What is file system journaling?

faezeh · 03-21-2005, 10:58 AM

Hi,
I have question regarding file system journaling,which is the difference between ext2 and ext3.
I think the difference is that when computer wants to be shut down and the file systems are unmounted
and the directory (mountpoint) wants to be transferred to the file system (storage device),if the fs system
type related to that mountpoint (directory) was ext3 either writes the complete changes to that storage device (fs)
or doesnot write at all but if the type of the fs related to that mountpoint was ext2 may sometimes write the changes
incomppletely,may umount the fs incompletely before shutting down.
thanks

marghorp · 03-21-2005, 01:04 PM

Yes, journaling is as it's name tells you a logging capability of the filesystem. If at any time there is some error, that causes the filesystem to stop working (being unable to write the changes to the harddisk), the journal is still the thing that can write those changes when the filesystem is operational again, and thus makes the recovery of the partition. In the end, it's a good thing to have.

faezeh · 03-23-2005, 01:51 AM

Thanks,
But can you tell me that when does writting on hard partition take place?
I believe that it only takes place when you try to umount the file system of that partition,so this file system journaling can only be useful when a problem occurs at umount time and this information can not be completely written on the hard partition,and then because the fstype is "ext3" when the next time the partition is mounted,the writting starts again from the point that it had ended.
Am I right?

michaelk · 03-23-2005, 05:51 PM

Here is one decent article on journaling filesystems:
http://www.linux-mag.com/2002-10/jfs_01.html

Data read from and written to the disk is buffered so data is not written immediately. When you unmount a drive any data still in the buffer is written to disk.

http://www.faqs.org/docs/linux_admin/buffer-cache.html