* UNIX is a trademark of Bell Laboratories.
** William N. Joy is currently employed by: Sun Microsystems, Inc, 2550 Garcia Avenue, Mountain View, CA 94043
*** Samuel J. Leffler is currently employed by: Lucasfilm Ltd., PO Box 2009, San Rafael, CA 94912
This work was done under grants from the National Science Foundation under grant MCS80-05144, and the Defense Advance Research Projects Agency (DoD) under ARPA Order No. 4031 monitored by Naval Electronic System Command under Contract No. N00039-82-C-0235.
** DEC, PDP, VAX, MASSBUS, and UNIBUS are trademarks of Digital Equipment Corporation.
* In practice, a file's size is constrained to be less than about one gigabyte.
** By ``partition'' here we refer to the subdivision of physical space on a disk drive. In the traditional file system, as in the new file system, file systems are really located in logical disk partitions that may overlap. This overlapping is made available, for example, to allow programs to copy entire disk drives containing multiple file systems.
* The actual number may vary from system to system, but is usually in the range 5-13.
** While it appears that the first cylinder group could be laid out with its super-block at the ``known'' location, this would not work for file systems with blocks sizes of 16 kilobytes or greater. This is because of a requirement that the first 8 kilobytes of the disk be reserved for a bootstrap program and a separate requirement that the cylinder group information begin on a file system block boundary. To start the cylinder group on a file system block boundary, file systems with block sizes larger than 8 kilobytes would have to leave an empty space between the end of the boot block and the beginning of the cylinder group. Without knowing the size of the file system blocks, the system would not know what roundup function to use to find the beginning of the first cylinder group.
* A program may be overwriting data in the middle of an existing file in which case space would already have been allocated.
* The first spill over point at 48 kilobytes is the point at which a file on a 4096 byte block file system first requires a single indirect block. This appears to be a natural first point at which to redirect block allocation. The other spillover points are chosen with the intent of forcing block allocation to be redirected when a file has used about 25% of the data blocks in a cylinder group. In observing the new file system in day to day use, the heuristics appear to work well in minimizing the number of completely filled cylinder groups.
** A UNIX command that is similar to the reading test that we used is ``cp file /dev/null'', where ``file'' is eight megabytes long.