Red Hat Enterprise Linux 5 Administration Unleashed

436CHAPTER 21Monitoring and Tuning the KernelFor ppc64 systems, edit the /etc/yaboot.conf file as root and add the following to theend of the line starting with append in the active boot stanza (remember to enable thechanges by executing /sbin/ybin after saving the changes to the file):crashkernel=128M@16MThe two values in this parameter represent the amount of memory to reserve for thesecondary kernel and the memory offset at which to start the reserved memory, respectively.Notice that at least 128 MB should be reserved for x86, x86_64, and ppc64 systems.At least 256 MB must be reserved for the ia64 architecture.The system must be rebooted with this new boot parameter so that the set amount ofmemory is reserved. After rebooting, notice that the amount of free memory is the totalamount of memory for the system minus the amount of reserved memory.Selecting Location for Dump FileNext, decide whether to save the dump file to a local or remote filesystem. Both local andremote filesystems have their advantages. Writing to the local filesystem doesn’t requirethe network connection to be functioning properly after a kernel crash. It can be muchfaster and more reliable depending on your network transfer speeds and the state of thenetwork card driver. Imagine the impact of saving a 36 GB file over the network. It couldcause the rest of the network to slow to a halt. If the system is in production, this couldmean failure for all other packet transfers. If network transfer is critical to the system,such as a system accepting orders or stock trades, slowing down the network isn’t acceptable.Also, if the network is not working properly on the system, the crash file might notever get written to the remote filesystem.TIPIf you need to send the crash file to someone such as Red Hat Support, you cancompress the file to make it smaller with the gzip or bzip utilities.However, writing to a local filesystem also means having enough dedicated disk space onthe system for the kernel dump file. If you have a network file server set up on yournetwork, it might be more convenient to write to a dedicated directory on it instead.Writing to a network location also has the advantage of being able to write all kerneldumps from all systems on the network to one central location.NOTEThe dump file can be quite large—the size of the physical memory plus a header field,so saving the file might take a considerable amount of time and require a significantamount of available disk space. Be sure you have plenty of free disk space in theconfigured location. It should be at least as big as the total memory for the system.