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chfile(8)
NAME
chfile - Changes attributes of AdvFS files
SYNOPSIS
/usr/sbin/chfile [-l on | off] [-L on | off] filename...
OPTIONS
-l on | off
Enables or disables (on | off) forced synchronous write requests to the
specified filename. By default, forced synchronous write requests to a
file are off.
-L on | off
Enables or disables (on | off) persistent atomic-write data logging on
the specified filename. By default, atomic-write data logging is off.
OPERAND
filename...
One or more file names.
DESCRIPTION
The chfile command lets you view or change 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:
Asynchronous I/O
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.
Forced synchronous I/O
When this mode is enabled, write requests to a file behave as if the
O_SYNC option 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.
Persistent atomic-write data logging I/O
When this mode is enabled, write requests to a file are asynchronous
and written to the AdvFS log file. Should a system crash during or
after a write system call, only complete write requests (up to 8192
bytes; see restrictions below) will be processed. This atomic operation
guarantees that all (or none) of a write buffer will be in the file.
For example, if 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 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.
When persistent atomic-write data logging is enabled on a file, writes
to the file are guaranteed to be flushed to disk in the order that they
are written by the application. This is generally not guaranteed to be
the case in POSIX-compliant file systems when files are opened in
asynchronous mode. For example, the following code does three writes
to a file:
open(file);
write(buffer A);
write(buffer B);
write(buffer C);
close(file);
If the file being written to did not use persistent atomic-write data
logging, and if the file were opened for asynchronous writes, then it
is possible that buffer C would be written to disk before buffer B,
that buffer B would be written to disk before buffer A, and so on. This
might lead to confusing file contents if the system were to crash while
the application was running.
But if persistent atomic-write data logging is used, even if the file
is open for asynchronous writes, the buffers would be guaranteed to be
written to disk in the order in which the application wrote them.
The -l and -L options are mutually exclusive. You cannot simultaneously
enable both forced synchronous writes and persistent 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 persistent atomic-write data logging by using the
chfile -L on command.
If you do not specify the options, the command displays the current state
of the file's I/O attribute.
The chfile command can be used on AdvFS files that have been remotely
mounted across NFS. You can run the chfile command on an NFS client to
examine or change the I/O mode of AdvFS files on the NFS server.
NOTES
If you cannot enable persistent atomic-write data logging (for example, if
you have frag files or are memory mapped) you can activate temporary on all
files in a mounted fileset for the duration of the mount by using the -o
adl option of the mount command. See AdvFS Administration for more
information.
Enabling persistent 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. See AdvFS Administration for more information.
RESTRICTIONS
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 persistent atomic-write data logging. Writes that are greater than
8192 bytes are written in segments that are at most 8192 bytes in length
with each segment an atomic-write. If an error should occur (disk full,
ulimit, and so on), the return value from the write will specify how much
was successfully written. 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 frag file as a way to reduce disk
fragmentation. For example, a file that does not use persistent 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 persistent 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
persistent 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 persistent atomic-write data
logging on it will fail.
Files that use persistent atomic-write data logging cannot be memory-mapped
through the mmap system call. The error ENOTSUP is returned if the
attempt is made. If a file has been memory-mapped through the mmap system
call, an attempt to activate persistent atomic-write data logging on it
fails with the same error.
EXIT STATUS
The utility returns a 0 (zero) on success and a 1 (one) on failure.
EXAMPLE
1. The following example queries 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
2. You can display similar information with the showfile command:
# showfile data_logging_file forced_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
3. The following code fragment queries the current I/O mode for an AdvFS
file:
.
.
.
advfs_opT myop;
int fd;
int io_mode;
.
.
.
myop.version = ADVFS_OP_CURR_VERSION;
myop.operation = ADVFS_AW_DATA_LOGGING;
myop.action = ADVFS_GET_INFO;
myop.info_buf = &io_mode;
myop.info_buf_size = sizeof(int);
ret = fcntl(fd, F_ADVFS_OP, &myop);
if (ret) {
perror("fcnt1 failed");
if (io_mode == ADVFS_ASYNC_IO)
printf("I/O mode is asynchronous.\n");
else if (io_mode == ADVFS_DATA_LOGGING_IO)
printf("I/O mode is atomic write data logging.\n");
else if (io_mode == ADVFS_SYNC_IO)
printf("I/O mode is forced synchronous writes.\n");
SEE ALSO
Commands: showfile(8), mount(8), chfsets(8)
Files: advfs(4)
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