This section describes parameters in function of input and output processes in the kernel.
Defines the maximum size of physical I/O requests. If a driver encounters a request larger than this size, the driver breaks the request into maxphys sized chunks. File systems can and do impose their own limit.
Signed integer
131,072 (sun4u or sun4v) or 57,344 (x86). The sd driver uses the value of 1,048,576 if the drive supports wide transfers. The ssd driver uses 1,048,576 by default.
stem-specific page size to MAXINT
Bytes
Yes, but many file systems load this value into a per-mount point data structure when the file system is mounted. A number of drivers load the value at the time a device is attached to a driver-specific data structure.
None
When doing I/O to and from raw devices in large chunks. Note that a DBMS doing OLTP operations issues large numbers of small I/Os. Changing maxphys does not result in any performance improvement in that case.
Unstable
Defines the “soft” limit on file descriptors that a single process can have open. A process might adjust its file descriptor limit to any value up to the “hard” limit defined by rlim_fd_max by using the setrlimit() call or by issuing the limit command in whatever shell it is running. You do not require superuser privilege to adjust the limit to any value less than or equal to the hard limit.
Signed integer
256
128 to MAXINT
File descriptors
No
Compared to rlim_fd_max. If rlim_fd_cur is greater than rlim_fd_max, rlim_fd_cur is reset to rlim_fd_max.
When the default number of open files for a process is not enough. Increasing this value means only that it might not be necessary for a program to use setrlimit to increase the maximum number of file descriptors available to it.
Unstable
Specifies the “hard” limit on file descriptors that a single process might have open. Overriding this limit requires superuser privilege.
Signed integer
65,536
128 to MAXINT
File descriptors
No
None
When the maximum number of open files for a process is not enough. Other limitations in system facilities can mean that a larger number of file descriptors is not as useful as it might be. For example:
A 32-bit program using standard I/O is limited to 256 file descriptors. A 64-bit program using standard I/O can use up to 2 billion descriptors. Specifically, standard I/O refers to the stdio(3C) functions in libc(3LIB).
select is by default limited to 1024 descriptors per fd_set. For more information, see the select(3C) man page. A 32-bit application code can be recompiled with a larger fd_set size (less than or equal to 65,536). A 64-bit application uses an fd_set size of 65,536, which cannot be changed.
An alternative to changing this on a system wide basis is to use the plimit command. If a parent process has its limits changed by plimit, all children inherit the increased limit. This alternative is useful for daemons such as inetd.
Unstable