3.7. 数据日志

Ordinarily, GFS2 writes only metadata to its journal. File contents are subsequently written to disk by the kernel's periodic sync that flushes file system buffers. An fsync() call on a file causes the file's data to be written to disk immediately. The call returns when the disk reports that all data is safely written.

Data journaling can result in a reduced fsync() time for very small files because the file data is written to the journal in addition to the metadata. This advantage rapidly reduces as the file size increases. Writing to medium and larger files will be much slower with data journaling turned on.

依赖 fsync() 同步文件数据的应用程序可能因使用数据日志而使性能有所提高。在被标记的目录及其所有子目录中生成的 GFS2 文件可自动启用数据日志。现有长度为 0 的文件也可以打开或者关闭其数据日志功能。

Enabling data journaling on a directory sets the directory to "inherit jdata", which indicates that all files and directories subsequently created in that directory are journaled. You can enable and disable data journaling on a file with the chattr command.

The following commands enable data journaling on the /mnt/gfs2/gfs2_dir/newfile file and then check whether the flag has been set properly.

# chattr +j /mnt/gfs2/gfs2_dir/newfile
# lsattr /mnt/gfs2/gfs2_dir
---------j--- /mnt/gfs2/gfs2_dir/newfile

以下命令禁用了 /mnt/gfs2/gfs2_dir/newfile 文件中的数据日志，并查看是否正确设定了标签。

# chattr -j /mnt/gfs2/gfs2_dir/newfile
# lsattr /mnt/gfs2/gfs2_dir
------------- /mnt/gfs2/gfs2_dir/newfile

You can also use the chattr command to set the j flag on a directory. When you set this flag for a directory, all files and directories subsequently created in that directory are journaled. The following set of commands sets the j flag on the gfs2_dir directory, then checks whether the flag has been set properly. After this, the commands create a new file called newfile in the /mnt/gfs2/gfs2_dir directory and then check whether the j flag has been set for the file. Since the j flag is set for the directory, then newfile should also have journaling enabled.

# chattr -j /mnt/gfs2/gfs2_dir
# lsattr /mnt/gfs2
---------j--- /mnt/gfs2/gfs2_dir
# touch /mnt/gfs2/gfs2_dir/newfile
# lsattr /mnt/gfs2/gfs2_dir
---------j--- /mnt/gfs2/gfs2_dir/newfile