Configuring dump space for systems with large physical memory

You can perform normal or selective dumps on systems with any amount of memory, including those with more than 4GB of memory. In a normal dump, all physical memory is dumped to disk. There are two types of selective dump: a selective dump that dumps only kernel mapped memory (default), or a more abbreviated selective dump that dumps only a portion of kernel mapped memory. The system dump memory image can be examined using crash(1M).

If you have experienced a system panic the relevant information for diagnosis is usually in the kernel pages. On a system with large memory, it can be more practical to perform a selective dump. This means that the dump is quicker and smaller at the expense of some information (user space pages). A selective dump is made at the time of the panic, but you must specify that you want a selective dump when the system is booted. Set the kernel tunable parameter SYSDUMP_SELECTIVE to ``1'' to obtain a selective dump for kernel mapped memory, or set it to ``2'' for an abbreviated, selective dump.

You might want to specify a selective dump if your system has a large amount of physical memory or if maximum use of disk space is required (keeping the swap/dump slice as small as possible to free up disk for filesystems).

If a system dump is directed to a dump device other than the default swap device, the dump will not be detected when the system boots up. To enable the system to detect a dump that is not in the swap area when the system boots, add the following entry into the file /etc/dumptab:

   /dev/dump       0       -

Use the following information to configure the swap/dump space for best results for the amount of memory on your system:

full dump

dump space should be equal to physical memory size (the dump will normally be less than this)

selective dump

dump space should be sized according to the following recommendations:

• physical memory is greater than 32MB, but less than 256MB:

     dump_space >= physical_memory_size/2
  

• physical memory is greater than 256MB:

     dump_space >= 128MB + SEGKMEM_PSE_BYTES + (general_memory_size - 256MB)/4
  

  where general_memory_size is the output of memsize -g.

For example: a system with 64MB of physical memory should have 32MB dump space; a system with 512MB of physical memory should have 192MB dump space; and, a system with 1GB of physical memory should have 320MB dump space.

---

NOTE: These recommendations are minimums, and under certain configurations and uses a selective dump could be as large as a full dump. These minimum sizes should only be used when disk space is at a premium.

Dump space should never be sized larger than 1.5GB for selective dumps, but can be larger for full dumps.

---