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atime, ctime and mtime in Unix filesystems

 

Three times tracked for each file in Unix are these:

atime - File Access Time

Access time shows the last time the data from a file was accessed - read by one of the Unix processes directly or through commands and scripts.

ctime - File Change Time

ctime indicates the last time a files metadata (inode) was changed. It changes when you change file's ownership or access permissions. The Linux man page for stat summarizes ctime behavior nicely: The field st_ctime is changed by writing or by setting inode information (i.e., owner, group, link count, mode, etc.).

mtime - File Modify Time

Last modification time shows time of the  last change to file's contents. It does not change with owner or permission changes, and is therefore used for tracking the actual changes to data of the file itself.

Find atime, ctime and mtime with ls

The simplest way to confirm the times associated with a file is to use ls command.

Timestamps are shown when using the long-format output of ls command, ls -l:

ubuntu# ls -l /tmp/file1
-rw-rr 1 greys root 9 2008-04-05 07:10 /tmp/file1

This is the default output of ls -l, which shows you the time of the last file modification - mtime. In our example, file /tmp/file1 was last changed around 7:10am.

If we want to see the last access time for this file, atime - you need to use -lu options for ls. The output will probably show some later time:

ubuntu# ls -lu /tmp/file1
-rw-rr 1 greys root 9 2008-04-05 07:27
In the example, it's 7:27am.

Lastly, ls -lc will show you the last time our file was changed, ctime:

ubuntu# ls -lc /tmp/file1
-rw-rr 1 greys root 9 2008-04-05 07:31 

Show atime, ctime and mtime with stat command

In Linux distributions, you will probably find a stat command, which can be used to show all of the times in a more convenient way, and among plenty of other useful information about your file:

ubuntu# stat /tmp/file1
File: `/tmp/file1′
Size: 9             Blocks: 8          IO Block: 4096   regular file
Device: 811h/2065d    Inode: 179420      Links: 1
Access: (0644/-rw-rr)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2008-04-05 07:27:51.000000000 +0100
Modify: 2008-04-05 07:10:14.000000000 +0100
Change: 2008-04-05 07:35:22.000000000 +0100

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[Aug 7, 2007] Linux Replacing atime

August 7, 2007 | KernelTrap 

Submitted by Jeremy on August 7, 2007 - 9:26am.

In a recent lkml thread, Linus Torvalds was involved in a discussion about mounting filesystems with the noatime option for better performance, "'noatime,data=writeback' will quite likely be *quite* noticeable (with different effects for different loads), but almost nobody actually runs that way."

He noted that he set O_NOATIME when writing git, "and it was an absolutely huge time-saver for the case of not having 'noatime' in the mount options. Certainly more than your estimated 10% under some loads."

The discussion then looked at using the relatime mount option to improve the situation, "relative atime only updates the atime if the previous atime is older than the mtime or ctime. Like noatime, but useful for applications like mutt that need to know when a file has been read since it was last modified."

Ingo Molnar stressed the significance of fixing this performance issue, "I cannot over-emphasize how much of a deal it is in practice. Atime updates are by far the biggest IO performance deficiency that Linux has today. Getting rid of atime updates would give us more everyday Linux performance than all the pagecache speedups of the past 10 years, _combined_." He submitted some patches to improve relatime, and noted about atime:

"It's also perhaps the most stupid Unix design idea of all times. Unix is really nice and well done, but think about this a bit: 'For every file that is read from the disk, lets do a ... write to the disk! And, for every file that is already cached and which we read from the cache ... do a write to the disk!'"


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