chfile - changes attributes of AdvFS files
/usr/sbin/chfile [-l on|off] [-L on|off] filename...
Enables or disables (on
off) forced synchronous write requests to the specified
filename. By default, forced synchronous write requests to a
Enables or disables (on
off) atomic write data logging on the specified
filename. By default, atomic write data logging is
One or more file names.
The chfile command lets you view or change attributes of an AdvFS file.
The only file attribute that can be set with the chfile command is the I/O mode that will be used when write requests are made to the file. There are three settings for this I/O mode: The default setting. Write requests are cached, the write system call returns to the calling program, and later (asynchronously), the data is written to the disk. When this mode is enabled, write requests to a file behave as if the O_SYNC flag had been set when the file was opened. The write system call returns a success value only after the data has been successfully written to disk. When this mode is enabled, write requests to a file are asynchronous. However, the write requests are also written to the AdvFS log file. Should a system crash during or after a write system call when this mode is enabled for the file, only complete write requests will be in the file on disk. This atomic operation guarantees that all (or none) of a write buffer will be in the file and that there will not be portions of the write request in the file. For example, suppose a write of an 8192-byte buffer was started and, during the write system call (or shortly thereafter) the system crashed. When the system was rebooted, either the entire 8192 bytes of data would be written to the file or none of it would have been written to the file. There would be no chance that some (but not all) bytes of the write request would be in the file.
The -l and -L options are mutually exclusive. You cannot simultaneously enable both forced synchronous writes and atomic write data logging on a file. However, you can override the current I/O mode for a file. For example, you can change a file's I/O mode setting from forced synchronous writes to atomic write data logging by using the chfile -L on command.
If you do not use the options, the command displays the current state of the file's I/O attribute.
command can be used on AdvFS files that
have been remotely mounted across NFS. You can run the
command on an NFS client to examine or change the I/O mode of AdvFS files
on the NFS server.
Enabling atomic write data logging for a file will retard performance
because the data is written to both the user file and the AdvFS log file.
Enabling forced synchronous writes to a file also can retard system performance.
You must be the owner of the file or the root user in order to change the attributes of the file with this command.
To use the chfile command on AdvFS files that are mounted across NFS, the NFS property list daemon, proplistd, must be running on the NFS client and the fileset must have been mounted on the client using the proplist option.
Only writes of up to 8192 bytes are guaranteed to be atomic for files that use atomic write data logging. When writing to an AdvFS file that has been mounted across NFS, a further restriction applies: the offset into the file of the write must be on an 8K page boundary, because NFS performs I/O on 8K page boundaries.
The showfile command does not display the I/O mode for files that are mounted across NFS. To display the I/O mode of these files, use the chfile command.
Usually AdvFS, when operating on small files that do not have a size that is a multiple of 8K, puts the last part of the files (their frags) into a special metadata file called the fileset frags file as a way to reduce disk fragmentation. For example, a file that does not use atomic write data logging and has had 20K of data written to it will occupy 20K of disk space (as displayed by the du command).
Files that use atomic write data logging are exempt from this behavior. As a result, they always have a disk usage (as displayed by the du command) that is a multiple of 8K. For example, a file that has atomic write data logging enabled and has had 20K of data written to it occupies 24K of disk space.
If a file has a frag, an attempt to activate atomic write data logging on it will fail.
Files that use atomic write data logging cannot be memory-mapped through
system call. The error ENOTSUP is returned if
the attempt is made. If a file has been memory-mapped through the
system call, an attempt to active atomic write data logging
on it fails with the same error.
The following example shows a directory with several different files and how you can use the chfile and showfile commands to query the file I/O mode. # ls -l total 8712 drwx------ 2 root system 8192 Nov 4 06:16 .tags -rwxr-xr-x 1 root system 8435752 Nov 5 08:43 data_logging_file -rw-r--r-- 1 root system 250880 Nov 5 08:44 forced_sync_file -rw-r--r-- 1 root system 195320 Nov 5 08:44 normal_async_file -rw-r----- 1 root operator 8192 Nov 4 06:16 quota.group
# chfile data_logging_file I/O type = atomic write data logging # chfile forced_sync_file I/O type = forced synchronous writes # chfile normal_async_file I/O type = normal asynchronous writes You can display similar information with the showfile command: # showfile data_logging_file foreced_sync_file normal_async_file
Id Vol PgSz Pages XtntType Segs SegSz I/O Perf File 8.8001 1 16 1030 simple ** ** ftx 100% data_logging_file 7.8008 2 16 31 simple ** ** sync 100% forced_sync_file 6.8001 1 16 24 simple ** ** async 100% normal_async_file