System Administration Guide: Devices and File Systems

Logical Block Size

The logical block size is the size of the blocks that the UNIX kernel uses to read or write files. The logical block size is usually different from the physical block size. The physical block size is usually 512 bytes, which is the size of the smallest block that the disk controller can read or write.

Logical block size is set to the page size of the system by default. The default logical block size is 8192 bytes (8 KB) for UFS file systems. The UFS file system supports block sizes of 4096 or 8192 bytes (4 or 8 KB). The recommended logical block size is 8 KB.

SPARC only –

You can specify only the 8192-byte block size on the sun-4u platform.

To choose the best logical block size for your system, consider both the performance you want and the available space. For most UFS systems, an 8-KB file system provides the best performance, offering a good balance between disk performance and the use of space in primary memory and on disk.

As a general rule, to increase efficiency, use a larger logical block size for file systems when most of the files are very large. Use a smaller logical block size for file systems when most of the files are very small. You can use the quot -c filesystem command on a file system to display a complete report on the distribution of files by block size.

However, the page size set when the file system is created is probably the best size in most cases.