0001 ==================================
0002 The s390 SCSI dump tool (zfcpdump)
0003 ==================================
0004
0005 System z machines (z900 or higher) provide hardware support for creating system
0006 dumps on SCSI disks. The dump process is initiated by booting a dump tool, which
0007 has to create a dump of the current (probably crashed) Linux image. In order to
0008 not overwrite memory of the crashed Linux with data of the dump tool, the
0009 hardware saves some memory plus the register sets of the boot CPU before the
0010 dump tool is loaded. There exists an SCLP hardware interface to obtain the saved
0011 memory afterwards. Currently 32 MB are saved.
0012
0013 This zfcpdump implementation consists of a Linux dump kernel together with
0014 a user space dump tool, which are loaded together into the saved memory region
0015 below 32 MB. zfcpdump is installed on a SCSI disk using zipl (as contained in
0016 the s390-tools package) to make the device bootable. The operator of a Linux
0017 system can then trigger a SCSI dump by booting the SCSI disk, where zfcpdump
0018 resides on.
0019
0020 The user space dump tool accesses the memory of the crashed system by means
0021 of the /proc/vmcore interface. This interface exports the crashed system's
0022 memory and registers in ELF core dump format. To access the memory which has
0023 been saved by the hardware SCLP requests will be created at the time the data
0024 is needed by /proc/vmcore. The tail part of the crashed systems memory which
0025 has not been stashed by hardware can just be copied from real memory.
0026
0027 To build a dump enabled kernel the kernel config option CONFIG_CRASH_DUMP
0028 has to be set.
0029
0030 To get a valid zfcpdump kernel configuration use "make zfcpdump_defconfig".
0031
0032 The s390 zipl tool looks for the zfcpdump kernel and optional initrd/initramfs
0033 under the following locations:
0034
0035 * kernel: <zfcpdump directory>/zfcpdump.image
0036 * ramdisk: <zfcpdump directory>/zfcpdump.rd
0037
0038 The zfcpdump directory is defined in the s390-tools package.
0039
0040 The user space application of zfcpdump can reside in an intitramfs or an
0041 initrd. It can also be included in a built-in kernel initramfs. The application
0042 reads from /proc/vmcore or zcore/mem and writes the system dump to a SCSI disk.
0043
0044 The s390-tools package version 1.24.0 and above builds an external zfcpdump
0045 initramfs with a user space application that writes the dump to a SCSI
0046 partition.
0047
0048 For more information on how to use zfcpdump refer to the s390 'Using the Dump
0049 Tools' book, which is available from IBM Knowledge Center:
0050 https://www.ibm.com/support/knowledgecenter/linuxonibm/liaaf/lnz_r_dt.html
