Veritas™ File System 5.0.1 Administrator's Guide

writing the new file contents to a temporary file and then renaming it on top of
the target file.
The delaylog mode
The default logging mode is delaylog. In delaylog mode, the effects of most
system calls other than write(2), writev(2), and pwrite(2) are guaranteed to be
persistent approximately 15 to 20 seconds after the system call returns to the
application. Contrast this with the behavior of most other file systems in which
most system calls are not persistent until approximately 30 seconds or more after
the call has returned. Fast file system recovery works with this mode.
The rename(2) system call flushes the source file to disk to guarantee the
persistence of the file data before renaming it. In the log and delaylog modes,
the rename is also guaranteed to be persistent when the system call returns. This
benefits shell scripts and programs that try to update a file atomically by writing
the new file contents to a temporary file and then renaming it on top of the target
file.
The tmplog mode
In tmplog mode, the effects of system calls have persistence guarantees that are
similar to those in delaylog mode. In addition, enhanced flushing of delayed
extending writes is disabled, which results in better performance but increases
the chances of data being lost or uninitialized data appearing in a file that was
being actively written at the time of a system failure. This mode is only
recommended for temporary file systems. Fast file system recovery works with
this mode.
Note: The term "effects of system calls" refers to changes to file system data and
metadata caused by the system call, excluding changes to st_atime.
See the stat(2) manual page.
Persistence guarantees
In all logging modes, VxFS is fully POSIX compliant. The effects of the fsync(2)
and fdatasync(2) system calls are guaranteed to be persistent after the calls
return. The persistence guarantees for data or metadata modified by write(2),
writev(2), or pwrite(2) are not affected by the logging mount options. The effects
of these system calls are guaranteed to be persistent only if the O_SYNC, O_DSYNC,
VX_DSYNC, or VX_DIRECT flag, as modified by the convosync= mount option, has
been specified for the file descriptor.
VxFS performance: creating, mounting, and tuning file systems
Mounting a VxFS file system
36