Understanding Custom File System Parameters
Before you choose to alter the default file system parameters assigned by the newfs command, you need to understand them. This section describes each of these parameters:
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 (usually 512 bytes), which is the size of the smallest block that the disk controller can read or write.
You can specify the logical block size of the file system. After the file system is created, you cannot change this parameter without rebuilding the file system. You can have file systems with different logical block sizes on the same disk.
By default, the logical block size is 8192 bytes (8KB) for UFS file systems. The UFS file system supports block sizes of 4096 or 8192 bytes (4KB or 8KB, with 8KB being the recommended logical block size).
To choose the best logical block size for your system, consider both the performance desired and the available space. For most UFS systems, an 8KB file system provides the best performance, offering a good balance between disk performance and use of space in primary memory and on disk.
Note
The sun4u architecture does not support the 4KB block size.
As a general rule, a larger logical block size increases efficiency for file systems in which most of the files are large. Use a smaller logical block size for file systems in which most of the files are small. You can use the quot -c file system command on a file system to display a complete report on the distribution of files by block size.
Fragment Size
As files are created or expanded, they are allocated disk space in either full logical blocks or portions of logical blocks called fragments. When disk space is needed to hold data for a file, full blocks are allocated first, and then one or more fragments of a block are allocated for the remainder. For small files, allocation begins with fragments.
The capability to allocate fragments of blocks to files rather than whole blocks saves space by reducing the fragmentation of disk space that results from unused holes in blocks.
You define the fragment size when you create a UFS file system. The default fragment size is 1KB. Each block can be divided into one, two, four, or eight fragments, resulting in fragment sizes from 512 bytes to 8192 bytes (for 4KB file systems only). The lower boundary is actually tied to the disk sector size, typically 512 bytes.
Note
The upper boundary might equal the full block size, in which case the fragment is not a fragment at all. This configuration might be optimal for file systems with large files when you are more concerned with speed than with space.
When choosing a fragment size, look at the trade-off between time and space: A small fragment size saves space but requires more time to allocate. As a general rule, a larger fragment size increases efficiency for file systems in which most of the files are large. Use a smaller fragment size for file systems in which most of the files are small.
Minimum Free Space
The minimum free space is the percentage of the total disk space held in reserve when you create the file system. Before Solaris 7, the default reserve was always 10 percent. In Solaris 7 and 8 and now Solaris 9, the minimum free space is automatically determined. This new method of calculating free space results in less wasted disk space on large file systems.
Free space is important because file access becomes less efficient as a file system gets full. As long as the amount of free space is adequate, UFS file systems operate efficiently. When a file system becomes full, using up the available user space, only root can access the reserved free space.
Commands such as df report the percentage of space available to users, excluding the percentage allocated as the minimum free space. When the command reports that more than 100 percent of the disk space in the file system is in use, some of the reserve has been used by root.
If you impose quotas on users, the amount of space available to the users does not include the free space reserve. You can change the value of the minimum free space for an existing file system by using the tunefs command.
Rotational Delay (Gap)
The rotational delay is the expected minimum time (in milliseconds) that it takes the CPU to complete a data transfer and initiate a new data transfer on the same disk cylinder. The default delay depends on the type of disk and is usually optimized for each disk type.
When writing a file, the UFS allocation routines try to position new blocks on the same disk cylinder as the previous block in the same file. The allocation routines also try to optimally position new blocks within tracks to minimize the disk rotation needed to access them.
To position file blocks so that they are “rotationally well-behaved,” the allocation routines must know how fast the CPU can service transfers and how long it takes the disk to skip over a block. By using options to the mkfs command, you can indicate how fast the disk rotates and how many disk blocks (sectors) it has per track. The allocation routines use this information to figure out how many milliseconds the disk takes to skip a block. Then, by using the expected transfer time (rotational delay), the allocation routines can position or place blocks so that the next block is just coming under the disk head when the system is ready to read it.
Place blocks consecutively only if your system is fast enough to read them on the same disk rotation. If the system is too slow, the disk spins past the beginning of the next block in the file and must complete a full rotation before the block can be read, which takes a lot of time. You should try to specify an appropriate value for the gap so that the head is located over the appropriate block when the next disk request occurs.
You can change the value of this parameter for an existing file system by using the tunefs command. The change applies only to subsequent block allocation, not to blocks already allocated. (tunefs is described in detail later in this chapter.)
Optimization Type
The optimization type is either space or time. When you select space optimization, disk blocks are allocated to minimize fragmentation and optimize disk use. Space optimization is the default.
When you select time optimization, disk blocks are allocated as quickly as possible, with less emphasis on their placement. Time is the default when you set the minimum free space to 10 percent or greater. With enough free space, the disk blocks can be allocated effectively with minimal fragmentation.
You can change the value of the optimization type parameter for an existing file system by using the tunefs command.
Number of I-nodes and Bytes per I-node
The number of I-nodes determines the number of files you can have in the file system because each file has one I-node. The number of bytes per I-node determines the total number of I-nodes created when the file system is made: the total size of the file system divided by the number of bytes per I-node. After the I-nodes are allocated, you cannot change the number without re-creating the file system.
The default number of bytes per I-node is 2048 (2KB), which assumes that the average size of each file is 2KB or greater. Most files are larger than 2KB. A file system with many symbolic links will have a lower average file size. If your file system will have many small files, you can give this parameter a lower value.
Note
Having too many I-nodes is much better than running out of them. If you have too few I-nodes, you could reach the maximum number of files on a disk slice that is practically empty.