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The s390 SCSI dump tool (zfcpdump)
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System z machines (z900 or higher) provide hardware support for creating system
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dumps on SCSI disks. The dump process is initiated by booting a dump tool, which
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has to create a dump of the current (probably crashed) Linux image. In order to
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not overwrite memory of the crashed Linux with data of the dump tool, the
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hardware saves some memory plus the register sets of the boot CPU before the
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dump tool is loaded. There exists an SCLP hardware interface to obtain the saved
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memory afterwards. Currently 32 MB are saved.
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This zfcpdump implementation consists of a Linux dump kernel together with
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a user space dump tool, which are loaded together into the saved memory region
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below 32 MB. zfcpdump is installed on a SCSI disk using zipl (as contained in
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the s390-tools package) to make the device bootable. The operator of a Linux
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system can then trigger a SCSI dump by booting the SCSI disk, where zfcpdump
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resides on.
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The kernel part of zfcpdump is implemented as a debugfs file under "zcore/mem",
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which exports memory and registers of the crashed Linux in an s390
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standalone dump format. It can be used in the same way as e.g. /dev/mem. The
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dump format defines a 4K header followed by plain uncompressed memory. The
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register sets are stored in the prefix pages of the respective CPUs. To build a
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dump enabled kernel with the zcore driver, the kernel config option
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CONFIG_CRASH_DUMP has to be set. When reading from "zcore/mem", the part of
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memory, which has been saved by hardware is read by the driver via the SCLP
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hardware interface. The second part is just copied from the non overwritten real
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memory.
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Since kernel version 3.12 also the /proc/vmcore file can also be used to access
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the dump.
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To get a valid zfcpdump kernel configuration use "make zfcpdump_defconfig".
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The s390 zipl tool looks for the zfcpdump kernel and optional initrd/initramfs
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under the following locations:
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* kernel: <zfcpdump directory>/zfcpdump.image
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* ramdisk: <zfcpdump directory>/zfcpdump.rd
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The zfcpdump directory is defined in the s390-tools package.
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The user space application of zfcpdump can reside in an intitramfs or an
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initrd. It can also be included in a built-in kernel initramfs. The application
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reads from /proc/vmcore or zcore/mem and writes the system dump to a SCSI disk.
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The s390-tools package version 1.24.0 and above builds an external zfcpdump
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initramfs with a user space application that writes the dump to a SCSI
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partition.
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For more information on how to use zfcpdump refer to the s390 'Using the Dump
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Tools book', which is available from
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http://www.ibm.com/developerworks/linux/linux390.
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