What is a boot archive

A boot archive is a subset of a root (/) file system. This boot archive contains all of the kernel modules, driver.conf files, in addition to a few configuration files. These files are located in the /etc directory. The files in the boot archive are read by the kernel before the root (/) file system is mounted. After the root (/) file system is mounted, the boot archive is discarded by the kernel from memory. Then, file I/O is performed against the root device

Note - In some releases, two primary boot archives (one 32-bit archive and one 64-bit archive) are created at installation time. The 32-bit archive is located in /platform/i86pc/boot_archive. The 64-bit archive is located in /platform/i86pc/amd64/boot_archive.

The files that make up the SPARC boot archives are located in the /platform directory.

The contents of this directory are divided into three groups of files:

Files that are required for a sun4u boot archive
Files that are required for a sun4v boot archive
Files that are required for a sun4us boot archive

The files that make up the x86 boot archives are located in the /platform/i86pc directory.

In certain releases, the contents of this directory are divided into two groups of files:

32-bit boot archive files, which are located in /platform/i86pc/boot_archive
64-bit boot archive files, which are located in /platform/i86pc/amd64/boot_archive

To list the files and directories that are included in the boot archives, use the bootadm list-archive command.

If any files in the archive are updated, the boot archive must be rebuilt. For modifications to take effect, the rebuild of the archive must take place before the next system reboot.

References

[1] http://docs.oracle.com/cd/E19963-01/html/821-1451/ggnke.html