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The second call to cat a file does not update the access time. I was expecting the access time to be updated ever time a file contents gets displayed.

I see the same behaviour if I open the file in web browser. Its access time does not get updated consistently.

Am I misunderstanding access time? What is different between the two cat calls?

$ touch test
$ stat test
  File: `test'
  Size: 0           Blocks: 0          IO Block: 4096   regular empty file
Device: 803h/2051d  Inode: 152694      Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1001/   aaron)   Gid: ( 1001/   aaron)
Access: 2012-08-21 11:05:40.586020996 +0200
Modify: 2012-08-21 11:05:40.586020996 +0200
Change: 2012-08-21 11:05:40.586020996 +0200
 Birth: -

$ vim test
$ stat test
  File: `test'
  Size: 5           Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d  Inode: 152694      Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1001/   aaron)   Gid: ( 1001/   aaron)
Access: 2012-08-21 11:05:52.890021630 +0200
Modify: 2012-08-21 11:06:31.606023626 +0200
Change: 2012-08-21 11:06:31.638023629 +0200
 Birth: -

$ cat test
test

$ stat test
  File: `test'
  Size: 5           Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d  Inode: 152694      Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1001/   aaron)   Gid: ( 1001/   aaron)
Access: 2012-08-21 11:06:44.662024298 +0200
Modify: 2012-08-21 11:06:31.606023626 +0200
Change: 2012-08-21 11:06:31.638023629 +0200
 Birth: -

$ cat test
test

$ stat test
  File: `test'
  Size: 5           Blocks: 8          IO Block: 4096   regular file
Device: 803h/2051d  Inode: 152694      Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1001/   aaron)   Gid: ( 1001/   aaron)
Access: 2012-08-21 11:06:44.662024298 +0200
Modify: 2012-08-21 11:06:31.606023626 +0200
Change: 2012-08-21 11:06:31.638023629 +0200
 Birth: -
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1 Answer 1

up vote 13 down vote accepted

http://en.wikipedia.org/wiki/Stat_(system_call)

Criticism of atime

Writing to a file changes its mtime and ctime, while reading a file changes its atime. As a result, on a POSIX-compliant system, reading a file causes a write, which has been criticized. This behaviour can usually be disabled by adding a mount option in /etc/fstab.

However, turning off atime updating breaks POSIX compliance, and some applications, notably the mutt mail reader (in some configurations), and some file usage watching utilities, notably tmpwatch. In the worst case, not updating atime can cause some backup programs to fail to back up a file.

Linux kernel developer Ingo Molnár called atime "perhaps the most stupid Unix design idea of all times," adding: "[T]hink 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!'" He further emphasized the performance impact thus:

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.

how to know if noatime or relatime is default mount option in kernel?

man mount
....
   relatime
          Update inode access times relative to  modify  or  change  time.
          Access time is only updated if the previous access time was ear‐
          lier than the current modify or change time. (Similar  to  noat‐
          ime,  but  doesn't break mutt or other applications that need to
          know if a file has been read since the last time  it  was  modi‐
          fied.)

          Since Linux 2.6.30, the kernel defaults to the behavior provided
          by this option (unless noatime was  specified), and the stricta‐
          time  option  is  required  to  obtain traditional semantics. In
          addition, since Linux 2.6.30, the file's  last  access  time  is
          always  updated  if  it  is more than 1 day old.
....

Which is how that particular partition was mounted and why cat does not update the access time as I expected.

share|improve this answer
    
More info about how ubuntu handles atime askubuntu.com/questions/2099/… –  nelaar Aug 22 '12 at 8:32
    
Here is an older discussion on how atime is affected by different mount options. linux.koolsolutions.com/2009/01/30/… –  nelaar Aug 22 '12 at 8:38

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