel parameter is
383	  specified, the s390x kdump kernel dynamically creates the header. The
384	  second mode has the advantage that for CPU and memory hotplug, kdump has
385	  not to be reloaded with kexec_load().
386	
387	* For s390x systems with many attached devices the "cio_ignore" kernel
388	  parameter should be used for the kdump kernel in order to prevent allocation
389	  of kernel memory for devices that are not relevant for kdump. The same
390	  applies to systems that use SCSI/FCP devices. In that case the
391	  "allow_lun_scan" zfcp module parameter should be set to zero before
392	  setting FCP devices online.
393	
394	Kernel Panic
395	============
396	
397	After successfully loading the dump-capture kernel as previously
398	described, the system will reboot into the dump-capture kernel if a
399	system crash is triggered.  Trigger points are located in panic(),
400	die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
401	
402	The following conditions will execute a crash trigger point:
403	
404	If a hard lockup is detected and "NMI watchdog" is configured, the system
405	will boot into the dump-capture kernel ( die_nmi() ).
406	
407	If die() is called, and it happens to be a thread with pid 0 or 1, or die()
408	is called inside interrupt context or die() is called and panic_on_oops is set,
409	the system will boot into the dump-capture kernel.
410	
411	On powerpc systems when a soft-reset is generated, die() is called by all cpus
412	and the system will boot into the dump-capture kernel.
413	
414	For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
415	"echo c > /proc/sysrq-trigger" or write a module to force the panic.
416	
417	Write Out the Dump File
418	=======================
419	
420	After the dump-capture kernel is booted, write out the dump file with
421	the following command:
422	
423	   cp /proc/vmcore <dump-file>
424	
425	You can also access dumped memory as a /dev/oldmem device for a linear
426	and raw view. To create the device, use the following command:
427	
428	    mknod /dev/oldmem c 1 12
429	
430	Use the dd command with suitable options for count, bs, and skip to
431	access specific portions of the dump.
432	
433	To see the entire memory, use the following command:
434	
435	   dd if=/dev/oldmem of=oldmem.001
436	
437	
438	Analysis
439	========
440	
441	Before analyzing the dump image, you should reboot into a stable kernel.
442	
443	You can do limited analysis using GDB on the dump file copied out of
444	/proc/vmcore. Use the debug vmlinux built with -g and run the following
445	command:
446	
447	   gdb vmlinux <dump-file>
448	
449	Stack trace for the task on processor 0, register display, and memory
450	display work fine.
451	
452	Note: GDB cannot analyze core files generated in ELF64 format for x86.
453	On systems with a maximum of 4GB of memory, you can generate
454	ELF32-format headers using the --elf32-core-headers kernel option on the
455	dump kernel.
456	
457	You can also use the Crash utility to analyze dump files in Kdump
458	format. Crash is available on Dave Anderson's site at the following URL:
459	
460	   http://people.redhat.com/~anderson/
461	
462	
463	To Do
464	=====
465	
466	1) Provide relocatable kernels for all architectures to help in maintaining
467	   multiple kernels for crash_dump, and the same kernel as the system kernel
468	   can be used to capture the dump.
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