The space used by a file can be a bit tricky to define. For simple files, the space used is straightforward, but space used by a sparse file is unique. A sparse file is one that has been created with 'holes' or unwritten parts. Consider writing a file with record #1, then by seeking to record one million, writing another record and then closing the file. The file has but two records and occupies only a couple of blocks but the missing parts are not stored nor counted in bdf(1) or du(1). Depending on the size of the file and the spareseness, the difference in apparent versus actual size may be quite large.
To locate sparse files, you must search for files where the difference is quite large between wc -c and the du(1) command. The du(1) command reads the file's disk space from the directory entries and would represent actual space on the disk. wc -c will read the file sequentially, and the filesystem will supply the missing data records as a stream of zeros. On tapes where compression is not available, the space used on the tape will be much more than the apparent disk space.
It is possible to create a sparse file with:
dd if=/etc/issue of=/var/tmp/sparse bs=2048k seek=1
where you will see the original file is just a few dozen bytes, the result with ls -l or wc -c shows a 2 meg file, but du(1) will show the file as occupying just a bit more than the original /etc/issue file. A cp(1) of the file will create a new file that is the same size (using ls -l or wc -c) but du(1) will now show a much larger size.
A common file that is often sparse is a core file from a crashed program. Another file that might be a sparse is /usr/lib/aliases.pag
(or /etc/mail/aliases.pag for 10.xx). And of course, any programmer could write special code that creates a sparse file.
bdf also shows inconsistent results for vxfs mount points versus the corresponding device file. For the HFS filesystem, inodes and superblocks are preset via newfs but for the Journaled File System there are no fixed inode locations. Instead, VxFS filesystems consist of a number of dynamic attributes that describe the filesystem. These dynamic attributes are called structural files and they are kept in a separate fileset (Fileset 1) from the user files (Fileset 999).
Since these structures are dynamic and can grow as more files are added to the filesystem, it is impossible to determine how much space will be used used by structural files and how much space will be used by user files. Unlike an HFS filesystem, where all the inodes are preallocated when the filesystem is created, a VxFS filesystem can continue to allocate more inodes as needed. However, each time a new block of inodes is allocated, the number of actual data blocks are reduced.
So bdf and du must guess as to the number of inodes that might be needed and the space available. The bdf 'used' column represents
space used by both user files and structural files. The bdf 'avail' column represents the Kbytes available after subtracting the actual Kbytes used and the estimated Kbytes needed for inodes.
Bill Hassell, sysadmin
