Determining the initial extent size for a file system

Veritas File System (VxFS) determines the size of the first extent that is allocated based on the first write to a new file. Normally, the first extent is the smallest power of 2 that is larger than the size of the first write. If that power of 2 is less than 8 KB (the default file system block size), the first extent that is allocated is 8 KB. After the initial extent is allocated, the file system increases the size of subsequent extents with each allocation as the file size is increased using extending writes.

The initial extent size is tunable, and can be changed using the System> option modify tunefstab command.

Increasing the initial extent size to a larger value helps to reduce file system fragmentation and improves I/O performance.

The best value for the initial extent size depends on the expected file sizes that are created by the application. The maximum value is 32768, which equates to a 256 MB extent allocation using the default 8 KB file system block size. Any over allocation of space is returned to the free space pool after the file is closed.

If the application creates a lot of small files with an exact size of 1 MB, then the initial extent size can be set to 128 (1 MB). If 1 MB is an approximate file size, then the initial extent size can be set to 64 (512 KB) instead. If most files are approximately 1 GB or greater in size, then the maximum value of 32768 can be used.