Answer

TARGET AUDIENCE:

Users running diskless or dataless clients in an AIX environment e.g. using AIX based clustered compute nodes in a xCAT environment.

OBJECTIVE:

This technote provides some background information about the NIM dump resource that is being created on a NIM master server and how to interpret the contents of /etc/tmiscsi/mappings describing the structure of the iSCSI disk.

OVERVIEW:

In the rare case of a dump of a diskless or dataless AIX client, it is necessary to write the dump image to a device that is not being overwritten by any other client sharing the same image. If a diskless or dataless AIX client is running on Power6 or later hardware, it is possible to use the firmware-assisted dump so that the dump can be written to the dump device after the reboot of the client following the crash. Assigning a dump resource to the client allows us to address this requirement.
This dump resource on the VIO server is presented to the diskless client as an iSCSI disk. That, in turn, requires some prerequisites to be met on the NIM master.

PROCEDURES:

To be able to create a dump resource, the iSCSI Target Software Device package needs to be installed on the NIM server - the fileset devices.tmiscsw.rte can be found on the Expansion Pack DVD for the AIX release of the NIM master.
Once the dump resource has been configured on the NIM master following the procedures in "Using the dump resource" in the AIX knowledge center the following additional devices are created:

• The iSCSI Target Mode Device Driver: tmsw0
  The tmsw0 device is the device providing the overall iSCSI target mode functionality. There are a number of attributes of the driver influencing the performance and behavior of the device driver:
  "Maximum number of sessions" specifies the number of sessions that can be opened at one time (i.e. limiting the maximum number of iSCSI clients connected to the NIM server at the same time).
  Default value is 200.
  "Maximum transfer size in one PDU" - this is the maximum transfer size allowed in one SCSI request. Default value is 262144,
  "Access Control enabled": If this is set to "on" (what is set by creating the dump resource) then the /etc/tmiscsi/access_lists file will be used to grant access for certain or all iSCSI initiators to the iSCSI targets.
• The iSCSI Target Mode Target: target0
  This is the logical device a given iSCSI initiator connects to. There can be multiple targets in one AIX LPAR (target0, target1 ... targetN). The iSCSI Target Name is identifying the target in the entire network and needs to be either a unique IQN or EUI name. Creating the dump resource will automatically assign an iSCSI Target Name "iqn.2009-10.com.ibm.master.nim:aix.nim.resouce.server:<hostname>"
  One can specify a number of additional characteristics, such as the maximum number of LUNs allowed, or the network interface(s) the target is available via to the initiators.
• iSCSI Target LUNs: luX
  These are the devices that will show up as an iSCSI disk on the iSCSI initiator. There can be up to 512 LUNs per target (lu0 ... lu512). The backing devices for an iSCSI LUN can be disks, logical volumes in a volume group, or local files in a JFS or JFS2 filesystem.
  

While for a dump resource the backing device for a LUN is a file, there is no Backing Device Name given in the characteristics of the device (as seen in the 'lsattr -El luX' output). Instead, the main characteristics of the backing device are given in /etc/tmiscsi/mappings as there are a number of files forming the iSCSI disk.
There can be a mixture of read-only and read-writeable files creating the disk. For the rules that describe this mapping, please see the comment lines in /etc/tmiscsi/mappings.

For the iSCSI dump resource an entry may look like this example:
lu1     0:1:RO:/tftpboot/%i.iplrecord
lu1     1:7:RO:/nim/dump_710_spot/dumpdiskhdr
lu1     8:2097144:RO:/tftpboot/%i.boot
lu1     2097152:104857600:RW:/nim/dump_710_spot/%i/dump

The /tftpboot/%i.iplrecord and /tftpboot/%i.boot entries describe the files that are necessary to perform a network boot - these files are created when the diskless or dataless client is created in the NIM environment. The dumpdiskhdr file represents a standard disk header. The last entry in the list specifies the file that is used to store the dump itself when the firmware-assisted dump is written upon reboot of the crashed client. This file is created only when a dump is written. It is important to have sufficient free space in the filesystem containing this file.

There are a number of limitations for the iSCSI disks on the client that is provided via a dump resource:

• The iSCSI disk cannot be used in a volume group (creation of a volume group fails).
• The iSCSI disk cannot be used for any other purposes then storing the dump image.

REFERENCES:

• Technote FAQ "Is iSCSI boot supported with AIX" https://www-03.ibm.com/support/techdocs/atsmastr.nsf/WebIndex/FQ127834
• Diskless and dataless clients http://www.ibm.com/support/knowledgecenter/ssw_aix_71/com.ibm.aix.install/nim_diskless_dataless_client.htm
• Using the dump resource

http://www.ibm.com/support/knowledgecenter/ssw_aix_71/com.ibm.aix.install/resource_dump.htm

CATEGORY:

AIXIOD

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