1==================================
2The s390 SCSI dump tool (zfcpdump)
3==================================
4
5System z machines (z900 or higher) provide hardware support for creating system
6dumps on SCSI disks. The dump process is initiated by booting a dump tool, which
7has to create a dump of the current (probably crashed) Linux image. In order to
8not overwrite memory of the crashed Linux with data of the dump tool, the
9hardware saves some memory plus the register sets of the boot CPU before the
10dump tool is loaded. There exists an SCLP hardware interface to obtain the saved
11memory afterwards. Currently 32 MB are saved.
12
13This zfcpdump implementation consists of a Linux dump kernel together with
14a user space dump tool, which are loaded together into the saved memory region
15below 32 MB. zfcpdump is installed on a SCSI disk using zipl (as contained in
16the s390-tools package) to make the device bootable. The operator of a Linux
17system can then trigger a SCSI dump by booting the SCSI disk, where zfcpdump
18resides on.
19
20The user space dump tool accesses the memory of the crashed system by means
21of the /proc/vmcore interface. This interface exports the crashed system's
22memory and registers in ELF core dump format. To access the memory which has
23been saved by the hardware SCLP requests will be created at the time the data
24is needed by /proc/vmcore. The tail part of the crashed systems memory which
25has not been stashed by hardware can just be copied from real memory.
26
27To build a dump enabled kernel the kernel config option CONFIG_CRASH_DUMP
28has to be set.
29
30To get a valid zfcpdump kernel configuration use "make zfcpdump_defconfig".
31
32The s390 zipl tool looks for the zfcpdump kernel and optional initrd/initramfs
33under the following locations:
34
35* kernel:  <zfcpdump directory>/zfcpdump.image
36* ramdisk: <zfcpdump directory>/zfcpdump.rd
37
38The zfcpdump directory is defined in the s390-tools package.
39
40The user space application of zfcpdump can reside in an intitramfs or an
41initrd. It can also be included in a built-in kernel initramfs. The application
42reads from /proc/vmcore or zcore/mem and writes the system dump to a SCSI disk.
43
44The s390-tools package version 1.24.0 and above builds an external zfcpdump
45initramfs with a user space application that writes the dump to a SCSI
46partition.
47
48For more information on how to use zfcpdump refer to the s390 'Using the Dump
49Tools' book, which is available from IBM Knowledge Center:
50https://www.ibm.com/support/knowledgecenter/linuxonibm/liaaf/lnz_r_dt.html
51