AIX Version 4.3 Differences Guide

4.6.1 Opening Files for Direct I/O
A new flag, O_DIRECT has been defined in fcntl.h. When an application opens a
file specifying or calling this flag through the fcntl() system call, I/O to this file will
be done using direct I/O.
4.6.1.1 Inode Flags
When a file is using direct I/O, the i_flag field in the inode is set with the IDIRECT
flag, defined in inode.h. Even so, it is not enough to simply have a flag in the
inode to determine if the file is using direct I/O or not. If a normal I/O application
opens the file while a direct I/O application currently has it open, then all I/O will
be done using normal I/O until the normal I/O reader or writer closes the file. A
count of direct I/O readers is maintained to determine if the direct I/O can be
resumed. A new field in the inode, i_diocnt, has been added for this purpose. This
field indicates if any application has the file opened for direct I/O.
4.6.2 JFS Function Calls for Direct I/O
There are only a few functions that were affected in the JFS for direct I/O to be
implemented. These functions are serialized by the inode lock and are described
below.
jfs_map() If a file opened for direct I/O is mapped, the IDIRECT flag is reset,
and all subsequent I/O will be done using normal I/O. If the mapped
file is then closed, direct I/O will be resumed.
jfs_close() Close semantics are closed with direct I/O. When the final close
occurs, (checked by the counts on gnode) the IDIRECT flag in the
inode is turned off. If a close is initiated by a normal I/O reader or
writer and another application opens the file for direct I/O, all cached
pages are flushed to disk, and direct I/O is resumed on the file.
jfs_dio() jfs_dio() is called from jfs_rdwr(). If the FDIRECT flag is set, jfs_dio()
evaluates if direct I/O can be done for a particular file. This function
performs all alignment and file state consistency checking.
If a read or write request cannot be done using direct I/O, jfs_dio()
returns a non-zero return code, and the request is done through
normal cached I/O.
4.6.3 System Archive Utilities
Archive commands are typical applications that can benefit from the use of direct
I/O. Therefore, the standard system commands tar, backup, restore, and cpio
have been enabled to use direct I/O. Since these commands are read-once
commands, that is, they do not reference the data again after it has been read
and written to media, the copyin() and copyout() characteristics of normal cached
I/O consume a lot of unnecessary CPU when these commands are executing.
The enabling of these commands has been accomplished by changing all calls to
open() and setting the O_DIRECT flag.
78 AIX Version 4.3 Differences Guide